LT1940/LT1940L
Dual Monolithic 1.4A,
1.1MHz Step-Down
Switching Regulator
U
FEATURES
DESCRIPTIO
The LT®1940 is a dual current mode PWM step-down
DC/DCconverterwithinternal2Apowerswitches.Bothcon-
verters are synchronized to a single 1.1MHz oscillator and
runwithoppositephases,reducinginputripplecurrent.The
output voltages are set with external resistor dividers, and
each regulator has independent shutdown and soft-start
circuits. Each regulator generates a power-good signal
when its output is in regulation, easing power supply se-
quencingandinterfacingwithmicrocontrollersandDSPs.
■
Wide Input Voltage Range
LT1940: 3.6V to 25V
LT1940L: 3.6V to 7V
■
Two 1.4A Output Switching Regulators with
Internal Power Switches
■
Constant 1.1MHz Switching Frequency
■
Anti-Phase Switching Reduces Ripple
■
Independent Shutdown/Soft-Start Pins
■
Independent Power Good Indicators Ease
Supply Sequencing
The LT1940’s 1.1MHz switching frequency allows the use
of tiny inductors and capacitors, resulting in a very small
dual1.4Aoutputsolution.Constantfrequencyandceramic
capacitors combine to produce low, predictable output
ripplevoltage. Withitswideinputrangeof3.6Vto25V, the
LT1940 regulates a wide variety of power sources, from
4-cellbatteriesand5Vlogicrailstounregulatedwalltrans-
formers,leadacidbatteriesanddistributed-powersupplies.
The LT1940L is intended to operate from regulated 5V
supplies. A current mode PWM architecture provides fast
transientresponsewithsimplecompensationcomponents
andcycle-by-cyclecurrentlimiting.Frequencyfoldbackand
thermal shutdown provide additional protection.
■
Uses Small Inductors and Ceramic Capacitors
■
Small 16-Lead Thermally Enhanced
TSSOP Surface Mount Package
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APPLICATIO S
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Disk Drives
■
DSP Power Supplies
■
Wall Transformer Regulation
Distributed Power Regulation
DSL Modems
Cable Modems
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■
■
, LTC and LT are registered trademarks of Linear Technology Corporation.
U
TYPICAL APPLICATIO
V
IN
7V TO 25V
4.7µF
V
CMDSH-3
CMDSH-3
IN
Efficiency vs Load Current
OUT2
5V
BOOST1 BOOST2
LT1940
100
1.4A
V
IN
= 12V
0.1µF
0.1µF
3.3µH
4.7µH
OUT1
3.3V
1.4A
90
80
70
60
SW1
SW2
V
= 5V
OUT
UPS140
16.5k
UPS140
30.1k
FB1
FB2
V
= 3.3V
OUT
V
C1
V
C2
10µF
15k
15k
10.0k
RUN/SS1
RUN/SS2
PG1
PG2
10.0k
100k
330pF
1nF
10µF
330pF
GND
0
0.5
1.0
1.5
LOAD CURRENT (A)
POWER
GOOD
1940 F01b
1940 F01
Figure 1. 3.3V and 5V Dual Output Step-Down Converter with Output Sequencing (LT1940)
1940fa
1