DN125 - Monolithic DC/DC Converters Break Speed Limits to Shrink Board Space

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Monolithic DC/DC Converters Break 1MHz to
Shrink Board Space – Design Note 125
Mitchell Lee
In the never-ending quest for board space, operating
frequency remains the most important variable in a
DC/DC converter design. Higher frequency equates
with smaller coils and capacitors. A new family of fast
monolithic converters that allows circuit designers to
reduce the size of their finished products is now available. Other improvements include quiescent currents
well below those of slower converters and a new switchdrive technique that reduces dynamic losses by at least
four fold over previous methods, virtually eliminating
these losses as a concern in efficiency calculations.
Table 1 shows the salient features of each member of
the family. Each is configured as a grounded-switch
step-up converter, but is equally useful in positive
and negative high efficiency buck, SEPIC, inverting
and flyback circuits. The converters are all based
on the LT ®1372 design, which operates at 500kHz,
draws 4mA quiescent current and contains a 1.5A,
0.5Ω switch. The LT1371 is designed for higher
power applications, with a 3A, 0.25Ω switch. Supply
current and operating frequency remain unchanged.
Reduced quiescent current (1mA) makes the LT1373
useful in low power designs or in applications where
the load current has a wide dynamic range. For the
ultimate in miniaturization, the LT1377 features 1MHz
operating frequency—especially helpful where post
filtering is employed.
Table 1. Family Characteristics
DEVICE
IQ
SWITCH
FREE RUNNING SYNCHRONIZATION
FREQUENCY
LIMIT
LT1371 4mA
3A
500kHz
800kHz
LT1372 4mA
1.5A
500kHz
800kHz
LT1373 1mA
1.5A
250kHz
360kHz
LT1377 4mA
1.5A
1MHz
1.6MHz
All devices share the same constant-frequency PWM
core, up to 90% duty cycle and 2.7V to 30V supply
range with a maximum switch rating of 35V. Unique
to these devices is a synchronization input that allows
the internal oscillator to be overridden by an external
clocking signal. The synchronization limit for each part
is also shown in Table 1. Another unique feature is a
second Feedback pin that allows direct regulation of
negative outputs.
A simple boost converter using the LT1372 is shown in
Figure 1. 350mA output current is available at 12V from
a 5V input. Adaptive switch drive and a 0.5Ω collector
resistance result in a peak efficiency of 87%, as shown in
Figure 2.
Figure 3 shows a buck-boost (SEPIC) converter built
around the 3-ampere LT1371. Inputs of 2.7V to 20V
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.
VIN
2.7V TO 11V
95
C1
47μF
L1*
10μH
VOUT = 12V
OFF
VIN
S/S
VSW
D1
MBRS130T3
85
VOUT
12V
LT1372
N/C
3
NFB
VC
FB
GND
1
C2
47nF
4.7nF
8
6, 7
2
R2
10.7k
R3
1.24k
+
C3
33μF
w2
R1
2k
80
VIN = 5V
75
70
VIN = 3V
65
60
*SUMIDA CD73-100KC
DN125 F01
Figure 1. Schematic Diagram: LT1372 Boost Converter
03/96/125_conv
EFFICIENCY (%)
+
5
ON 4
VIN = 9V
90
10
100
OUTPUT CURRENT (mA)
1000
DN125 F02
Figure 2. Efficiency of Boost Converter Shown in Figure 1
are converted to a 5V regulated output at up to 1.8A
(see Figure 4). In spite of handling 9W output power,
the 500kHz operating frequency of the LT1371 allows
a 0.37-inch toroidal core to be used for the coupled
inductor, with excellent efficiency. In shutdown, the
output is completely disconnected from the input source.
250mA. This converter topology exhibits inherently low
output ripple and noise and uses a single-core coupled
inductor. Operating at 250kHz allows the use of relatively
small filter components. The speed-to-power ratio of the
LT1373 is quite high; with only 1mA quiescent current,
it maintains higher efficiency at light loads.
The latest generation of disk drives has adopted magneto-resistive (MR) read-write heads. These operate
with a low noise bias supply of – 3V. Figure 5 shows a
Cuk-configured LT1373 capable of generating –3V at
At the other end of the spectrum is the 1MHz LT1377. It
has the same high speed-to-power ratio as the LT1373.
In Figure 6 the LT1377 is used as a 50mA charger for 4
to 6 NiCd cells, operating from a 5V input. The charger
is clamped against excessive output voltage at 11V,
and maintains constant output current from 0V to 11V.
MBRS340T3
OUTPUT
5V
•
L1*
10μH
+
INPUT
2.7V
TO 20V
100μF
20V
OS-CON
33μF
20V
OS-CON
•
VIN
VSW
S/S
LT1371
NFB
FB
VC
GND
OFF ON
+
+
NC
150μF
6.3V
OS-CON
3.6k
1.2k
20k
47nF
*HURRICANE ELECTRONICS LAB HL-8798
COILTRONICS CTX10-4
4.7nF
1MHz operation is also useful in radio applications where
a 455kHz IF is present, as it gives one octave separation
from that critical frequency. Figure 7 shows the LT1377
configured as a high efficiency buck converter, with a
5V, 1A output. A 20μH inductor is used in this application to maintain a low ripple current (10%), thus easing
output filtering requirements.
L1
4.7μH*
DN125 F03
Figure 3. 5V, 9W Converter Operates Over Wide
Input Range with Good Efficiency
C3
100μF
16V
4
VIN = 5V
C1
22μF
10V
OUTPUT (mA)
R2
2k
+
C2
47nF
1000
GND
GND
6
7
C4
1nF
Q2
Si9400DY
R3
1k
50mA
(11V MAX)
*COILCRAFT
DO-1608-472
DN125 F06
500
Figure 6. Battery Charger
0
0
10
5
8V TO 30V
INPUT
20
15
INPUT (V)
3
•
4
C2
47μF
16V
L1*
VIN
5V
•1
2
5
+
4
7
6
VSW
VIN
S/S
LT1373
GND
*SUMIDA CLS62-100L
**MOTOROLA MBR0520LT3
NFB
VC
GND
4
VOUT
–3V
250mA
R1
1k
1%
C6
0.1μF
8
3
1
+
D1**
+
R4
5.1k
C4
10nF
R2
4.99k
1%
NC
+
C1
2.2μF
+
D1
1N5818
DN125 F04
Figure 4. Maximum Available Output Current
of LT1371 9W Converter (Figure 3)
C1
22μF
10V
VC
Q1
2N3906
D2
10V
400mW
VSW
2
FB
LT1377
1
R1
12Ω
+
8
5
VIN
S/S
2000
1500
D1
MBR0520L
3
S/S
100μF
5
8
VIN
VSW
LT1377
NFB
SG
FB
VC
6
PG
1
4.7nF
3.57k
1.24k
1N4148 10Ω
100nF
7
2k
47nF
C3
47μF
16V
2
20μH*
+
*COILTRONICS CTX20-2P
**SANYO VIDEO COMPONENTS
MBRS130
5V
1A
150μF
6.3V
OS-CON**
DN125 F07
†
PATENTS MAY APPLY
DN125 F05
Figure 7. 1MHz LT1377-Based Buck Converter
Figure 5. Low Ripple 5V to –3V “Cuk”† Converter
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn125f_conv LT/GP 0396 155K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 1996
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