LT1941
Triple Monolithic
Switching Regulator
U
FEATURES
DESCRIPTIO
■
Wide Input Range: 3.5V to 25V
Three Switching Regulators with Internal Power
The LT®1941 is a triple current mode DC/DC converter
with internal power switches. Two of the regulators are
step-down converters with 3A and 2A power switches.
The third regulator can be configured as a boost, inverter
or SEPIC converter and has a 1.5A power switch. All three
converters are synchronized to a 1.1MHz oscillator. The
two step-down converters run with opposite phases,
reducing input ripple current. The output voltages are set
with external resistor dividers and each regulator has
independent shutdown and soft-start circuits. Each regu-
lator generates a power good signal when its output is in
regulation, easing power supply sequencing and interfac-
ing with microcontrollers and DSPs.
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Switches: 3A Step-Down, 2A Step-Down,
1.5A Inverting/Boost
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Antiphase Switching Reduces Ripple
■
Independent Shutdown/Soft-Start Pins
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Independent Power Good Indicators Ease Supply
Sequencing
■
Input Voltage Power Good Indicators Monitor Input
Supply
■
Uses Small Inductors and Ceramic Capacitors
■
Constant 1.1MHz Switching Frequency
■
Thermally Enhanced 28-Lead TSSOP Package
The high switching frequency offers several advantages
by permitting the use of small inductors and ceramic
capacitors. Small inductors and capacitors lead to a very
small triple output solution. The constant switching fre-
quency, combined with low impedance ceramic capaci-
tors, result in low, predictable output ripple. With its wide
input voltage range of 3.5V to 25V, the LT1941 regulates
a broad array of power sources from 4-cell batteries and
5V logic rails to unregulated wall transformers, lead acid
batteries and distributed-power supplies.
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APPLICATIO S
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Cable Modems
DSL Modems
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Distributed Power Regulation
Wall Transformer Regulation
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Disk Drives
DSP Power
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, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
V
OUT1
V
OUT2
V
IN
4.7V TO 14V
130k 100k
100k
100k
100k
PGOOD1
5GOOD PGOOD2
12GOOD PGOOD3
BOOST1 BOOST2
LT1941
PGOOD1
Start-Up Waveforms
with Sequencing
5GOOD
PGOOD2
12GOOD
PGOOD3
RUN/SS
0.22µF
0.22µF
10.7k
3µH
33µF
22µH
3.3µH
V
OUT1
1.8V
2V/DIV
SW1
SW2
V
3.3V
1.4A
13.7k
OUT2
2.4A
V
OUT1
FB1
FB2
2V/DIV
3300pF
1000pF
1.5nF
V
C1
V
C2
V
OUT2
22µF
5V/DIV
10k 2.49k
22nF
7.32k 3.3k
1.5nF
RUNSS1 RUNSS2
V
OUT3
10V/DIV
SW3
NFB
BIAS1
BIAS2
1µF
22µH
10µF
I
VIN(AVE)
1A/DIV
V
*
OUT3
–12V
V
C3
13.7k
133k
PGOOD2
5V/DIV
350mA
FB3
RUNSS3
1.5k
1941 F01b
GND
1.5nF
10µF
1941 F01
*240mA AT V = 5V, 550mA AT V = 12V
IN IN
Figure 1. Triple Output Power Supply: 3.3V, 1.8V, –12V
1941f
1