AN29 - Some Thoughts on DC/DC Converters

Transcription

Application Note 29
October 1988
Some Thoughts on DC/DC Converters
Jim Williams and Brian Huffman
INTRODUCTION
Many systems require that the primary source of DC power
be converted to other voltages. Battery driven circuitry is
an obvious candidate. The 6V or 12V cell in a laptop computer must be converted to different potentials needed for
memory, disc drives, display and operating logic. In theory,
AC line powered systems should not need DC/DC converters
because the implied power transformer can be equipped
with multiple secondaries. In practice, economics, noise
requirements, supply bus distribution problems and other
constraints often make DC/DC conversion preferable. A
common example is logic dominated, 5V powered systems
utilizing ±15V driven analog components.
The range of applications for DC/DC converters is large,
with many variations. Interest in converters is commensurately quite high. Increased use of single supply powered
systems, stiffening performance requirements and battery
operation have increased converter usage.
Historically, efficiency and size have received heavy emphasis. In fact, these parameters can be significant, but
often are of secondary importance. A possible reason
behind the continued and overwhelming attention to size
and efficiency in converters proves surprising. Simply
put, these parameters are (within limits) relatively easy to
achieve! Size and efficiency advantages have their place,
but other system-oriented problems also need treatment.
Low quiescent current, wide ranges of allowable inputs,
substantial reductions in wideband output noise and cost
effectiveness are important issues. One very important
converter class, the 5V to ±15V type, stresses size and
efficiency with little emphasis towards parameters such
as output noise. This is particularly significant because
wideband output noise is a frequently encountered problem
with this type of converter. In the best case, the output noise
mandates careful board layout and grounding schemes.
In the worst case, the noise precludes analog circuitry
from achieving desired performance levels (for further
discussion see Appendix A, “The 5V to ±15V Converter
— A Special Case”). The 5V to ±15V DC/DC conversion
requirement is ubiquitous, and presents a good starting
point for a study of DC/DC converters.
5V TO ±15V CONVERTER CIRCUITS
Low Noise 5V to ±15V Converter
Figure 1’s design supplies a ±15V output from a 5V input.
Wideband output noise measures 200 microvolts peakto-peak, a 100× reduction over typical designs. Efficiency
at 250mA output is 60%, about 5% to 10% lower than
conventional types. The circuit achieves its low noise
performance by minimizing high speed harmonic content
in the power switching stage. This forces the efficiency
trade-off noted, but the penalty is small compared to the
benefit.
The 74C14 based 30kHz oscillator is divided into a 15kHz
2-phase clock by the 74C74 flip-flop. The 74C02 gates and
10k-0.001μF delays condition this 2-phase clock into nonoverlapping, 2-phase drive at the emitters of Q1 and Q2
(Figure 2, Traces A and B, respectively). These transistors
provide level shifting to drive emitter followers Q3-Q4. The
Q3-Q4 emitters see 100Ω-0.003μF filters, slowing drive
to output MOSFETs Q5-Q6. The filter’s effects appear at
the gates of Q5 and Q6 (Traces C and D, respectively). Q5
and Q6 are source followers, instead of the conventional
common source connection. This limits transformer rise
time to the gate terminals filtered slew rate, resulting in
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
an32f
AN29-1
Application Note 29
22k
1N5817
+
+
10
100
+V
74C14
POINT
“A”
(SEE TEXT)
BOOST
OUTPUT
≈17VDC
Q
74C74
D
Q
0.001
2
1N5817
LT1054
5
D3
1N5817
8
BOOST
D2
1N5817
47
10k
0.001
74C02
4
TURBO BURST
5V
10k
Q5
0.001
74C02
74C14
74C14
Q1
7
2
8
470
0.001
LT1086
3
100
9
1k
10k
LEVEL SHIFTS
100Ω
K2
Q2
D
249*
= ±15 COMMON
100
= +5 GROUND
10k
150k
LT337A
5V
Q4
0.003
1k
–4VDC
22μF
OUT
COMMON
100μF
Q6
DIRVERSEDGE
SHAPING
1.37k*
10μF
OUTPUT
S
10μF
+
L1
D1
1N5817
0.003
+15
OUT
+
124*
100μF
Q3
100Ω
150k
15kHz, 5μs
NON-OVERLAP
S
1
MUR 120
(ALL)
+
K1
6
+
74C14
74C14
D
10μF
* = 1% FILM RESISTOR
FET = MTP3055E-MOTOROLA
PNP = 2N3906
NPN = 2N3904
L1 = PULSE ENGINEERING, INC. #PE-61592
= FERRITE BEAD, FERRONICS #21-110J
+
CLK-NON
OVERLAP
GENERATOR
5VIN
(4.5V TO 5.5V)
+
1μF
–15
OUT
+
CK
3
2.74k*
AN29 F01
+
Figure 1. Low Noise 5V to ±15V Converter
put. The 470Ω-0.001μF damper in L1’s output maintains
loading during switching, aiding low noise performance.
The ferrite beads in the gate leads eliminate parasitic RF
oscillations associated with follower configurations.
A = 20V/DIV
B = 20V/DIV
C = 20V/DIV
D = 20V/DIV
E = 10V/DIV
F = 10V/DIV
HORIZ = 20μs/DIV
AN29 F02
Figure 2. 5V to ±15V Low Noise Converter Waveforms
well controlled waveforms at the sources of Q5 and Q6
(Traces E and F, respectively). L1 sees complimentary,
slew limited drive, eliminating the high speed harmonics
normally associated with this type converter. L1’s output
is rectified, filtered and regulated to obtain the final out-
The source follower configuration eases controlling L1’s
edge rise times, but complicates gate biasing. Special
provisions are required to get the MOSFETs fully turned
on and off. Source follower connected Q5 and Q6 require
voltage overdrive at the gates to saturate. The 5V primary
supply cannot provide the specified 10V gate — channel
bias required for saturation. Similarly, the gates must be
pulled well below ground to turn the MOSFETs off. This
is so because L1’s behavior pulls the sources negative
when the devices turn off. Turn-off bias is bootstrapped
from the negative side of Q6’s source waveform. D1 and
the 22μF capacitor produce a –4V potential for Q3 and Q4
to pull down to. Turn-on bias is generated by a 2-stage
an32f
AN29-2
Application Note 29
boost loop. The 5V supply is fed via D3 to the LT®1054
switched capacitor voltage converter (switched capacitor
voltage converters are discussed in Appendix B, “Switched
Capacitor Voltage Converters — How They Work”). The
LT1054 configuration, set up as a voltage doubler, initially
provides about 9V boost to point “A” at turn-on. When
the converter starts running L1 produces output (“Turbo
Boost” on schematic) at windings 4-6 which is rectified by
D2, raising the LT1054’s input voltage. This further raises
point “A” to the 17V potential noted on the schematic.
These internally generated voltages allow Q5 and Q6 to
receive proper drive, minimizing losses despite their source
follower connection. Figure 3, an AC-coupled trace of the
15V converter output, shows 200μVP-P noise at full power
(250mA output). The –15V output shows nearly identical characteristics. Switching artifacts are comparable in
amplitude to the linear regulators noise. Further reduction
in switching based noise is possible by slowing Q5 and
Q6 rise times. This, however, necessitates reducing clock
rate and increasing non-overlap time to maintain available
output power and efficiency. The arrangement shown
represents a favorable compromise between output noise,
available output power, and efficiency.
A = 100μV/DIV
(AC-COUPLED)
HORIZ = 5μs/DIV
AN29 F03
Figure 3. Output Noise of the Low Noise 5V to ±15V Converter.
Appendix H Shows a Modern IC Low Noise Regulator
Ultralow Noise 5V to ±15V Converter
Residual switching components and regulator noise set
Figure 1’s performance limits. Analog circuitry operating
at the very highest levels of resolution and sensitivity may
require the lowest possible converter noise. Figure 4’s
converter uses sine wave transformer drive to reduce
harmonics to negligible levels. The sine wave transformer
drive combines with special output regulators to produce
less than 30μV of output noise. This is almost 7× lower
than the previous circuit and approaches a 1000× improvement over conventional designs. The trade off is efficiency
and complexity.
A1 is set up as a 16kHz Wein bridge oscillator. The single
power supply requires biasing to prevent A1’s output from
saturating at the ground rail. This bias is established by
returning the undriven end of the Wein network to a DC
potential derived from the LT1009 reference. A1’s output
is a pure sine wave (Figure 5, Trace A) biased off ground.
A1’s gain must be controlled to maintain sine wave output.
A2 does this by comparing A1’s rectified and filtered positive output peaks with an LT1009 derived DC reference.
A2’s output, biasing Q1, servo controls A1’s gain. The
0.22μF capacitor frequency compensates the loop, and the
thermally mated diodes minimize errors due to rectifier
temperature drift. These provisions fix A1’s AC and DC
output terms against supply and temperature changes.
A1’s output is AC coupled to A3. The 2k –820Ω divider
re-biases the sine wave, centering it inside A3’s input
common mode range even with supply shifts. A3 drives a
power stage, Q2-Q5. The stages common emitter outputs
and biasing permit 1VRMS (3VP-P) transformer drive, even
at VSUPPLY = 4.5V. At full converter output loading the
stage delivers 3 ampere peaks but the waveform is clean
(Trace B), with low distortion (Trace C). The 330μF coupling
capacitor strips DC and L3 sees pure AC. Feedback to A3 is
taken at the Q4-Q5 collectors. The 0.1μF unit at this point
suppresses local oscillations. L3’s secondary RC network
adds additional high frequency damping.
Without control of quiescent current the power stage
will encounter thermal runaway and destroy itself. A4
measures DC output current across Q5’s emitter resistor
and servo controls Q6 to fix quiescent current. A divided
portion of the LT1009 reference sets the servo point at
A4’s negative input and the 0.33μF feedback capacitor
stabilizes the loop.
L3’s rectified and filtered outputs are applied to regulators
designed for low noise. A5 and A7 amplify the LT1021’s
filtered 10V output up to 15V. A6 and A8 provide the –15V
output. The LT1021 and amplifiers give better noise performance than three terminal regulators. The Zener-resistor
network clips overvoltages due to start-up transients.
an32f
AN29-3
Q1
–
A1
LT1006
= 1N4934
= 1N4148
= ±15 COMMON
= +5 GROUND
UNMARKED NPN = 2N3904
* = 1% METAL FILM RESISTOR
†
= THF337K006P1G
*
THERMALLY
MATED
750Ω*
–
8
A3
LT1006
+
430Ω
220Ω
1k
Q6
Q3
2N2905
Q2
2N2219
+
3.1k
10k
5V
20k
0.33
–
10k
1N4001
22μF
0.22
0.1Ω
Q5
MJE3055
0.1
330μF†
Q4
MJE2955
+
L4
100μH
IQ
CONTROL
LOOP
620Ω
10k
+
1
8
L3
4
+
+
A8
LT1010
1N5260B
*
*
5k
+
*
220μF
4.99k*
10k*
220μF
A7
LT1010
0.1
–15VOUT
L2
47μF
25μH
330Ω
OUT
LT1021
IN 10V
19V
UNREG
8
A5
1/2 LT1013
–
0.005 *
1μF
10k*
330Ω
A6
1/2 LT1013
–
–19V
UNREG
3
4
5
Figure 4. Ultralow Noise Sine Wave Drive 5V to ±15V Converter
LT1009
2.5
0.1
22μF
100Ω
50Ω
50Ω
A4
1/2 LT1013
1k
1k*
820Ω
2k
1/2 LT1013
200k
0.22
1μF
5V
68Ω
1N4001
+
0.22
680Ω
10k
0.01
430Ω
POWER
AMP
–
A2
OSCILLATOR
STAB. LOOP
2k
270Ω
(SELECTED
VALUE—
SEE TEXT)
47μF
+
8
220Ω
16kHz
OSCILLATOR
L1, L2 = PULSE ENGINEERING, INC. #PE-92100
1k
+
1k
1k
+
0.01
AN29 F04
0.1
47μF
+
+
+
AN29-4
+
5VIN
(4.5V TO 5.5V)
10k*
OUT
COMMON
15VOUT
L1
25μH
Application Note 29
an32f
Application Note 29
A = 2V/DIV
B = 2V/DIV
C = 1% DISTORTION
D = 20μV/DIV
HORIZ = 50μs/DIV
AN29 F05
Figure 5. Waveforms for the Sine Wave Driven Converter.
Note that Output Noise (Trace D) is Only 30μVP-P
L1 and L2 combine with their respective output capacitors to aid low noise characteristics. These inductors are
outside the feedback loop, but their low copper resistance
does not significantly degrade regulation. Trace D, the 15V
output at full load, shows less than 30μV (2ppm) of noise.
The most significant trade-off in this design is efficiency.
The sine wave transformer drive forces substantial power
loss. At full output (75mA), efficiency is only 30%.
Before use, the circuit should be trimmed for lowest
distortion (typically 1%) in the sine wave delivered to
L3. This trim is made by selecting the indicated value at
A1’s negative input. The 270Ω value shown is nominal,
with a typical variance of ±25%. The sine wave’s 16kHz
frequency is a compromise between the op amps available gain bandwidth, magnetics size, audible noise, and
minimization of wideband harmonics.
Single Inductor 5V to ±15V Converter
Simplicity and economy are another dimension in 5V to
±15V conversion. The transformer in these converters is
usually the most expensive component. Figure 6’s unusual
drive scheme allows a single, 2-terminal inductor to replace
the usual transformer at significant cost savings. Tradeoffs include loss of galvanic isolation between input and
output and lower power output. Additionally, the regulation
technique employed causes about 50mV of clock related
output ripple.
The circuit functions by periodically and alternately allowing
each end of the inductor to flyback. The resultant positive
and negative peaks are rectified and filtered. Regulation
is obtained by controlling the number of flyback events
during the respective output’s flyback interval.
The leftmost logic inverter produces a 20kHz clock (Trace A,
Figure 7) which feeds a logic network composed of additional inverters, diodes and the 74C90 decade counter. The
counter output (Trace B) combines with the logic network
to present alternately phased clock bursts (Traces C and D)
to the base resistors of Q1 and Q2. When φ1 (Trace B)
is unclocked it resides in its high state, biasing Q2 and
Q4 on. Q4’s collector effectively grounds the “bottom” of
L1 (Trace H). During this interval φ2 (Trace A) puts clock
bursts into Q1’s base resistor. If the –15V output is too
low servo comparator C1A’s output (Trace E) is high, and
Q1’s base can receive pulsed bias. If the converse is true
the comparator will be low, and the bias gated away via
Q1’s base diode. When Q1 is able to bias, Q3 switches,
resulting in negative going flyback events at the “top” of
L1 (Trace G). These events are rectified and filtered to
produce the –15V output. C1A regulates by controlling
the number of clock pulses that switch the Q1-Q3 pair.
The LT1004 serves as a reference. Trace J, the AC-coupled
–15V output, shows the effect of C1A’s regulating action.
The output stays within a small error window set by C1A’s
switched control loop. As input voltage and loading conditions change C1A adjusts the number of clock pulses
allowed to bias Q1-Q3, maintaining loop control.
When the φ1 and φ2 signals reverse state the operating
sequence reverses. Q3’s collector (Trace G) is pulled high
with Q2-Q4 switching controlled by C1B’s servo action.
Operating waveforms are similar to the previous case.
Trace F is C1B’s output, Trace H is Q4’s collector (L1’s “bottom”) and Trace I is the AC-coupled 15V output. Although
the two regulating loops share the same inductor they
operate independently, and asymmetrical output loading is
not deleterious. The inductor sees irregularly spaced shots
of current (Trace K), but is unaffected by its multiplexed
operation. Clamp diodes prevent reverse biasing of Q3
and Q4 during transient conditions. The circuit provides
±25mA of regulated power at 60% efficiency.
Low Quiescent Current 5V to ±15V Converter
A final area in 5V to ±15V converter design is reduction
of quiescent current. Typical units pull 100mA to 150mA
of quiescent current, unacceptable in many low power
systems.
an32f
AN29-5
Application Note 29
150k*
5V
12.4k*
+
300Ω
C1A
1/2 LT1018
HP5082-2810
10k
5V
–
2k
100Ω
32k
K2
10k
4.7k
Q3
2N5023
Q1
2N3906
–15VOUT
+
100
1000pF
5
L1
145μH
10k
74C90
÷10
5V
K1
12
4.7k
Q2
2N3904
100Ω
10k
Q4
2N3507
2k
5V
300Ω
15VOUT
+
100
137k*
+
C1B
1/2 LT1018
–
12.4k*
1k
5V
AN29 F06
LT1004
1.2V
= 1N4148
= 74C14
L1 = PULSE ENGINEERING, INC. # PE-92105
Figure 6. Single Inductor 5V to ±15V Regulated Converter
A = 5V/DIV
B = 5V/DIV
C = 10V/DIV
D = 10V/DIV
E = 10V/DIV
F = 10V/DIV
G = 20V/DIV
H = 20V/DIV
I = 0.05V/DIV (AC-COUPLED)
J = 0.05V/DIV (AC-COUPLED)
K = 1A/DIV
HORIZ = 100μs/DIV
AN29 F07
Figure 7. Waveforms for the Single Inductor, Dual-Output,
Regulated Converter
Figure 8’s design supplies ±15V outputs at 100mA while
consuming only 10mA quiescent current. The LT1070
switching regulator (for a complete description of this
device, see Appendix C, “Physiology of the LT1070”) drives
L1 in flyback mode. A damper network clamps excessive
flyback voltages. Flyback events at L1’s secondary are
half-wave rectified and filtered, producing positive and
negative outputs across the 47μF capacitors. The positive
16V output is regulated by a simple loop. Comparator
C1A balances a sample of the positive output with a 2.5V
reference obtained from the LT1020. When the 16V output (Trace A, Figure 9) is too low, C1A switches (Trace B)
high, turning off the 4N46 opto-isolator. Q1 goes off, and
the LT1070’s control pin (VC) pulls high (Trace C). This
causes full duty cycle 40kHz switching at the VSW pin
(Trace D). The resultant energy into L1 forces the 16V
output to ramp quickly positive, turning off C1A’s output.
an32f
AN29-6
Application Note 29
5VIN
(4.5V TO 5.5V)
16V PRE-REG
1N4148
1
1.2k
2W
0.47
L1
9
1.2M*
+
C1A
1/2 LT1017
216k*
8
+
47μF
8
5
+
MUR120
20M
47μF
3
9
3
GND
VIN
–
OUT
–1N
LT1070
COMP
PNP
+IN
7
FB
VIN
NC
FB
VSW
LT1070
VC
82k
0.001μF
3M*
10k
15VOUT
100mA
11
4
500k*
6
0.001μF
2.5M*
+
10μF
500k*
2.5V COMP
REF OUT NPN
7
1N4148
5V
2
+
10μF
100k
4N46
–15VOUT
100mA
GND
Q2
VN2222
10k
–16V UNREG
10k
Q1
2N3906
470k
TO
REF OUT
TO 16V
PRE-REG
0.002
470k
390k
= +5 GROUND
= ±15 COMMON
UPDATE
Burst Mode regulators
can achieve lower IQ
OPTIONAL
(SEE TEXT)
TO
–16V UNREG
3.2M
–
TO
–15V
1.5M
+
1.5k
C1B
1/2 LT1017
5.6M
47k
AN29 F08
Figure 8. Low IQ, Isolated 5V to ±15V Converter
A = 100mV/DIV
(AC-COUPLED ON
16VDC LEVEL)
B = 20V/DIV
C = 2V/DIV
D = 20V/DIV
HORIZ = 5ms/DIV
AN29 F09
Figure 9. Waveforms for the Low IQ 5V to ±15V Converter
The 20M value combined with the 4N46’s slow response
(note the delay between C1A going high and the VC pin
rise) gives about 40mV of hysteresis. The LT1070’s onoff duty cycle is load dependent, saving significant power
when the converter is lightly loaded. This characteristic
is largely responsible for the 10mA quiescent current.
The opto-isolator preserves the converters input-output
isolation. The LT1020, a low quiescent current regulator
with low dropout, further regulates the 16V line, giving
the 15V output. The linear regulation eliminates the 40mV
ripple and improves transient response. The –16V output
tends to follow the regulated –16V line, but regulation
is poor. The LT1020’s auxiliary onboard comparator is
compensated to function as an op amp by the RC damper
at Pin 5. This amplifier linearly regulates the –16V line.
MOSFET Q2 provides low dropout current boost, sourcing
the –15V output. The –15V output is stabilized with the
op amp by comparing it with the 2.5V reference via the
500k-3M current summing resistors. 1000pF capacitors
frequency compensate each regulating loop. This converter
functions well, providing ±15V outputs at 100mA with
only 10mA quiescent current. Figure 10 plots efficiency
versus a conventional design over a range of loads. For
high loads results are comparable, but the low quiescent
circuit is superior at lower current.
A possible problem with this circuit is related to the poor
regulation of the –16V line. If the positive output is lightly
loaded L1’s magnetic flux is low. Heavy negative output
loading under this condition results in the –16V line falling
below its output regulators dropout value. Specifically, with
no load on the 15V output only 20mA is available from
the –15V output. The full 100mA is only available from
an32f
AN29-7
Application Note 29
output (the VC pin) uses an RC damper for stable loop
compensation.
100
90
EFFICIENCY (%)
80
This circuit works well but pulls 9mA of quiescent current.
If battery capacity is limited by size or weight this may be
too high. How can this figure be reduced while retaining
high current performance?
LOW QUIESCENT
CURRENT DESIGN
70
60
50
CONVENTIONAL
DESIGN
40
30
VIN
6V
20
L1*
50μH
10
0
0
10 20 30 40 50 60 70 80 90 100
OUTPUT CURRENT (mA)
AN29 F10
MUR8100
VIN
VSW
470μF
LT1070
Figure 10. Efficiency vs Load for the Low IQ Converter
GND
the –15V output when the 15V output is supplying more
than 8mA. This restriction is often acceptable, but some
situations may not tolerate it. The optional connection in
Figure 8 (shown in dashed lines) corrects the difficulty.
C1B detects the onset of –16V line decay. When this occurs its output pulls low, loading the 16V line to correct
the problem. The biasing values given permit correction
before the negative linear regulator drops out.
MICROPOWER QUIESCENT CURRENT CONVERTERS
Many battery-powered applications require very wide
ranges of power supply output current. Normal conditions
require currents in the ampere range, while standby or
“sleep” modes draw only microamperes. A typical laptop
computer may draw 1 to 2 amperes running while needing only a few hundred microamps for memory when
turned off. In theory, any DC/DC converter designed
for loop stability under no-load conditions will work. In
practice, a converter’s relatively large quiescent current
may cause unacceptable battery drain during low output
current intervals.
Figure 11 shows a typical flyback based converter. In this
case the 6V battery is converted to a 12V output by the
inductive flyback voltage produced each time the LT1070’s
VSW pin is internally switched to ground (for commentary
on inductor selection in flyback converters see Appendix D,
“Inductor Selection for Flyback Converters”). An internal
40kHz clock produces a flyback event every 25μs. The
energy in this event is controlled by the IC’s internal error amplifier, which acts to force the feedback (FB) pin to
a 1.23V reference. The error amplifiers high impedance
+
VC
VOUT
12V
10.7k
FB
1.24k
1k
+
1μF
AN29 F11
*PULSE ENGINEERING, INC
#PE-51515
Figure 11. 6V to 12V, 2 Amp Converter with 9mA Quiescent Current
A solution is suggested by considering an auxiliary VC pin
function. If the VC pin is pulled within 150mV of ground the
IC shuts down, pulling only 50 microamperes. Figure 12’s
special loop exploits this feature, reducing quiescent current to only 150 microamperes. The technique shown is
particularly significant, with broad implication in battery
powered systems. It is easily applied to a wide variety of
DC/DC converters, meeting an acknowledged need across
a wide spectrum of applications.
Figure 12’s signal flow is similar to Figure 11, but additional
circuitry appears between the feedback divider and the VC
pin. The LT1070’s internal feedback amplifier and reference
are not used. Figure 13 shows operating waveforms under
no-load conditions. The 12V output (Trace A) ramps down
over a period of seconds. During this time comparator
A1’s output (Trace B) is low, as are the 74C04 paralleled
inverters. This pulls the VC pin (Trace C) low, putting the
IC in its 50μA shutdown mode. The VSW pin (Trace D) is
high, and no inductor current flows. When the 12V output
drops about 20mV, A1 triggers and the inverters go high,
pulling the VC pin up and turning on the regulator. The
VSW pin pulses the inductor at the 40kHz clock rate, causing the output to abruptly rise. This action trips A1 low,
forcing the VC pin back into shutdown. This “bang-bang”
control loop keeps the 12V output within the 20mV ramp
an32f
AN29-8
Application Note 29
–
6VIN
(4.5V TO 8V)
+
47μF
L1
50μH
A2
1/2 LT1017
3.6M*
1.2M*
MUR405
+
VSW
LT1070 FB
VC
“LOW BATT”
+
12VOUT
C1
2700μF
R1
1M*
R7
100k
C3
1500pF
NC
GND
R2
120k*
6V
R6
200Ω
UPDATE
Micropower regulators using
Burst Mode operation are available
–
C2
47μF
+
+
A1
1/2 LT1017
10pF**
R3
2M
R4
10k
R5
180k
6V
** = OPTIONAL. SEE TEXT
= 1N4148
LT1004
1.2V
AN29 F12
= 74C04
Figure 12. 6V to 12V, 2 Amp Converter with 150μA Quiescent Current
hysteresis window set by R3-R4. Diode clamps prevent
VC pin overdrive. Note that the loop oscillation period of
4 to 5 seconds means the R6-C2 time constant at VC is
not a significant term. Because the LT1070 spends almost
all of the time in shutdown, very little quiescent current
(150μA) is drawn.
Figure 14 shows the same waveforms with the load increased to 3mA. Loop oscillation frequency increases to
keep up with the loads sink current demand. Now, the VC
pin waveform (Trace C) begins to take on a filtered appearance. This is due to R6-C2’s 10ms time constant. If
A = 0.02V/DIV
(AC-COUPLED)
B = 5V/DIV
C = 2V/DIV
D = 10V/DIV
HORIZ = 1s/DIV
AN29 F13
Figure 13. Low IQ Converter Waveforms with No Load
(Traces B and D Retouched for Clarity)
an32f
AN29-9
Application Note 29
A formal stability analysis for this circuit is quite complex,
but some simplifications lend insight into loop operation.
At 100μA loading (120kΩ) C1 and the load form a decay
time constant exceeding 300 seconds. This is orders of
magnitude larger than R7-C3, R6-C2, or the LT1070’s
40kHz commutation rate. As a result, C1 dominates the
loop. Wideband A1 sees phase shifted feedback, and very
low frequency oscillations similar to Figure 13’s occur1.
Although C1’s decay time constant is long, its charge
time constant is short because the circuit has low sourcing impedance. This accounts for the ramp nature of the
oscillations.
Increased loading reduces the C1 load decay time constant. Figure 16’s plot reflects this. As loading increases,
the loop oscillates at a higher frequency due to C1’s decreased decay time. When the load impedance becomes
low enough C1’s decay time constant ceases to dominate
the loop. This point is almost entirely determined by R6
and C2. Once R6 and C2 “take over” as the dominant time
constant the loop begins to behave like a linear system.
In this region (e.g. above about 75mA, per Figure 16) the
LT1070 runs continuously at its 40kHz rate. Now, the R7C3 time constant becomes significant, performing as a
simple feedback lead2 to smooth output response. There is
A = 0.02V/DIV
(AC-COUPLED)
B = 5V/DIV
C = 2V/DIV
D = 10V/DIV
AN29 F14
HORIZ = 20ms/DIV
Figure 14. Low IQ Converter Waveforms at Light Loading
A = 0.02V/DIV
B = 5V/DIV
C = 2V/DIV
D = 10V/DIV
AN29 F15
HORIZ = 20μs/DIV
Figure 15. Low IQ Converter Waveforms at 1 Amp Loading
550
LINEAR REGION
500
EXTENDS TO
2.5A
450
LOOP FREQUENCY (Hz)
the load continues to increase, loop oscillation frequency
will also increase. The R6-C2 time constant, however,
is fixed. Beyond some frequency, R6-C2 must average
loop oscillations to DC. Figure 15 shows the same circuit
points at 1 ampere loading. Note that the VC pin is at DC,
and repetition rate has increased to the LT1070’s 40kHz
clock frequency. Figure 16 plots what is occurring, with
a pleasant surprise. As output current rises, loop oscillation frequency also rises until about 500Hz. At this point
the R6-C2 time constant filters the VC pin to DC and the
LT1070 transitions into “normal” operation. With the VC
pin at DC it is convenient to think of A1 and the inverters as a linear error amplifier with a closed-loop gain set
by the R1-R2 feedback divider. In fact, A1 is still duty
cycle modulating, but at a rate far above R6-C2’s break
frequency. The phase error contributed by C1 (which was
selected for low loop frequency at low output currents) is
dominated by the R6-C2 roll off and the R7-C3 lead into
A1. The loop is stable and responds linearly for all loads
beyond 80mA. In this high current region the LT1070 is
desirably “fooled” into behaving like Figure 11’s circuit.
400
350
300
250
IQ = 150μA
0.2Hz
200
150
100
50
0
0
20
60
40
OUTPUT (mA)
80
100
AN29 F16
Figure 16. Figure 12’s Loop Frequency vs Output Current.
Note Linear Loop Operation Above 80mA
1Some layouts may require substantial trace area to A1’s inputs. In such
cases the optional 10pF capacitor shown ensures clean transitions at A1’s
output.
2“Zero Compensation” for all you technosnobs out there.
an32f
AN29-10
Application Note 29
constant decay3 (“rattling” is perhaps more appropriate)
is visible as Trace B approaches steady state between the
4th and 5th vertical divisions.
A = 10V/DIV
A2 functions as a simple low-battery detector, pulling low
when VIN drops below 4.8V.
B = 0.1V/DIV
(AC-COUPLED)
AN29 F17
HORIZ = 5ms/DIV
Figure 17. Load Transient Response for Figure 12’s
Low IQ Regulator
100
90
EFFICIENCY (%)
80
TYPICAL OPERATING
REGION
70
60
50
15mA
40
3mA
30
650μA
20
IQ = 150μA
10
0
0
0.5
TYPICAL STANDBY
REGION
1.5
2
1
OUTPUT CURRENT (A)
2.5
AN29 F18
Figure 18. Efficiency vs Output Current for Figure 12.
Standby Efficiency is Poor, But Power Loss Approaches
Battery Self Discharge
a fundamental trade-off in the selection of the R7-C3 lead
network values. When the converter is running in its linear
region they must dominate the DC hysteresis deliberately
generated by R3-R4. As such, they have been chosen for
the best compromise between output ripple at high load
and loop transient response.
Despite the complex dynamics transient response is quite
good. Figure 17 shows performance for a step from no
load to 1 ampere. When Trace A goes high a 1 ampere load
appears across the output (Trace B). Initially, the output
sags almost 150mV due to slow loop response time (the
R6-C2 pair delay VC pin response). When the LT1070 comes
on (signaled by the 40kHz “fuzz” at the bottom extreme
of Trace B) response is reasonably quick and surprisingly
well behaved considering circuit dynamics. The multi-time
Figure 18 plots efficiency versus output current. High
power efficiency is similar to standard converters. Low
power efficiency is somewhat better, although poor in
the lowest ranges. This is not particularly bothersome,
as power loss is very small.
This loop provides a controlled, conditional instability
instead of the more usually desirable (and often elusive)
unconditional stability. This deliberately introduced characteristic lowers converter quiescent current by a factor of
60 without sacrificing high power performance. Although
demonstrated in a boost converter, it is readily exportable
to other configurations. Figure 19a’s step-down (buck
mode) configuration uses the same basic loop with almost
no component changes. P-channel MOSFET Q1 is driven
from the LT1072 (a low power version of the LT1070) to
convert 12V to a 5V output. Q2 and Q3 provide current
limiting, while Q4 supplies turn off drive to Q1. the lower
output voltage mandates slightly different hysteresis biasing than Figure 12, accounting for the 1MΩ value at the
comparators positive input. In other respects the loop and
its performance are identical. Figure 19b uses the loop in
a transformer based multi-output converter. Note that the
floating secondaries allow a –12V output to be obtained
with a positive voltage regulator.
Low Quiescent Current Micropower 1.5V to 5V
Converter
Figure 20 extends our study of low quiescent current converters into the low voltage, micropower domain. In some
circumstances, due to space or reliability considerations, it
is preferable to operate circuitry from a single 1.5V cell. This
eliminates almost all ICs as design candidates. Although
it is possible to design circuitry which runs directly from
a single cell (see LTC® Application Note 15, “Circuitry For
Single Cell Operation”) a DC/DC converter permits using
higher voltage ICs. Figure 20’s design converts a single
3Once again, “multi-pole settling” for those who adore jargon.
an32f
AN29-11
Application Note 29
L1
100μH
Q1
IRF-9531
0.4Ω
12VIN
(8V TO 16V)
100Ω
MUR405
Q4
2N3904
Q2
2N3906
5VOUT
+
1k
1500pF
2700μF
1M
1M*
100k
+
VIN
VSW
LT1072 FB
VC
2k
340k*
NC
GND
Q3
2N3904
200Ω
1/2 LT1017
+
1N4148
10k
+
1N4148
12VIN
–
10μF
47μF
1N4148
10k
470k
12VIN
LT1004
1.2V
10pF**
AN29 F19a
= 74C04
UPDATE
Burst Mode regulators
can achieve lower IQ
Figure 19a. The Low Quiescent Current Loop Applied to a Buck Converter
1.5V cell to a 5V output with only 125μA quiescent current.
Oscillator C1A’s output is a 2kHz square wave (Trace D,
Figure 21). The configuration is conventional, except that
the biasing accommodates the narrow common mode
range dictated by the 1.5V supply. To maintain low power,
C1A’s integrating capacitor is small, with only 50mV of
swing. The parallel connected sides of C2 drive L1. When
the 5V output (Trace A) coasts down far enough C1B
goes low (Trace B), pulling both C2 positive inputs close
to ground. C1A’s clock now appears at the paralleled C2
outputs (Trace C), forcing energy into L1. The paralleled
outputs minimize saturation losses. L1’s flyback pulses,
rectified and stored in the 47μF capacitor, form the circuits
DC output. C1B on-off modulates C2 at whatever duty
cycle is required to maintain the circuits 5V output. The
LT1004 is the reference, with the resistor divider at C1B’s
positive input setting the output voltage. Schottky clamping
of C2’s outputs prevents negative going overdrives due to
parasitic L1 behavior.
an32f
AN29-12
Application Note 29
MUR120
L1
LT1086
+
7
1.2k
11k
470μF
t
8
12VIN
22μF
12V
+
10μF
9
+
MUR120
LT1086
4
10
t
2
2k
2W
1.2k
+
t
470μF
+
10μF
11k
11
–12V
VOUT
5V
1A
MBR360
0.2μF
3
+
5
t
C1
2700μF
L1
t
1
6
MUR120
VC
+
NC
C3
0.005
12VIN
R6
200Ω
A1
1/2 LT1017
C2
47μF
R2
453k*
+
GND
R7
10k
74C04 (5)
VSW
LT1071 FB
R1
1M*
–
VIN
R3
1M
10pF**
1N4148
1N4148
R4
10k
R5
180k
12VIN
LT1004
1.2
AN29 F19b
Figure 19b. Multi-Output, Transformer Coupled Low Quiescent Current Converter
The 1.2V LT1004 reference biasing is bootstrapped to the
5V output, permitting circuit operation down to 1.1V. A
10M bleed to supply ensures start-up. The 1M resistors
divide down the 1.2V reference, keeping C1B inside common mode limits. C1B’s positive feedback RC pair sets
about 100mV hysteresis and the 22pF unit suppresses
high frequency oscillation.
The micropower comparators and very low duty cycles at
light load minimize quiescent current. The 125μA figure
noted is quite close to the LT1017’s steady-state currents.
As load increases the duty cycle rises to meet the demand,
requiring more battery power. Decrease in battery voltage
produces similar behavior. Figure 22 plots available output
current versus battery voltage. Predictably, the highest
power is available with a fresh cell (e.g., 1.5V to 1.6V),
although regulation is maintained down to 1.15V for 250μA
loading. The plot shows that the test circuit continued to
regulate below this point, but this cannot be relied on in
practice (LT1017 VMIN = 1.15V). The low supply voltage
makes saturation and other losses in this circuit difficult
to control. As such, efficiency is about 50%.
an32f
AN29-13
Application Note 29
VIN
(1.1V TO 2V)
NC
L1
4
3.9M
5VOUT
HP5082-2810
0.001
3
4.3M*
+
NC
–
150pF
C1A
1/2 LT1017
1.5M
+
3.9M
5
2
6
1
47μF
C2B
1/2 LT1017
150k
22pF
C1B
1/2 LT1017
–
+
2M
10M
+
–
240k
619k*
1.5V
10M
1M*
470k
360k
1M*
VIN
LT1004
1.2V
10M
PNP = 2N3906
NPN = 2N3904
L1 = TRIAD # SP-29
390k
1.5V
–
C2A
1/2 LT1017
+
HP-5082-2810
1N4148
HP5082-2810
4
OPTIONAL FOR
NEGATIVE
OUTPUT
(SEE TEXT)
+
0.001
2.2μF
620k
1
+
22pF
1.5V
5
C2B
6
5.1M
3
–
1.2M*
47k
C1B
10k
VIN
47μF
LT1004
1.2V
+
C2A
10M
TO 390kΩ
OF C2B
AN29 F20
OUTPUT (μA) AVAILABLE AT VOUT = 5V
Figure 20. 800μA Output 1.5V to 5V Converter
A = 100mV/DIV
(AC-COUPLED ON
5VDC LEVEL)
B = 2V/DIV
C = 2V/DIV
D = 2V/DIV
HORIZ = 5ms/DIV
AN29 F21
850
800 IQ = 125μA
750
700
VOUT = 5V
650
EFFICIENCY ≈ 50%
600
550
500
450
400
350
300
250
200
LT1017
150
GUARANTEED MINIMUM
100
OPERATING VOLTAGE
50
0
0
1.05 1.15 1.25 1.35 1.45 1.55
INPUT VOLTAGE (V)
AN29 F22
Figure 21. Waveforms for Low Power 1.5V to 5V Converter
Figure 22. Output Current Capability vs Input Voltage for Figure 20
an32f
AN29-14
Application Note 29
The optional connection in Figure 20 (shown in dashed
lines) takes advantage of the transformers floating secondary to furnish a –5V output. Drive circuitry is identical, but
C1B is rearranged as a current summing comparator. The
LT1004’s bootstrapped positive bias is supplied by L1’s
primary flyback spikes.
at high power. If lowest quiescent current is necessary the
technique detailed back in Figure 12 is applicable.
The circuit is essentially a flyback regulator, similar to
Figure 11. The LT1070’s low saturation losses and ease
of use permit high power operation and design simplicity. Unfortunately, this device has a 3V minimum supply
requirement. Bootstrapping its supply pin from the 5V
output is possible, but requires some form of start-up
mechanism. Dual comparator C1 and the transistors form
a start-up loop. When power is applied C1A oscillates
(Trace A, Figure 24) at 5kHz. Q1 biases, driving Q2’s base
hard. Q2’s collector (Trace B) pumps L1, causing voltage
step-up flyback events. These events are rectified and
stored in the 500μF capacitor, producing the circuit’s DC
output. C1B is set up so it (Trace C) goes low when circuit
200mA Output 1.5V to 5V Converter
Although useful, the preceding circuit is limited to low
power operation. Some 1.5V powered systems (survival
2-way radios, remote, transducer fed data acquisition
systems, etc.) require much more power. Figure 23’s
design supplies a 5V output with 200mA capacity. Some
sacrifice in quiescent current is made in this circuit. This is
predicated on the assumption that it operates continuously
22
+
1.5VIN
L1
25μH
220μF
+
VIN
1N5823
Q2
2N3507
VSW
500μF
3.74k*
LT1070
FB
GND
VC
5VOUT
+
665Ω*
1.5VIN
1k
+
OPTIONAL IF
VSUPPLY CAN EXCEED 1.7V
+V
6.8μF
10k
0.01
10k
47k
1.5VIN
1N4148
100Ω
1k
2Ω
Q1
2N2907
TO
C1B “+”
INPUT
1.5V
–
1k
75k
LT1004
1.2V
100k
C1A
1/2 LT1018
+
AN29 F23
200k
1.5VIN
HP5082-2810
–
68k
C1B
1/2 LT1018
39k
576Ω*
100k
1.5VIN
UPDATE
LT1172 can be used
in place of LT1070
+
47k
Figure 23. 200mA Output 1.5V to 5V Converter
an32f
AN29-15
Application Note 29
output crosses about 4.5V. When this occurs C1A’s integration capacitor is pulled low, stopping it from oscillating.
Under these conditions Q2 can no longer drive L1, but the
LT1070 can. This behavior is observable at the LT1070’s
VSW pin (the junction of L1, Q2’s collector and the LT1070),
Trace D. When the start-up circuit goes off, the LT1070 VIN
pin has adequate supply voltage and it begins operation.
This occurs at the 4th vertical division of the photograph.
There is some overlap between start-up loop turn-off and
LT1070 turn-on, but it has no detrimental effect. Once the
circuit is running it functions similarly to Figure 11.
The start-up loop must be carefully designed to function
over a wide range of loads and battery voltages. Start-up
currents exceed 1 ampere, necessitating attention to Q2’s
saturation and drive characteristics. The worst case is a
nearly depleted battery and heavy output loading. Figure 25
shows circuit output starting into a 100mA load at VBATTERY
= 1.2V. The sequence is clean, and the LT1070 takes over at
the appropriate point. In Figure 26, loading is increased to
200mA. Start-up slope decreases, but starting still occurs.
The abrupt slope increase (6th vertical division) is due to
overlapping operation of the start-up loop and the LT1070.
Figure 27 plots input-output characteristics for the circuit.
Note that the circuit will start into all loads with VBATTERY =
1.2V. Start-up is possible down to 1.0V at reduced loads.
Once the circuit has started, the plot shows it will drive full
200mA loads down to VBATTERY = 1.0V. Reduced drive is
possible down to VBATTERY = 0.6V (a very dead battery)!
Figures 28 and 29, dynamic XY crossplot versions of
Figure 27, are taken at 20 and 200 milliamperes, respectively. Figure 30 graphs efficiency at two supply voltages
over a range of output currents. Performance is attractive, although at lower currents circuit quiescent power
degrades efficiency. Fixed junction saturation losses are
responsible for lower overall efficiency at the lower supply
voltage. Figure 31 shows quiescent current increasing as
supply decays. Longer inductor current charge intervals
are necessary to compensate the decreased supply voltage.
A = 5V/DIV
B = 10V/DIV
C = 2V/DIV
VERT = 1V/DIV
D = 1V/DIV
(AC-COUPLED ON
5VDC LEVEL)
AN29 F24
Figure 24. High Power 1.5V to 5V Converter Start-Up Sequence
VERT = 1V/DIV
HORIZ = 2ms/DIV
AN29 F25
Figure 25. High Power 1.5V to 5V Converter Turn-On Into a
100mA Load at VBATT = 1.2V
HORIZ = 2ms/DIV
AN29 F26
Figure 26. High Power 1.5V to 5V Converter Turn-On Into a
200mA Load at VBATT = 1.2V
MINIMUM INPUT VOLTAGE TO MAINTAIN VOUT = 5V
HORIZ = 2ms/DIV
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
START
RUN
0
20 40 60 80 100 120 140 160 180 200
OUTPUT CURRENT (mA)
AN29 F27
Figure 27. Input-Output Data for Figure 23
an32f
AN29-16
Application Note 29
100
VOUT = 5V
90
80
EFFICIENCY (%)
VERT = OUTPUT
= 1V/DIV
HORIZ = INPUT = 0.15V/DIV
70
VIN = 1.5V
60
VIN = 1.2V
50
40
30
AN29 F28
20
10
Figure 28. Input-Output XY Characteristics of the
1.5V to 5V Converter at 20mA Loading
0
0
20 40 60 80 100 120 140 160 180 200
OUTPUT CURRENT (mA)
AN29 F30
Figure 30. Efficiency vs Operating Point for Figure 23
1.50
1.45
VERT = OUTPUT
= 1V/DIV
HORIZ = INPUT = 0.15V/DIV
AN29 F29
Figure 29. Input-Output XY Characteristics of the
1.5V to 5V Converter at 200mA Loading
SUPPLY VOLTAGE (V)
1.40
1.35
1.30
1.25
1.20
1.15
1.10
1.05
1.00
HIGH EFFICIENCY CONVERTERS
50
55
65
70
75
60
QUIESCENT CURRENT (mA)
80
AN29 F31
High Efficiency 12V to 5V Converter
Efficiency is sometimes a prime concern in DC/DC converter design (see Appendix E, “Optimizing Converters
for Efficiency”). In particular, small portable computers
frequently use a 12V primary supply which must be converted down to 5V. A 12V battery is attractive because it
offers long life when all trade-offs and sources of loss are
considered. Figure 32 achieves 90% efficiency. This circuit
can be recognized as a positive buck converter. Transistor
Q1 serves as the pass element. The catch diode is replaced
with a synchronous rectifier, Q2, for improved efficiency.
The input supply is nominally 12V but can vary from 9.5V
to 14.5V. Power losses are minimized by utilizing low
source-to-drain resistance, 0.028Ω, NMOS transistors
for the catch diode and pass element. The inductor, Pulse
Engineering PE-92210K, is made from a low loss core
material which squeezes a little more efficiency out of the
circuit. Also, keeping the current sense threshold voltage
low minimizes the power lost in the current limit circuit.
Figure 31. IQ vs Supply Voltage for Figure 23
Figure 33 shows the operating waveforms. Q5 drives the
synchronous rectifier, Q2, when the VSW pin (Trace A) is
turned “off”. Q2 is turned off through D1 and D2 when the
VSW pin is “on”. To turn on Q1, the gate (Trace B) must
be driven above the input voltage. This is accomplished
by bootstrapping the capacitor, C1, off the drain of Q2
(Trace C). C1 charges up through D1 when Q2 is turned on.
When Q2 is turned off, Q3 is able to conduct, providing a
path for C1 to turn Q1 on. During this time, current flows
through Q1 (Trace D) through the inductor (Trace E) and
into the load. To turn Q1 off, the VSW pin must be “off”.
Q5 is now able to turn on Q4 and the gate of Q1 is pulled
low through D3 and the 50Ω resistor. This resistor is used
to reduce the voltage noise generated by fast switching
characteristics of Q1. When Q2 is conducting (Trace F),
Q1 must be off. The efficiency will be decreased if both
transistors are conducting at the same time. The 220Ω
an32f
AN29-17
Application Note 29
12VIN
(9.5V TO 14.5V)
47pF
50k
0.018Ω
100Ω
1k
Q8**
2N3906
Q7**
2N3906
1N4148
5k
R1
619Ω
5k
Q3
2N2222
Q5
2N2222
+
220μF
D3
1N4148
D1
1N4148
VIN
C1
0.1μF
Q1
P50N05E
L1
100μH
VSW
LT1072CN8
E2
FB
VC
GND
E1
50Ω
3.01k*
1N4148
Q6
2N2222
LT1004-2.5
9k
100Ω
VOUT
5V
5A
+
1000μF
Q9
2N2222
3.5k
1k
220Ω
1μF
220Ω
Q4
VN2222
1k*
D2
1N4148
Q2
P50N05E
AN29 F32
L1 = PULSE ENGINEERING, INC. # PE-92210K
* = 1% FILM RESISTORS
** = USE MATCHING OF 20mV AT 200μA
= P50N05E
IRFZ44
(MOTOROLA) (INTERNATIONAL RECTIFIER)
Figure 32. 90% Efficiency Positive Buck Converter with Synchronous Switch
100
A = 20V/DIV
SYNC SWITCHES
B = 20V/DIV
90
EFFICIENCY (%)
C = 20V/DIV
D = 2A/DIV
E = 2A/DIV
80
PMOS AND DIODES
(SEE FIGURE 42)
70
PNP AND DIODES
(SEE FIGURE 42)
60
F = 2A/DIV
HORIZ = 10μs/DIV
AN29 F33
Figure 33. Waveforms for 90% Efficiency Buck Converter
resistors and D2 are used to minimize the overlap of the
switch cycles. Figure 34 shows the efficiency versus load
plot for the circuit as shown. The other plots are for nonsynchronously switched buck regulators (see indicated
Figures).
Short circuit protection is provided by Q6 through Q9.
A 200μA current source is generated from an LT1004,
Q6 and the 9k resistor. This current flows through R1
and generates a threshold voltage of 124mV for the
comparator, Q7 and Q8. When the voltage drop across the
50
0
1
3
2
ILOAD (A)
4
5
AN29 F34
Figure 34. Efficiency vs Load for Figure 32. The Synchronous
Switches Give Higher Efficiency than Simple FET or Bipolar
Transistors and Diodes
0.018Ω sense resistor exceeds 124mV, Q8 is turned on.
The LT1072’s VSW pin goes off when the VC pin is pulled
below 0.9V. This occurs when Q8 forces Q9 to saturate.
An RC damper suppresses line transients that might
prematurely turn on Q8.
an32f
AN29-18
Application Note 29
MBR1060
4
L1
t
+
5
5VOUT
1000μF 100mA TO 1A
(SEE TEXT)
MBR1060
12VIN
680Ω
0.47μF
2k
8
3
t
+
22μF
MBR360
VIN
+
0.1μF 1
t
6
3.40k*
VSW
100μF
1k
LT1070
FB
GND
VC
1.07k*
1k
1μF
AN29 F35
MBR1060 = MOTOROLA
Figure 35. High Efficiency Flux Sensed Isolated Converter
High Efficiency, Flux Sensed Isolated Converter
Figure 35’s 75% efficiency is not as good as the previous
circuit, but it has a fully floating output. This circuit uses
a bifilar wound flux sensing secondary to provide isolated
voltage feedback. In operation the LT1070’s VSW pin (Trace
A, Figure 36) pulses L1’s primary, producing identical waveforms at the floating power and flux sensing secondaries
(Traces B and C). Feedback occurs from the flux sense
winding via the diode and capacitive filter. The 1k resistor provides a bleed current, while the 3.4k-1.07k divider
sets output voltage. The diode partially compensates the
diode in the power output winding, resulting in an overall
temperature coefficient of about 100ppm/°C. The oversize
diode aids efficiency, although significant improvement
(e.g., 5% to 10%) is possible if synchronous rectification
is employed, as in Figure 32. The primary damper network
is unremarkable, although the 2k-0.1μF network has been
added to suppress excessive ringing at low output current.
A = 10V/DIV
B = 10V/DIV
C = 10V/DIV
HORIZ = 5μs/DIV
AN29 F36
Figure 36. Waveforms for Flux Sensed Converter
This ringing is not deleterious to circuit operation, and the
network is optional. Below about 10% loading non-ideal
transformer behavior introduces significant regulation error. Regulation stays within ±100mV from 10% to 100%
of output rating, with excursion exceeding 900mV at no
load. Figure 37’s circuit trades away isolation for tight
regulation with no output loading restrictions. Efficiency
is the same.
an32f
AN29-19
Application Note 29
MBR1060
12VIN
680Ω
0.47μF 8
5VOUT
1A
L1
4
t
+
1000μF
MBR360
VIN
+
1
t
3.01k*
5
VSW
100μF
LT1070
FB
GND
VC
1k*
1k
1μF
AN29 F37
Figure 37. Non-Isolated Version of Figure 35
WIDE RANGE INPUT CONVERTERS
Wide Range Input –48V to 5V Converter
Often converters must accommodate a wide range of
inputs. Telephone lines can vary over considerable tolerances. Figure 38’s circuit uses an LT1072 to supply a 5V
output from a telecom input. The raw telecom supply is
nominally –48V but can vary from –40V to –60V. This range
of voltages is acceptable to the VSW pin but protection is
required for the VIN pin (VMAX = 60V). Q1 and the 30V
Zener diode serve this purpose, dropping VIN’s voltage to
acceptable levels under all line conditions.
Here the “top” of the inductor is at ground and the LT1072’s
ground pin at –V. The feedback pin senses with respect
to the ground pin, so a level shift is required from the 5V
output. Q2 serves this purpose, introducing only –2mV/°C
drift. This is normally not objectionable in a logic supply.
It can be compensated with the optional appropriately
scaled diode-resistor shown in Figure 38.
Frequency compensation uses an RC damper at the VC
pin. The 68V Zener is a type designed to clamp and absorb
excessive line transients which might otherwise damage
the LT1072 (VSW maximum voltage is 75V)
Figure 39 shows operating waveforms at the VSW pin.
Trace A is the voltage and Trace B the current. Switching
is crisp, with well controlled waveforms. A higher current
version of this circuit appears in LTC Application Note 25,
“Switching Regulators For Poets.”
3.5V to 35VIN–5VOUT Converter
Figure 40’s approach has an even wider input range. In
this case it produces either a –5V or 5V output (shown in
dashed lines). This circuit is an extension of Figure 11’s
basic flyback topology. The coupled inductor allows the
option for buck, boost, or buck-boost converters. This
circuit can operate down to 3.5V for battery applications
while accepting 35V inputs.
Figure 41 shows the operating waveforms for this circuit.
During the VSW (Trace A) “on” time, current flows through
the primary winding (Trace B). No current is transferred
to the secondary because the catch diode, D1, is reverse
biased. The energy is stored in the magnetic field. When
the switch is turned “off” D1 forward biases and the energy
is transferred to the secondary winding. Trace C is the
voltage seen on the secondary and Trace D is the current
an32f
AN29-20
Application Note 29
3k
1/2W
+
220Ω
Q1
2N5550
100μF
68V**
L1
100μH
1k
OPTIONAL
LOW DRIFT FEEDBACK
CONNECTION (SEE TEXT)
1N5936
30V
2.2μF
+
*
+
VIN
330μF
VSW
3.9k
1%
Q2
2N5401
LT1072HV
VC
3.01k
1%
1.1k
1%
2k
INPUT
–48V
(–40V TO –60V)
FROM
5V OUTPUT
Q2
2N5401
TO
FB PIN
FB
GND
5VOUT
0.5A
0.22
*
TO
–48V
MUR410 (MOTOROLA)
Q3
2N5401
1k
1%
3.01k
1%
AN29 F38
**
1.5KE68A (MOTOROLA)
Figure 38. Wide Range Input Converter
flowing through it. This is not an ideal transformer so
not all of the primary windings energy is coupled into the
secondary. The energy left in the primary winding causes
the overvoltage spikes seen on the VSW pin (Trace E). This
phenomenon is modeled by a leakage inductance term
which is placed in series with the primary winding. When
the switch is turned “off” current continues to flow in the
inductor causing the snubber diode to conduct (Trace F).
The snubber diode current falls to zero as the inductor
loses its energy. The snubber network clamps the voltage
spike. When the snubber diode current reaches zero, the
VSW pin voltage settles to a potential related to the turns
ratio, output voltage and input voltage.4
The feedback pin senses with respect to ground, so Q1
through Q3 provides the level shift from the –5V output.
Q1 introduces a –2mV/°C drift to the circuit. This effect can
be compensated by a circuit similar to the one shown in
Figure 38. Line regulation is degraded due to Q3’s output
impedance. If this is a problem, an op amp must be used
to perform the level shift (see AN19, Figure 29).
A = 50V/DIV
B = 0.5A/DIV
HORIZ = 5μs/DIV
AN29 F39
Figure 39. Waveforms for Wide Range Input Converter
Wide Range Input Positive Buck Converter
Figure 42 is another example of a positive buck converter.
This is a simpler version compared to the synchronous
switch buck, Figure 32. However, efficiency isn’t as high
(see Figure 34). If the PMOS transistor is replaced with
a Darlington PNP transistor (shown in dashed lines)
efficiency decreases further.
4Application Note AN19, “LT1070 Design Manual,” page 25
an32f
AN29-21
Application Note 29
VIN = 3.5V TO 35V
5V
0.68μF
510Ω
1W
3
n=1
MBR360
4
(MOTOROLA)
VIN
+
1k*
1%
•
1k*
1%
L1
Q3
2N3906
Q2
2N3906
MBR360
VIN
VSW
100μF
L1
2
n=1
LT1070
5VOUT
1A
+
1000μF
•
4
FB
GND
3
1
VSW
FB
VC
1k
1k*
1k*
1μF
2
L1
Q1
2N2222
+
1000μF
n=1
•
3.01k*
3.32k*
1%
OPTIONAL (SEE TEXT)
AN29 F40
1
–5VOUT
1A
D1
MBR360
Figure 40. Wide Range Input Positive-to-Negative Flyback Converter
A = 20V/DIV
B = 4A/DIV
C = 10V/DIV
D = 4A/DIV
E = 20V/DIV
F = 2A/DIV
A, B, C, D HORIZ = 10μs/DIV
E, F HORIZ = 1μs/DIV
AN29 F41
Figure 41. Waveforms for Wide Range Input Positive
–5V Output Flyback Converter
Figure 43a shows the operating waveforms for this circuit.
The pass transistor’s (Q1) drive scheme is similar to the one
shown in Figure 32. During the VSW (Trace A) “on” time,
the gate of the pass transistor is pulled down through D1.
This forces Q1 to saturate. Trace B is the voltage seen on
the drain of Q1 and Trace C is the current passing through
Q1. The supply current flows through the inductor (Trace D)
and into the load. During this time energy is being stored
in the inductor. When voltage is applied to the inductor,
current does not instantly rise. As the magnetic field builds
up, the current builds. This is seen in the inductor current
waveform (Trace D). When the VSW pin is “off,” Q2 is able
to conduct and turns Q1 off. Current can no longer flow
through Q1, instead D2 is conducting (Trace E). During
this period some of the energy stored in the inductor will
be transferred to the load. Current will be generated from
the inductor as long as there is any energy in it. This can
be seen in Figure 43a. This is known as continuous mode
operation. If the inductor is completely discharged, no
current will be generated (see Figure 43b). When this
happens neither switch, Q1 or D2, is conducting. The
inductor looks like a short and the voltage on the cathode
of D2 will settle to the output voltage. These “boingies”
can be seen in Trace B of Figure 43b. This is known as
discontinuous mode operation. Higher input voltages
can be handled with the gate-source Zener clamped by
D2. The 400 milliwatt Zener’s current must be rescaled
by adjusting the 50Ω value. Maximum gate-source voltage is 20V. The circuit will function up to 35VIN. At inputs
beyond 35V all semiconductor breakdown voltages must
be considered.
an32f
AN29-22
Application Note 29
Q1
IRF9Z30
(HEAT SINK)
0.1Ω
D2
12V
1N759
(OPT)
1k
1k
2N3906
D1
1N4148
Q2
2N2222A
5VOUT
5A
D2
MBR735
(MOTOROLA)
100μF
+
VIN
12V TO 35V
L1
170μH
51Ω
1W
VIN
+
E1
100μF
3.01k*
VSW
FB
LT1072CN8
E2
GND
1k*
VC
1N4148
1k
5.1k
2N2222
1k
100pF
1μF
AN29 F42
2N6667
(HEAT SINK)
L1 = PULSE ENGINEERING, INC # PE-92113
1k
OPTIONAL
(SEE TEXT)
2N2222
1N4148
100Ω
1W
Figure 42. Positive Buck Converter
A = 10V/DIV
A = 10V/DIV
B = 10V/DIV
B = 10V/DIV
C = 0.5A/DIV
C = 2A/DIV
D = 0.5A/DIV
D = 2A/DIV
E = 0.5A/DIV
E = 2A/DIV
HORIZ = 10μs/DIV
AN29 F43a
Figure 43a. Waveforms for Wide Range Input Positive Buck
Converter (Continuous Mode)
HORIZ = 10μs/DIV
AN29 F43b
Figure 43b. Waveforms for Wide Range Input Positive Buck
Converter (Discontinuous Mode)
an32f
AN29-23
Application Note 29
Q1
IRF9Z30
28V NOMINAL
(15V TO 35V)
1k
Q2
2N2222A
7.5V
1N755
L1
330μH
D3
MBR360
(MOTOROLA)
D4
MBR360
28V
250mA
+
1000μF
1N4148
220Ω
1W
+
D1
1N4148
100μF
D2
MBR360
26.1k
VSW
FB
E1
LT1072CN8
E2
VC
GND
VIN
1.21k*
1k
1μF
AN29 F44
L1 = PULSE ENGINEERING, INC # PE-52627
Figure 44. Positive Buck-Boost Converter
Buck-Boost Converter
The buck boost topology is useful when the input voltage can either be higher or lower than the output. In this
example, Figure 44, this is accomplished with a single
inductor instead of a transformer, as in Figure 40 (optional).
However, the input voltage range only extends down to
15V and can reach to 35V. If the maximum 1.25A switch
current rating of the LT1072 is exceeded an LT1071 or
LT1070 can be used instead. At high power levels package
thermal characteristics should be considered.
The operation of the circuit is similar to the positive buck
converter, Figure 42. The gate drive to the pass transistor
is derived the same way except the gate-source voltage is
clamped. Remember, the gate-source maximum voltage
rating is specified at ±20V. Figure 45 shows the operating
waveforms. When the VSW pin is “on” (Trace A), the pass
transistor, Q1, is saturated. The gate voltage (Trace B) is
clamped by the Zener diode. Trace C is the voltage on the
drain of Q1 and Trace D is the current through it. This
is where the similarities between the two circuits end.
Notice the inductor is pulled to within a diode drop, D2
above ground, instead of being tied to the output (see
A = 20V/DIV
B = 10V/DIV
C = 20V/DIV
D = 2A/DIV
E = 2A/DIV
F = 2A/DIV
HORIZ = 10μs/DIV
AN29 F45
Figure 45. Waveforms for Positive Buck-Boost Converter
Figure 42). In this case, the inductor has the input voltage
applied across it, except for a Vbe and saturation losses.
D4 is reverse biased and blocks the output capacitor from
discharging into the VSW pin. When the VSW pin is “off” Q1
and D2 cease to conduct. Since the current in the inductor
(Trace E) continues to flow, D3 and D4 are forward biased
and the energy in the inductor is transferred into the load.
Trace F is the current through D3. Also, D2 keeps Q1 from
staying on if the circuit is operating in buck mode. D1, on
the other hand, blocks current from flowing into the gate
drive circuit when operating in boost mode.
an32f
AN29-24
Application Note 29
VREF ≈ 1.8V
INPUT
L1
335μH
Q1
2N6667
LT1083
IN
+
MR1122
10k
10,000μF
1N914
28V
1k
OUT
ADJ
470
+
240*
OUTPUT
10μF
2k
0.001
VREF ≈ 1.8V
1M
4N28
LT1011
10k
–
4
1
AN29 F46
10k
+
28V
1N914
Figure 46. High Power Linear Regulator with Switching Pre-Regulator
A = 200mV/DIV
(AC-COUPLED)
B = 50V/DIV
C = 20V/DIV
D = 5A/DIV
HORIZ = 500μs/DIV
AN29 F47
Figure 47. Switching Pre-Regulated Linear Regulators Waveforms
Wide Range Switching Pre-Regulated Linear
Regulator
In a sense, linear regulators can be considered extraordinarily wide range DC/DC converters. They do not face
the dynamic problems switching regulators encounter
under varying ranges of input and output. Excess energy
is simply dissipated at heat. This elegantly simplistic
energy management mechanism pays dearly in terms of
efficiency and temperature rise. Figure 46 shows a way a
linear regulator can more efficiently control high power
under widely varying input and output conditions.
The regulator is placed within a switched-mode loop that
servo-controls the voltage across the regulator. In this
arrangement the regulator functions normally while the
switched-mode control loop maintains the voltage across it
at a minimal value, regardless of line, load or output setting
changes. Although this approach is not quite as efficient as
a classical switching regulator, it offers lower noise and the
fast transient response of the linear regulator. The LT1083
functions in the conventional fashion, supplying a regulated
output at 7.5A capacity. The remaining components form
the switched-mode dissipation limiting control. This loop
forces the potential across the LT1083 to equal the 1.8V
value of VREF . The opto-isolator furnishes a convenient
way to single end the differentially sensed voltage across
the LT1083. When the input of the regulator (Trace A,
Figure 47) decays far enough, the LT1011 output (Trace B)
switches low, turning on Q1 (Q1 collector is Trace C). This
allows current flow (Trace D) from the circuit input into the
10,000μF capacitor, raising the regulator’s input voltage.
When the regulator input rises far enough, the comparator
goes high, Q1 cuts off and the capacitor ceases charging. The MR1122 damps the flyback spike of the current
limiting inductor. The 0.001μF-1M combination sets loop
hysteresis at about 100mVP-P . This free-running oscillation control mode substantially reduces dissipation in
the regulator, while preserving its performance. Despite
changes in the input voltage, different regulated outputs or
load shifts, the loop always ensures minimum dissipation
in the regulator.
an32f
AN29-25
Application Note 29
100
90 POUT = 12W
POUT = 85W
VOUT = 12V VIN = 15V
EFFICIENCY (%)
80
POUT = 105W
70
POUT = 15W
POUT = 5W
60
VOUT = 15V VIN = 28V
50
40
VOUT = 5V
VIN = 15V
VOUT = 5V
VIN = 28V
POUT = 35W
POUT = 35W
POUT = 5W
30
VOUT = 5V VIN = 15V LT1083 WITH NO PRE-REGULATOR. THEORETICAL LIMITS ONLY.
DISSIPATION LIMITED
20
VOUT = 5V VIN = 28V LT1083 WITH NO PRE-REGULATOR. THEORETICAL LIMITS ONLY.
DISSIPATION LIMITED
10
0
0
1
2
4
3
OUTPUT (A)
5
7
6
AN29 F48
Figure 48. Efficiency vs Output Current for Figure 46 at Various Operating Points
Figure 48 plots efficiency at various operating points. Junction losses and the loop enforced 1.8V across the LT1083
are relatively small at high output voltages, resulting in
good efficiency. Low output voltages do not fare as well,
but compare very favorably to the theoretical data for the
LT1083 with no pre-regulator. At the higher theoretical
dissipation levels the LT1083 will shut down, precluding
practical operation.
15V
22μF
+
*
L1
3
0.47
7
1k
1/2W
1
MUR120
VIN
10M
1%
(SEE NOTES)
VSW
FB
High Voltage Converter—1000VOUT , Nonisolated
Photomultiplier tubes, ion generators, gas-based detectors, image intensifiers and other applications need high
voltages. Converters frequently supply these potentials.
Generally, the limitation on high voltage is transformer
insulation breakdown. A transformer is almost always
used because a simple inductor forces excessive voltages on the semiconductor switch. Figure 49’s circuit,
reminiscent of Figure 11’s basic flyback configuration, is
a 15V to 1000VOUT converter. The LT1072 controls output by modulating the flyback energy into L1, forcing its
feedback (FB) pin to 1.23V (the internal reference value).
In this example loop compensation is heavily overdamped
by the VC pin capacitor. L1’s damper network limits flyback
spikes within the VSW pin’s 75V rating.
Fully Floating, 1000VOUT Converter
Figure 50 is similar to Figure 49 but features a fully floating
output. This provision allows the output to be referenced
off system ground, often desirable for noise or biasing
reasons. Basic loop action is as before, except that the
LT1072
GND
VOUT
1000V
5W
8
15V
HIGH VOLTAGE CONVERTERS
0.1μF
2000V
VC
12.4k 1%
(IDEAL VALUE–PAD AS REQUIRED
FOR 1000VOUT)
2μF
10M = MAX-750-22 VICTOREEN, INC.
*
= SEMTEC, FM-50
Figure 49. Nonisolated 15V to 1000V Converter
LT1072’s internal error amplifier and reference are replaced
with galvanically isolated equivalents. Power for these
components is bootstrapped from the output via source
follower Q1 and its 2.2M ballast resistor. A1 and the LT1004,
micropower components, minimize dissipation in Q1 and
its ballast. Q1’s gate bias, tapped from the output divider
string, produces about 15V at its source. A1 compares the
scaled divider output with the LT1004 reference. The error
signal, A1’s output, drives the opto-coupler. Photocurrent
is kept low to save power. The opto-coupler output pulls
down on the VC pin, closing a loop. Frequency compensation at the VC pin and A1 stabilizes the loop.
an32f
AN29-26
Application Note 29
15VIN
1k
0.47
L1
1
MUR120
VIN
NC
3
7
1000VOUT
5W
0.1μF
2000V
0.1
10M (SEE NOTES)
1%
D
8
Q1
VN2222
S
VSW
200k*
180k 7
FB
LT1072
VC
2.2M
+
8
A1
LT1006
GND
3.6k
4N46
200k
–
10k
4
1M
1k
2μF
5k
OUTPUT
ADJUST
0.68
LT1004
1.2V
10k*
AN29 F50
= INPUT GROUND
= OUTPUT COMMON
10M = VICTOREEN MAX-750-22
= SEMTEC, FM-50
Figure 50. Isolated Output 15V to 1000V Converter
The transformers isolated secondary and optical feedback
produce a regulated, fully galvanically floating output.
Common mode voltages of 2000V are acceptable.
20,000VCMV Breakdown Converter
Figure 50’s common mode breakdown limits are imposed
by transformer and opto-coupler restrictions. Isolation
amplifiers, transducer measurement at high common mode
voltages (e.g., winding temperature of a utility company
transformer and ESD sensitive applications) require high
breakdowns. Additionally, very precise floating measurements, such as signal conditioning for high impedance
bridges, can require extremely low leakage to ground.
Achieving high common mode voltage capability with
minimal leakage requires a different approach. Magnetics is usually considered the only approach for isolated
transfer of appreciable amounts of electrical energy.
Transformer action is, however, achievable in the acoustic
domain. Some ceramic materials will transfer electrical
energy with galvanic isolation. Conventional magnetic
transformers work on an electrical-magnetic-electrical
basis using the magnetic domain for electrical isolation.
The acoustic transformer uses an acoustic path to get
isolation. The high voltage breakdown and low electrical
conductance associated with ceramics surpasses isolation
characteristics of magnetic approaches. Additionally, the
acoustic transformer is simple. A pair of leads bonded to
each end of the ceramic material forms the device. Insulation resistance exceeds 1012Ω, with primary-secondary
capacitances of 1pF to 2pF. The material and its physical
configuration determine its resonant frequency. The device
may be considered as a high Q resonator, similar to a quartz
crystal. As such, drive circuitry excites the device in the
positive feedback path of a wideband gain element. Unlike
a crystal, drive circuitry is arranged to pass substantial
current through the ceramic, maximizing power into the
transformer.
an32f
AN29-27
Application Note 29
2k
15V 15V
–
0.002
+
2k
Q1
2N3904
LT1011
1N4148
PIEZOCERAMIC TRANSFORMERS
AVAILABLE FROM CHANNEL
INDUSTRIES, INC. SANTA BARBARA, CA.
100Ω
PRIMARY
470pF
PIEZOCERAMIC
TRANSFORMER
1k
15V
1N4148
680Ω
3
SECONDARY
VIN
VOUT
10V
FLOATING
OUTPUT
2
0.001μF
LT1020
+
10μF
GND
FB
1.5M*
+
100μF
11
500k*
AN29 F51
FLOATING
OUTPUT
COMMON
Figure 51. 15V to 10V Converter with 20,000V Isolation
A = 10V/DIV
1
B = 20mA/DIV
+
2
C1
10μF
3
4
C = 20V/DIV
8
LT1054
VIN
2μF
+
7
6
5
C2
–VOUT
100μF
+
HORIZ = 2μs/DIV
AN29 F52
AN29 F53
Figure 52. Waveforms for the 20,000V Isolation Converter
Figure 53. A Basic Switched-Capacitor Converter
In Figure 51, the piezo-ceramic transformer is in the LT1011
comparators positive feedback loop. Q1 is an active pull-up
for the LT1011, an open-collector device. The 2k-0.002μF
path biases the negative input. Positive feedback occurs at
the transformers resonance, and oscillation commences
(Trace A, Figure 52 is Q1’s emitter). Similar to quartz
crystals, the transformer has significant harmonic and
overtone modes. The 100Ω-470pF damper suppresses
spurious oscillations and “mode hopping.” Drive current
(Trace B) approximates a sine wave, with peaking at the
transitions. The transformer looks like a highly resonant
filter to the resultant acoustic wave propagated in it. The
secondary voltage (Trace C) is sinusoidal. Additionally, the
transformer has voltage gain. The diode and 10μF capacitor convert the secondary voltage to DC. The LT1020 low
quiescent current regulator gives a stabilized 10V output.
Output current for the circuit is a few milliamperes. Higher
currents are possible with attention to transformer design.
an32f
AN29-28
Application Note 29
2
3.5V ≤ VIN ≤ 15V
CIN = COUT = 100μF
1N4001
1N4001
+VOUT
TJ = 125°C
1
+
+
10μF
100μF
1
8
2
0
–VIN
TJ = –55°C
TJ = 25°C
•
6
4
• INDICATES GUARANTEED TEST POINT
5
AN29 F55
2μF
10 20 30 40 50 60 70 80 90 100
OUTPUT CURRENT (mA)
0
7
LT1054
3
+
VOLTAGE LOSS (V)
•
AN29 F54
Figure 54. Losses for the Basic Switched-Capacitor Converter
Figure 55. Switched-Capacitor –VIN to +VOUT Converter
VIN ≥ 6V
8
9
3
GND
VIN
–IN
OUT
LT1020
5
+
10μF
1
8
2
7
3
4
LT1054
10k
COMP
PNP
+IN
REF
OUT
7
4
0.002
6
0.001μF
1M*
5
COMP
NPN
FB
2
500k*
11
5VOUT
100mA
0.001μF
+
500k*
10μF
6
+
499k*
100k
10μF
100μF
AN29 F56
–5VOUT
75mA
+
VN2222
100k
Figure 56. High Current Switched-Capacitor 6V to ±5V Converter
SWITCHED-CAPACITOR BASED CONVERTERS
Inductors are used in converters because they can store
energy. This stored magnetic energy, released and expressed in electrical terms, is the basis of converter operation. Inductors are not the only way to store energy with
efficient release expressed in electrical terms. Capacitors
store charge (already an electrical quantity) and as such,
can be used as the basis for DC/DC conversion. Figure 53
shows how simple a switched-capacitor based converter
can be (the fundamentals of switched-capacitor based
conversion are presented in Appendix B, “Switched-Capacitor Voltage Converters—How They Work”). The LT1054
provides clocked drive to charge C1. A second clock phase
discharges C1 into C2. The internal switching is arranged
so C1 is “flipped” during the discharge interval, producing a negative output at C2. Continuous clocking allows
C2 to charge to the same absolute value as C1. Junction
and other losses preclude ideal results, but performance
is quite good. This circuit will convert VIN to –VOUT with
losses shown in Figure 54. Adding an external resistive
divider allows regulated output (see Appendix B).
With some additional steering diodes this configuration
can effectively run “backwards” (Figure 55), converting
a negative input to a positive output. Figure 56’s variant
gives low dropout linear regulation for 5V and –5V outputs from 6VIN. The LT1020-based dual output regulation
an32f
AN29-29
Application Note 29
scheme is adapted from Figure 8. Figure 57 uses diode
steering to get voltage boost, providing ≈2VIN. Bootstrapping this configuration with Figure 54’s basic circuit leads
to Figure 58, which converts a 5V input to 12V and –12V
outputs. As might be expected output current capacity is
traded for the voltage gain, although 25mA is still available.
Figure 59, another boost converter, employs a dedicated
version of Figure 58 (the LT1026) to get regulated ±7V
from a 6V input. The LT1026 generates unregulated ±11V
rails from the 6V input with the LT1020 and associated
components (again, purloined from Figure 8) producing
regulation. Current and boost capacity are reduced from
Figure 58’s levels, but the regulation and simplicity are
noteworthy. Figure 60 combines the LT1054’s clocked
switched-capacitor charging with classical diode voltage
multiplication, producing positive and negative outputs.
At no load ±13V is available, falling to ±10V with each
side supplying 10mA.
High Power Switched-Capacitor Converter
Figure 61 shows a high power switched-capacitor converter
with a 1A output capacity. Discrete devices permit high
power operation.
The LTC1043 switched-capacitor building block provides
non-overlapping complementary drive to the Q1-Q4 power
MOSFETs. The MOSFETs are arranged so that C1 and
C2 are alternately placed in series and then in parallel.
During the series phase, the 12V supply current flows
through both capacitors, charging them and furnishing
load current. During the parallel phase, both capacitors
deliver current to the load. Traces A and B, Figure 62, are
the LTC1043-supplied drives to Q3 and Q4, respectively.
Q1 and Q2 receive similar drive from Pins 3 and 11. The
diode-resistor networks provide additional non-overlapping drive characteristics, preventing simultaneous drive
to the series-parallel phase switches. Normally, the output
would be one-half of the supply voltage, but C1 and its
associated components close a feedback loop, forcing
the output to 5V. With the circuit in the series phase, the
output (Trace C) heads rapidly positive. When the output
exceeds 5V, C1 trips, forcing the LTC1043 oscillator pin
(Trace D) high. This truncates the LTC1043’s triangle wave
oscillator cycle. The circuit is forced into the parallel phase
and the output coasts down slowly until the next LTC1043
clock cycle begins. C1’s output diode prevents the triangle
down-slope from being affected and the 100pF capacitor
provides sharp transitions. The loop regulates the output to
VIN
3.5V TO 15V
1N4001
+
VOUT
+
1N4001
+
100μF
+
2μF
10μF
–
1
2
VIN = 3.5V TO 15V
VOUT ≈ 2VIN – (VL + 2VDIODE)
VL = LT1054 VOLTAGE LOSS
8
LT1054
7
3
6
4
5
AN29 F57
Figure 57. Voltage Boost Switched-Capacitor Converter
VIN = 5V
+
5μF
1
VOUT ≈ 12V
IOUT = 25mA
8
1N914
+
1N914
+
100μF
+
2
10μF
3
4
LT1054
#1
10μF
7
1
8
2
7
20k
6
+
10μF
2N2219
5
3
100μF
LT1054
#2
1N5817
6
5μF
+
1k
4
5
+
100μF
VOUT ≈ –12V
IOUT = 25mA
+
AN29 F58
Figure 58. Switched-Capacitor 5V to ±12V Converter
an32f
AN29-30
Application Note 29
5V by feedback controlling the turn-off point of the series
phase. The circuit constitutes a large scale switched-capacitor voltage divider which is never allowed to complete
a full cycle. The high transient currents are easily handled
by the power MOSFETs and overall efficiency is 83%.
Williams, J., “Power Conditioning Techniques for Batteries,” Linear Technology Corporation, Application Note 8.
Tektronix, Inc., CRT Circuit, Type 453 Operating Manual,
p. 3-16.
Pressman, A. I., “Switching and Linear Power Supply,
Power Converter Design,” Hayden Book Co., Hasbrouck
Heights, New Jersey, 1977, ISBN 0-8104-5847-0.
REFERENCES
Williams, J., “Conversion Techniques Adopt Voltages to
your Needs,” EDN, November 10, 1982, p. 155.
Chryssis, G., “High Frequency Switching Power Supplies,
Theory and Design,” McGraw Hill, New York, 1984, ISBN
0-07-010949-4.
Williams, J., “Design DC/DC Converters to Catch Noise
at the Source,” Electronic Design, October 15, 1981, p.
229.
Sheehan, D., “Determine Noise of DC/DC Converters,”
Electronic Design, September 27, 1973.
Nelson, C., “LT1070 Design Manual,” Linear Technology
Corporation, Application Note 19.
Bright, Pittman, and Royer, “Transistors as On-Off Switches
in Saturable Core Circuits,” Electronic Manufacturing,
October, 1954.
Williams, J., “Switching Regulators for Poets,” Linear
Technology Corporation, Application Note 25.
1μF
+
8
2
7
3
LT1026
6
4
+
1μF
8
6VIN
9
3
GND
VIN
–IN
OUT
LT1020
5
5
1μF
100k
COMP
PNP
+IN
REF
OUT
7
4
+
0.002μF
0.001μF
1M*
COMP
NPN
FB
2
500k*
11
7VOUT
20mA
0.001μF
+
100μF
270k*
6
+
360k*
10μF
100k
AN29 F59
VN2222
–7VOUT
20mA
*1% METAL FILM RESISTOR
100k
≈–11V NO LOAD
Figure 59. Switched-Capacitor Based 6V to ±7V Converter
10μF
10μF
+
+
C1
10μF
8
2
7
3
100μF
5V
6
5
10μF
+
10μF
10μF
100μF
+
= 1N4148
10μF
–VOUT
+
+
LT1054
4
+
+VOUT
+
1
+
1μF
1
+
+
≈11V NO LOAD
AN29 F60
10μF
Figure 60. Switched-Capacitor Charge Pump Based Voltage Multiplier
an32f
AN29-31
Application Note 29
12VIN
12VIN
–
6
8
4
12VIN
1k
11
LT1004
1.2V REFERENCE
+
7
C1
LT1011
1
LTC1043
22k
8
2k
100pF
S
Q1
D
470μF
S Q3 D
+
470μF
1k
38k
VOUT
5V
1A
12
12k
12V
13
14
6
5
12V
S Q2 D
1k
2
D
Q4
S
1k
3
18
AN29 F61
15
ALL DIODES ARE 1N4148
Q1, Q2, Q3 = IRF9531 P-CHANNEL
Q4 = IRF533 N-CHANNEL
12VIN
17
16
4
12V
180pF
Figure 61. High Power Switched-Capacitor Converter
A = 20V/DIV
B = 20V/DIV
C = 0.1V/DIV
(AC-COUPLED)
D = 10V/DIV
HORIZ = 20μs/DIV
AN29 F62
Figure 62. Waveforms for Figure 61
an32f
AN29-32
Application Note 29
APPENDIX A
The 5V to ±15V Converter—A Special Case
Five volt logic supplies have been standard since the introduction of DTL logic over twenty years ago. Preceding and
during DTL’s infancy the modular amplifier houses standardized on ±15V rails. As such, popular early monolithic
amplifiers also ran from ±15V rails (additional historical
perspective on amplifier power supplies appears in AN11’s
appended section, “Linear Power Supplies—Past, Present
and Future”). The 5V supply offered process, speed and
density advantages to digital ICs. The ±15V rails provided
a wide signal processing range to the analog components.
These disparate needs defined power supply requirements
for mixed analog-digital systems at 5V and ±15V. In systems with large analog component populations the ±15V
supply was and still is usually derived from the AC line.
Such line derived ±15V power becomes distinctly undesirable in predominantly digital systems. The inconvenience,
difficulty and cost of distributing analog rails in heavily
digital systems makes local generation attractive. 5V to
±15V DC/DC converters were developed to fill this need
and have been with us for about as long as 5V logic.
Figure A1 is a conceptual schematic of a typical converter.
The 5V input is applied to a self-oscillating configuration
composed of transistors, a transformer and a biasing
network. The transistors conduct out of phase, switching
(Figure A2, Traces A and C are Q1’s collector and base,
while Traces B and D are Q2’s collector and base) each
time the transformer saturates.5 Transformer saturation
causes a quickly rising, high current to flow (Trace E).
This current spike, picked up by the base drive winding,
switches the transistors. Transformer current abruptly
drops and then slowly rises until saturation again forces
switching. This alternating operation sets transistor duty
cycle at 50%. The transformers secondary is rectified,
filtered and regulated to produce the output.
This configuration has a number of desirable features. The
complementary high frequency (typically 20kHz) square
wave drive makes efficient use of the transformer and allows
relatively small filter capacitors. The self-oscillating primary
drive tends to collapse under overload, providing desirable short-circuit characteristics. The transistors switch
in saturated mode, aiding efficiency. This hard switching,
combined with the transformer’s deliberate saturation does,
however, have a drawback. During the saturation interval
a significant, high frequency current spike is generated
5This type of converter was originally described by Royer, et al. See
References.
5VIN
INPUT
FILTER
4
Q1
5
POWER
SWITCHING
Q2
LINEAR
REGULATORS
OUTPUT
+VREG
15V
6
1
C1
OUT
COMMON
R2
R1
2
+
3
–VREG
–15V
AN29 FA1
+
BASE BIASING
AND DRIVE
Figure A1. Conceptual Schematic of a Typical 5V to ±15V Converter
an32f
AN29-33
Application Note 29
A = 20V/DIV
A = 10V/DIV
B = 20V/DIV
C = 2V/DIV
B = 2A/DIV
D = 2V/DIV
E = 5A/DIV
F = 0.02V/DIV
C = 10mV/DIV
HORIZ = 5μs/DIV
AN29 FA2
HORIZ = 500ns/DIV
AN29 FA3
Figure A2. Typical 5V to ±15V Saturating Converters Waveforms
Figure A3. Switching Details of Saturating Converter
(again, Trace E). This spike causes noise to appear at the
converter outputs (Trace F is the AC-coupled 15V output).
Additionally, it pulls significant current from the 5V supply.
The converters input filter partially smooths the transient,
but the 5V supply is usually so noisy the disturbance is
acceptable. The spike at the output, typically 20mV high, is
a more serious problem. Figure A3 is a time and amplitude
expansion of Figure A2’s Traces B, E and F. It clearly shows
the relationship between transformer current (Trace B,
Figure A3), transistor collector voltage (Trace A, Figure A3)
and the output spike (Trace C, Figure A3). As transformer
current rises, the transistor starts coming out of saturation. When current rises high enough the circuit switches,
causing the characteristic noise spike. This condition is
exacerbated by the other transistors concurrent switching,
causing both ends of the transformer to simultaneously
conduct current to ground.
drive, ensuring transistor saturation under heavy loading
but wasting power at lighter loads. Adaptive bias schemes
will mitigate this problem, but increase complexity and
almost never appear in converters of this type.
Selection of transistors, output filters and other techniques
can reduce spike amplitude, but the converters inherent
operation ensures noisy outputs.
This noisy operation can cause difficulties in precision
analog systems. IC power supply rejection at the high
harmonic spike frequency is low, and analog system errors frequently result. A 12-bit SAR A-to-D converter is
a good candidate for such spike-noise caused problems.
Sampled data ICs such as switched capacitor filters and
chopper amplifiers often show apparent errors which are
due to spike induced problems. “Simple” DC circuits can
exhibit baffling “instabilities” which in reality are spike
caused problems masquerading as DC shifts.
The drive scheme is also responsible for high quiescent
current consumption. The base biasing always supplies full
The noise problem is, however, the main drawback of this
approach to 5V to ±15V conversion. Careful design, layout,
filtering and shielding (for radiated noise) can reduce noise,
but cannot eliminate it.
Some techniques can help these converters with the noise
problem. Figure A4 uses a “bracket pulse” to warn the
powered system when a noise pulse is about to occur.
Ostensibly, noise sensitive operations are not carried
out during the bracket pulse interval. The bracket pulse
(Trace A, Figure A5) drives a delayed pulse generator
which triggers (Trace B) the flip-flop. The flip-flop output
biases the switching transistors (Q1 collector is Trace C).
The output noise spike (Trace D) occurs within the bracket
pulse interval. The clocked operation can also prevent
transformer saturation, offering some additional noise
reduction. This scheme works well, but presumes the
powered system can tolerate periodic intervals where
critical operations cannot take place.
In Figure A6 the electronic tables are turned. Here, the
host system silences the converter when low noise is
required. Traces B and C are base and collector drives
for one transistor while Traces D and E show drive to the
other device. The collector peaking is characteristic of
saturating converter operation. Output noise appears on
Trace F. Trace A’s pulse gates off the converter’s base bias,
stopping switching. This occurs just past the 6th vertical
division. With no switching, the output linear regulator sees
the filter capacitor’s pure DC and noise disappears.
an32f
AN29-34
Application Note 29
OVERLAP PULSE
OUTPUT
Q
OVERLAP
PULSE
GENERATOR
DELAYED
PULSE
Q1
TO
RECTIFIERS,
FILTERS AND
REGULATORS
5V
÷2
FLIP-FLOP
Q
Q2
AN29 FA4
Figure A4. Overlap Generator Provides a “Bracket Pulse” Around Noise Spikes
A = 5V/DIV
B = 20V/DIV
A = 5V/DIV
C = 1A/DIV
B = 10V/DIV
D = 20V/DIV
C = 5V/DIV
E = 1A/DIV
D = 20mV/DIV
HORIZ = 500ns/DIV
AN29 FA5
F = 50mV/DIV
(AC-COUPLED ON
15V LEVEL)
HORIZ = 20μs/DIV
AN29 FA6
Figure A5. Waveforms for the Bracket Pulse Based Converter
Figure A6. Detail of the Strobed Operation Converter
This arrangement also works nicely but assumes the control
pulse can be conveniently generated by the system. It also
requires larger filter capacitors to supply power during the
low noise interval.
Other methods involve clock synchronization, timing skewing and other schemes which prevent noise spikes from
coinciding with sensitive operations. While useful, none
of these arrangements offer the flexibility of the inherently
noise free converters shown in the text.
APPENDIX B
Switched Capacitor Voltage Converters—How They
Work
To understand the theory of operation of switched capacitor
converters, a review of a basic switched capacitor building
block is helpful.
In Figure B1, when the switch is in the left position,
capacitor C1 will charge to voltage V1. The total charge
on C1 will be Q1 = C1V1. The switch then moves to the
right, discharging C1 to voltage V2. After this discharge
time, the charge on C1 is Q2 = C1V2. Note that charge
has been transferred from the source, V1, to the output,
V2. The amount of charge transferred is:
If the switch is cycled f times per second, the charge
transfer per unit time (i.e., current) is:
1 = f • Q = f • C1(V1 – V2)
To obtain an equivalent resistance for the switchedcapacitor network we can rewrite this equation in terms
of voltage and impedance equivalence:
1=
V1– V2 V1– V2
=
1
REQUIV
fC1
Q = Q1 – Q2 = C1(V1 – V2)
an32f
AN29-35
Application Note 29
A new variable, REQUIV , is defined such that REQUIV = 1/fC1.
Thus, the equivalent circuit for the switched-capacitor
network is as shown in Figure B2. The LT1054 and other
switched-capacitor converters have the same switching
action as the basic switched-capacitor building block. Even
though this simplification doesn’t include finite switch
on-resistance and output voltage ripple, it provides an
intuitive feel for how the device works.
These simplified circuits explain voltage loss as a function of frequency. As frequency is decreased, the output
impedance will eventually be dominated by the 1/fC1 term
and voltage losses will rise.
Note that losses also rise as frequency increases. This is
caused by internal switching losses which occur due to
some finite charge being lost on each switching cycle. This
charge loss per-unit-cycle, when multiplied by the switching
frequency, becomes a current loss. At high frequency this
loss becomes significant and voltage losses again rise.
The oscillators of practical converters are designed to
run in the frequency band where voltage losses are at
V1
V2
f
C1
C2
a minimum. Figure B3 shows the block diagram of the
LT1054 switched-capacitor converter.
The LT1054 is a monolithic, bipolar, switched-capacitor voltage converter and regulator. It provides higher output current then previously available converters with significantly
lower voltage losses. An adaptive switch drive scheme
optimizes efficiency over a wide range of output currents.
Total voltage loss at 100mA output current is typically 1.1V.
This holds true over the full supply voltage range of 3.5V
to 15V. Quiescent current is typically 2.5mA.
The LT1054 also provides regulation. By adding an external
resistive divider, a regulated output can be obtained. This
output will be regulated against changes in input voltage
and output current. The LT1054 can also be shut down by
grounding the feedback pin. Supply current in shutdown
is less than 100μA.
The internal oscillator of the LT1054 runs at a nominal
frequency of 25kHz. The oscillator pin can be used to adjust the switching frequency, or to externally synchronize
the LT1054.
VREF
VIN
6
8
RL
2.5V
REF
R
+
AN29 FB1
Figure B1. Switched-Capacitor Building Block
DRIVE
–
1
CAP+ 2
FEEDBACK/
SHUTDOWN
REQUIV
V1
+
V2
OSC
R
1
REQUIV =
fC1
Q
C2
CIN*
Q
CAP– 4
RL
7
OSC
DRIVE
DRIVE
AN29 FB2
Figure B2. Switched-Capacitor Equivalent Circuit
3 GND
+
*EXTERNAL CAPACITORS
COUT*
5 –VOUT
DRIVE
AN29 FB3
Figure B3. LT1054 Switched-Capacitor Converter Block Diagram
an32f
AN29-36
Application Note 29
APPENDIX C
Physiology of the LT1070
The LT1070 is a current mode switcher. This means that
switch duty cycle is directly controlled by switch current
rather than by output voltage. Referring to Figure C1, the
switch is turned on at the start of each oscillator cycle. It
is turned off when switch current reaches a predetermined
level. Control of output voltage is obtained by using the
output of a voltage-sensing error amplifier to set current
trip level. This technique has several advantages. First,
it has immediate response to input voltage variations,
unlike ordinary switchers which have notoriously poor
line transient response. Second, it reduces the 90° phase
shift at mid-frequencies in the energy storage inductor.
This greatly simplifies closed-loop frequency compensation under widely varying input voltage or output load
conditions. Finally, it allows simple pulse-by-pulse current
limiting to provide maximum switch protection under output overload or short conditions. A low dropout internal
regulator provides a 2.3V supply for all internal circuitry on
the LT1070. This low dropout design allows input voltage
to vary from 3V to 60V with virtually no change in device
performance. A 40kHz oscillator is the basic clock for all
internal timing. It turns on the output switch via the logic
and driver circuitry. Special adaptive antisat circuitry detects onset of saturation in the power switch and adjusts
driver current instantaneously to limit switch saturation.
This minimizes driver dissipation and provides very rapid
turn-off of the switch.
A 1.2V bandgap reference biases the positive input of
the error amplifier. The negative input is brought out for
output voltage sensing. This feedback pin has a second
function; when pulled low with an external resistor,
VIN
16V
2.3V
REG
SWITCH
OUT
FLYBACK
ERROR
AMP
40kHz
OSC
LOGIC
MODE
SELECT
5A
75V
SWITCH
DRIVER
ANTI-SAT
COMP
FB
–
+
ERROR
AMP
VC
+
SHUTDOWN
CIRCUIT
1.24V REF
+
CURRENT
AMP
0.02Ω
–
GAIN ≈ 6
0.15V
AN29 FC1
Figure C1. LT1070 Internal Details
an32f
AN29-37
Application Note 29
it programs the LT1070 to disconnect the main error
amplifier output and connects the output of the flyback
amplifier to the comparator input. The LT1070 will then
regulate the value of the flyback pulse with respect to the
supply voltage. This flyback pulse is directly proportional
to output voltage in the traditional transformer-coupled
flyback topology regulator. By regulating the amplitude
of the flyback pulse the output voltage can be regulated
with no direct connection between input and output. The
output is fully floating up to the breakdown voltage of
the transformer windings. Multiple floating outputs are
easily obtained with additional windings. A special delay
network inside the LT1070 ignores the leakage inductance
spike at the leading edge of the flyback pulse to improve
output regulation.
The error signal developed at the comparator input is
brought out externally. This pin (VC) has four different
functions. It is used for frequency compensation, current
limit adjustment, soft-starting, and total regulator shutdown. During normal regulator operation this pin sits at a
voltage between 0.9V (low output current) and 2.0V (high
output current). The error amplifiers are current output
(gm) types, so this voltage can be externally clamped
for adjusting current limit. Likewise, a capacitor-coupled
external clamp will provide soft-start. Switch duty cycle
goes to zero if the VC pin is pulled to ground through a
diode, placing the LT1070 in an idle mode. Pulling the VC
pin below 0.15V causes total regulator shutdown with only
50μA supply current for shutdown circuitry biasing. For
more details, see Linear Technology Application Note 19,
Pages 4-8.
APPENDIX D
Inductor Selection for Flyback Converters
While saturation is a prime concern, cost, heating, size,
availability and desired performance are also significant.
Electromagnetic theory, although applicable to these issues,
can be confusing, particularly to the non-specialist.
Figure D1 shows a typical flyback based converter utilizing
the LT1070 switching regulator. A simple approach may
be employed to determine the appropriate inductor. A very
useful tool is the #845 inductor kit6 shown in Figure D2.
This kit provides a broad range of inductors for evaluation
in test circuits such as Figure D1.
6Available from Pulse Engineering, Inc., P.O. Box 12235, San Diego, CA
92112, 619-268-2400
22μF
VIN
GND
Practically speaking, an empirical approach is often a
good way to address inductor selection. It permits real
time analysis under actual circuit operating conditions
using the ultimate simulator—a breadboard. If desired,
inductor design theory can be used to augment or confirm
experimental results.
TEST
INDUCTOR
5VIN
+
A common problem area in DC/DC converter design is the
inductor, and the most common difficulty is saturation. An
inductor is saturated when it cannot hold any more magnetic
flux. As an inductor arrives at saturation it begins to look
more resistive and less inductive. Under these conditions
current flow is limited only by the inductor’s DC copper
resistance and the source capacity. This is why saturation
often results in destructive failures.
VSW
LT1070
MBR735 (MOTOROLA)
10.7k
12V OUTPUT
FB
VC
1k
+
+
1.24k
470μF
1μF
AN29 FD1
Figure D1. Basic LT1070 Flyback Converter Test Circuit
an32f
AN29-38
Application Note 29
Figure D3 was taken with a 450μH value, high core capacity inductor installed. Circuit operating conditions such as
input voltage and loading are set at levels appropriate to
the intended application. Trace A is the LT1070’s VSW pin
voltage while Trace B shows its current. When VSW pin
voltage is low, inductor current flows. The high inductance means current rises relatively slowly, resulting in
the shallow slope observed. Behavior is linear, indicating
no saturation problems. In Figure D4, a lower value unit
with equivalent core characteristics is tried. Current rise
is steeper, but saturation is not encountered. Figure D5’s
selected inductance is still lower, although core characteristics are similar. Here, the current ramp is quite
pronounced, but well controlled. Figure D6 brings some
informative surprises. This high value unit, wound on a
low capacity core, starts out well but heads rapidly into
saturation, and is clearly unsuitable.
The described procedure narrows the inductor choice
within a range of devices. Several were seen to produce
acceptable electrical results, and the “best” unit can be
further selected on the basis of cost, size, heating and other
parameters. A standard device in the kit may suffice, or a
derived version can be supplied by the manufacturer.
Using the standard products in the kit minimizes specification uncertainties, accelerating the dialogue between user
and inductor vendor.
A = 20V/DIV
B = 1A/DIV
HORIZ = 5μs/DIV
AN29 FD4
Figure D4. Waveforms for 170μH, High Capacity Core Unit
A = 20V/DIV
B = 1A/DIV
Figure D2. Model 845 Inductor Selection Kit from Pulse
Engineering, Inc. (Includes 18 Fully Specified Devices)
HORIZ = 5μs/DIV
AN29 FD5
A = 20V/DIV
A = 20V/DIV
B = 1A/DIV
B = 1A/DIV
HORIZ = 5μs/DIV
AN29 FD3
HORIZ = 5μs/DIV
AN29 FD6
Figure D6. Waveforms for 500μH, Low Capacity Core Inductor
(Note Saturation Effects)
an32f
AN29-39
Application Note 29
APPENDIX E
Optimizing Converters for Efficiency
Squeezing the utmost efficiency out of a converter is a
complex, demanding design task. Efficiency exceeding 80%
to 85% requires some combination of finesse, witchcraft
and just plain luck. Interaction of electrical and magnetic
terms produces subtle effects which influence efficiency.
A detailed, generalized method for obtaining maximum
converter efficiency is not readily described but some
guidelines are possible.
Losses fall into several loose categories including junction,
ohmic, drive, switching and magnetic losses.
Semiconductor junctions produce losses. Diode drops
increase with operating current and can be quite costly
in low voltage output converters. A 700mV drop in a
5V output converter introduces more than 10% loss.
Schottky devices will cut this nearly in half, but loss is still
appreciable. Germanium (rarely used) is lower still, but
switching losses negate the low DC drop at high speeds.
In very low power converters Germanium’s reverse leakage may be equally oppressive. Synchronously-switched
rectification is more complex, but can sometimes simulate
a more efficient diode (see text Figure 32). When evaluating such a scheme remember to include both AC and DC
drive losses in efficiency estimates. DC losses include
base or gate current in addition to DC consumption in
any driver stage. AC losses might include the effects of
gate (or base) capacitance, transition region dissipation
(the switch spends some time in its linear region) and
power lost due to timing skew between drive and actual
switch action.
Transistor saturation losses are also a significant term.
Channel and collector-emitter saturation losses become
increasingly significant as operating voltages decrease. The
most obvious way to minimize these losses is to select low
saturation components. In some cases this will work, but
remember to include the drive losses (usually higher) for
lower saturation devices in overall loss estimates. Actual
losses caused by saturation effects and diode drops is
sometimes difficult to ascertain. Changing duty cycles
and time variant currents make determination tricky. One
simple way to make relative loss judgements is to measure
device temperature rise. Appropriate tools here include
thermal probes and (at low voltages) the perhaps more
readily available human finger. At lower power (e.g., less
dissipation, even though loss percentage may be as great)
this technique is less effective. Sometimes deliberately
adding a known loss to the component in question and
noting efficiency change allows loss determination.
Ohmic losses in conductors are usually only significant
at higher currents. “Hidden” ohmic losses include socket
and connector contact resistance and equivalent series
resistance (ESR) in capacitors. ESR generally drops with
capacitor value and rises with operating frequency, and
should be specified on the capacitor data sheet. Consider
the copper resistance of inductive components. It is often
necessary to evaluate trade-offs of an inductors copper
resistance versus magnetic characteristics.
Drive losses were mentioned, and are important in obtaining
efficiency. MOSFET gate capacitance draws substantial AC
drive current per cycle, implying higher average currents as
frequency goes up. Bipolar devices have lower capacitance,
but DC base current eats power. Large area devices may
appear attractive for low saturation, but evaluate drive
losses carefully. Usually, large area devices only make
sense when operating at a significant percentage of rated
current. Drive stages should be thought out with respect
to efficiency. Class A type drives (e.g., resistive pull-up
or pull-down) are simple and fast, but wasteful. Efficient
operation usually requires active source-sink combinations
with minimal cross conduction and biasing losses.
Switching losses occur when devices spend significant
amounts of time in their linear region relative to operating
frequency. At higher repetition rates transition times can
become a substantial loss source. Device selection and
drive techniques can minimize these losses.
Magnetics design also influences efficiency. Design of
inductive components is well beyond the scope of this appended section, but issues include core material selection,
wire type, winding techniques, size, operating frequency,
current levels, temperature and other issues.
an32f
AN29-40
Application Note 29
Some of these topics are discussed in Linear Technology
Application Note 19, but there is no substitute for access
to a skilled magnetics specialist. Fortunately, the other
categories mentioned usually dominate losses, allowing
good efficiencies to be obtained with standard magnetics.
Custom magnetics are usually only employed after circuit
losses have been reduced to lowest practical levels.
APPENDIX F
Instrumentation for Converter Design
Instrumentation for DC/DC converter design should be
selected on the basis of flexibility. Wide bandwidths, high
resolution and computational sophistication are valuable
features, but are usually not required for converter work.
Typically, converter design requires simultaneous observation of many circuit events at relatively slow speeds.
Single ended and differential voltage and current signals
are of interest, with some measurements requiring fully
floating inputs. Most low level measurement involves
AC signals and is accommodated with a high sensitivity
plug-in. Other situations call for observation of small,
slowly changing (e.g., 0.1Hz to 10Hz) events on top of DC
levels. This range falls outside the AC-coupled cut-off of
most oscilloscopes, mandating differential DC nulling or
“slide-back” plug-in capability. Other requirements include
high impedance probes, filters and oscilloscopes with
very versatile triggering and multi-trace capability. In our
converter work we have found a number of particularly
noteworthy instruments in several categories.
Probes
For many measurements standard 1× and 10× scope
probes are fine. In most cases the ground strap may be
used, but low level measurements, particularly in the presence of wideband converter switching noise, should be
taken with the shortest possible ground return. A variety
of probe tip grounding accessories are available, and are
usually supplied with good quality probes (see Figure F1).
In some cases, directly connecting the breadboard to the
‘scope may be necessary (Figure F2).
Wideband FET probes are not normally needed, but a
moderate speed, high input impedance buffer probe is quite
useful. Many converter circuits, especially micropower
designs, require monitoring of high impedance nodes. The
10MΩ loading of standard 10× probes usually suffices,
but sensitivity is traded away. 1× probes retain sensitivity,
but introduce heavier loading. Figure F3 shows an almost
absurdly simple, but useful, circuit which greatly aids
probe loading problems. The LT1022 high speed FET op
amp drives an LT1010 buffer. The LT1010’s output allows
cable and probe driving and also biases the circuit’s input
shield. This bootstraps the input capacitance, reducing its
effect. DC and AC errors of this circuit are low enough for
almost all converter work, with enough bandwidth for most
circuits. Built into a small enclosure with its own power
supply, it can be used ahead of a ‘scope or DVM with good
results. Pertinent specifications appear in the diagram.
Figure F4 shows a simple probe filter which sets high and
low bandwidth restrictions. This circuit, placed in series
with the ‘scope input, is useful for eliminating switching
artifacts when observing circuit nodes.
An isolated probe allows fully floating measurements,
even in the presence of high common mode voltages. It
is often desirable to look across floating points in a circuit.
The ability to directly observe an ungrounded transistor’s
saturation characteristics or monitor waveforms across a
floating shunt makes this probe valuable. One probe, the
Signal Acquisition Technologies, Inc. Model SL-10, has
10MHz bandwidth and 600V common mode capability.
Current probes are an indispensable tool in converter
design. In many cases current waveforms contain more
valuable information than voltage measurements. The
clip-on types are quite convenient. Hall effect based versions respond down to DC, with bandwidths of 50MHz.
Transformer types are faster, but roll off below several
hundred cycles (Figure F5). Both types have saturation
limitations which, when exceeded, cause odd results
on the CRT, confusing the unwary. The Tektronix P6042
(and the more recent AM503) Hall type and P6022/134
transformer based type give excellent results. The HewlettPackard 428B clip-on current probe responds from DC
an32f
AN29-41
Application Note 29
Figure F1. Proper Probing Technique for Low Level Measurements in the Presence of High Frequency Noise
Figure F2. Direct Connections to the Oscilloscope Give Best Low Level Measurements.
Note Ground Reference Connection to the Differential Plug-In’s Negative Input
an32f
AN29-42
Application Note 29
to only 400Hz, but features 3% accuracy over a 100μA
to 10A range. This instrument, useful for determining efficiency and quiescent current, eliminates shunt caused
measurement errors.
the more modern 7603), equipped with a type 1A4 (2 dual
trace 7A18s required for the 7603) plug-in, has four full
capability input channels with flexible triggering and superb
CRT trace clarity. This instrument, or its equivalent, will
handle a wide variety of converter circuits with minimal
restrictions. The Tektronix 556 offers an extraordinary
array of features valuable in converter work. This dual
beam instrument is essentially two fully independent
oscilloscopes sharing a single CRT. Independent vertical,
horizontal and triggering permit detailed display of almost
any converters operation. Equipped with two type 1A4
Oscilloscopes and Plug-Ins
The oscilloscope plug-in combination is an important
choice. Converter work almost demands multi-trace capability. Two channels are barely adequate, with four far
preferable. The Tektronix 2445/6 offers four channels, but
two have limited vertical capability. The Tektronix 547 (and
18V
10k
1000pF
–
LT1022
CLIP
INPUTS
OUTPUT
LT1010
+
AN29 FF3
–18V
INPUT CAPACITANCE ≈ 8pF
IB = 50pA
GBW = 8.5MHz
SLEW = 23V/μs
OFFSET VOLTAGE = 250μV
OFFSET TEMPERATURE DRIFT = 5μV/°C
Figure F3. A Simple High Impedance Probe
OUT
160pF
100kHz
0.0016
10kHz
HIGH PASS
0.016
1kHz
0.16μF
100Hz
BNC INPUT
(TO PROBE)
16k
1.6μF
10Hz
SCOPE
LOW PASS
1μF
0.1
0.01
BNC OUTPUT
(TO SCOPE)
0.001
1M
CSMALL
TYPICALLY
9pF TO 22pF
100pF
AN29 FF4
10Hz
100Hz
1kHz
10kHz
100kHz
OUT
Figure F4. Oscilloscope Filter
an32f
AN29-43
Application Note 29
plug-ins, the 556 will display eight real time inputs. The
independent triggering and time bases allow stable display
of asynchronous events. Cross beam triggering is also
available, and the CRT has exceptional trace clarity.
Two oscilloscope plug-in types merit special mention.
At low level, a high sensitivity differential plug-in is indispensable. The Tektronix 1A7 and 7A22 feature 10μV
sensitivity, although bandwidth is limited to 1MHz. The
A = 100mA/DIV
B = 100mA/DIV
HORIZ = 2ms/DIV
AN29 FF5
Figure F5. Hall (Trace A) and Transformer (Trace B) Based
Current Probes Responding to Low Frequency
units also have selectable high and low pass filters and
good high frequency common mode rejection. Tektronix
types W, 1A5 and 7A13 are differential comparators. They
have calibrated DC nulling (“slideback”) sources, allowing observation of small, slowly moving events on top of
common mode DC.
Voltmeters
Almost any DVM will suffice for converter work. It should
have current measurement ranges and provision for
battery operation. The battery operation allows floating
measurements and eliminates possible ground loop errors. Additionally, a non-electronic (VOM) voltmeter (e.g.,
Simpson 260, Triplett 630) is a worthwhile addition to the
converter design bench. Electronic voltmeters are occasionally disturbed by converter noise, producing erratic
readings. A VOM contains no active circuitry, making it
less susceptible to such effects.
APPENDIX G
The Magnetics Issue
Magnetics is probably the most formidable issue in
converter design. Design and construction of suitable
magnetics is a difficult task, particularly for the non-specialist. It is our experience that the majority of converter
design problems are associated with magnetics requirements. This consideration is accented by the fact that
most converters are employed by non-specialists. As a
purveyor of switching power ICs we incur responsibility
towards addressing the magnetics issue (our publicly
spirited attitude is, admittedly, capitalistically influenced).
As such, it is LTC’s policy to use off-the-shelf magnetics
in our circuits. In some cases, available magnetics serve
a particular design. In other situations the magnetics have
been specially designed, assigned a part number and
made available as standard product. In these endeavors
our magnetics supplier and partner is;
Pulse Engineering, Inc.
P.O. Box 12235
7250 Convoy Court
San Diego, California 92112
619-268-2400
Figure G1. Magnetics for LTC Applications Circuits are Designed
and Supplied as Standard Product by Pulse Engineering, Inc.
In many circumstances a standard product is suitable
for production. Other cases may require modifications or
changes which Pulse Engineering can provide. Hopefully,
this approach serves the needs of all concerned.
an32f
AN29-44
Application Note 29
APPENDIX H
15V represents the maximum allowable input supply. Many
applications require higher voltage inputs; the circuit in
Figure H1 uses a cascoded3 output stage to achieve such
high voltage capability. This 24V to 5V (VIN = 20V–50V)
converter is reminiscent of previous LT1533 circuits,
except for the presence of Q1 and Q2.4 These devices,
interposed between the IC and the transformer, constitute
a cascoded high voltage stage. They provide voltage gain
while isolating the IC from their large drain voltage swings.
LT1533 Ultralow Noise Switching Regulator for High
Voltage or High Current Applications
The LT1533 switching regulator1, 2 achieves 100μV output noise by using closed-loop control around its output
switches to tightly control switching transition time. Slowing down switch transitions eliminates high frequency
harmonics, greatly reducing conducted and radiated noise.
The part’s 30V, 1A output transistors limit available power.
It is possible to exceed these limits while maintaining low
noise performance by using suitably designed output
stages.
Normally, high voltage cascodes are designed to simply
supply voltage isolation. Cascoding the LT1533 presents
special considerations because the transformer’s instantaneous voltage and current information must be accurately
transmitted, albeit at lower amplitude, to the LT1533. If
this is not done, the regulator’s slew-control loops will
not function, causing a dramatic output noise increase.
The AC-compensated resistor dividers associated with the
High Voltage Input Regulator
The LT1533’s IC process limits collector breakdown to
30V. A complicating factor is that the transformer causes
the collectors to swing to twice the supply voltage. Thus,
6
T1
7
5
8
24VIN
(20V TO 50V)
+
4
10μF
9
3
MBRS140
10
1
0.002μF
220Ω
10k
Q3
MPSA42
Q4
2N2222
10k
Q1
L3
100μH
OPTIONAL
SEE TEXT
+
220μF
100μF
12
0.002μF
Q2
1k
5VOUT
+
10k
220Ω
L1
100μH
1k
2
+
4.7μF
2
14
4
3
1500pF
11
5
18k
6
15
COL A
VIN
0.01μF
MBRS140
SYNC
DUTY
SHDN
CT
LT1533
L2
PGND
RT
NFB
10
11
COL B
VC
RVSL
FB
16
8
7
7.5k
1%
GND
RCSL
9
12
13
2.49k
1%
12k
10k
AN29 FH1
AN70 F40
L1, L3: COILTRONICS CTX100-3
L2: 22nH TRACE INDUCTANCE, FERRITE BEAD OR
INDUCTOR COILCRAFT B-07T TYPICAL
Q1, Q2: MTD6N15
T1: COILTRONICS VP4-0860
Figure 1H. A Low Noise 24V to 5V Converter (VIN = 20V–50V): Cascoded MOSFETs
Withstand 100V Transformer Swings, Permitting the LT1533 to Control 5V/2A Output
an32f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
AN29-45
Application Note 29
The follower loss limits efficiency to about 68%. Higher
input voltages minimize follower-induced loss, permitting
efficiencies in the low 70% range.
Q1–Q2 gate-drain biasing serve this purpose, preventing
transformer swings coupled via gate-channel capacitance
from corrupting the cascode’s waveform-transfer fidelity.
Q3 and associated components provide a stable DC termination for the dividers while protecting the LT1533 from
the high voltage input.
Figure H4 shows noise performance. Ripple measures 4mV
(Trace A) using a single LC section, with high frequency
content just discernible. Adding the optional second LC
section reduces ripple to below 100μV (trace B), and high
frequency content is seen to be inside 180μV (note ×50
vertical scale-factor change).
Figure H2 shows that the resultant cascode response is
faithful, even with 100V swings. Trace A is Q1’s source;
traces B and C are its gate and drain, respectively. Under
these conditions, at 2A output, noise is inside 400μV peak.
1Witt, Jeff. The LT1533 Heralds a New Class of Low Noise Switching
Regulators. Linear Technology VII:3 (August 1997).
2Williams, Jim. LTC Application Note 70: A Monolithic Switching Regulator
with 100μV Output Noise. October 1997.
3The term “cascode,” derived from “cascade to cathode,” is applied
to a configuration that places active devices in series. The benefit may
be higher breakdown voltage, decreased input capacitance, bandwidth
improvement or the like. Cascoding has been employed in op amps,
power supplies, oscilloscopes and other areas to obtain performance
enhancement.
4This circuit derives from a design by Jeff Witt of Linear Technology Corp.
5Operating the slew loops from follower base current was suggested by
Bob Dobkin of Linear Technology Corp.
Current Boosting
Figure H3 boosts the regulator’s 1A output capability
to over 5A. It does this with simple emitter followers
(Q1–Q2). Theoretically, the followers preserve T1’s voltage
and current waveform information, permitting the LT1533’s
slew-control circuitry to function. In practice, the transistors must be relatively low beta types. At 3A collector
current, their beta of 20 sources ≈150mA via the Q1–Q2
base paths, adequate for proper slew-loop operation.5
A = 20V/DIV
B = 5V/DIV
AC-COUPLED
A = 5mV/DIV
C = 100V/DIV
B = 100μV/DIV
AN29 FH2
10μs/DIV
Figure H2. MOSFET-Based Cascode Permits the Regulator to
Control 100V Transformer Swings While Maintaining a Low
Noise 5V Output. Trace A Is Q1’s Source, Trace B Is Q1’s Gate
and Trace C Is the Drain. Waveform Fidelity Through Cascode
Permits Proper Slew-Control Operation
Figure H4. Waveforms for Figure H3 at 10W Output: Trace A
Shows Fundamental Ripple with Higher Frequency Residue
Just Discernible. The Optional LC Section Results in Trace B’s
180μVP-P Wideband Noise Performance
1N4148
330Ω
5V
1N5817
0.05Ω
T1
Q1
+
4.7μF
14
11
3
1500pF
4
5
18k
6
0.003μF
VIN
SHDN
COL A
DUTY
COL B
SYNC
CT
PGND
LT1533
RVSL
RT
RCSL
10
0.01μF
VC
GND
NFB
9
8
FB
+
2
4.7μF
15
Q2
0.05Ω
330Ω
16
L2
7
680Ω
L1
300μH
12V
L3
33μH
+
(
OPTIONAL FOR
LOWEST RIPPLE
)
+
100μF
100μF
1N5817
1N4148
13 10k
12 10k
AN29 FH4
2μs/DIV
R1
21.5k
1%
AN29 FH3
R2
2.49k
1%
L1: COILTRONICS CTX300-4
L2: 22nH TRACE INDUCTANCE, FERRITE BEAD OR
INDUCTOR. COILCRAFT B-07T TYPICAL
L3: COILTRONICS CTX33-4
Q1, Q2: MOTOROLA D45C1
T1: COILTRONICS CTX-02-13949-X1
: FERRONICS FERRITE BEAD 21-110J
Figure H3. A 10W Low Noise 5V to 12V Converter: Q1–Q2 Provide 5A Output Capacity While Preserving the LT1533’s Voltage/Current
Slew Control. Efficiency Is 68%. Higher Input Voltages Minimize Follower Loss, Boosting Efficiency Above 71%
an32f
AN29-46
Linear Technology Corporation
IM/GP 689 5K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1989

AN29 - Some Thoughts on DC/DC Converters

Transcription

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