Final Electrical Specifications
LT1940
Dual Monolithic 1.4A,
1.1MHz Step-Down Switching Regulator
August 2002
DESCRIPTIO
The LT
®
1940 is a dual current mode PWM step-down
DC/DC converter with internal 2A power switches. Both
converters are synchronized to a single 1.1MHz oscillator
and run with opposite phases, reducing input ripple cur-
rent. 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 sequencing and interfacing with microcontrollers
and DSPs.
The LT1940’s 1.1MHz switching frequency allows the use
of tiny inductors and capacitors, resulting in a very small
dual 1.4A output solution. Constant frequency and ce-
ramic capacitors combine to produce low, predictable
output ripple voltage. With its wide input range of 3.6V to
25V, the LT1940 regulates a wide variety of power sources,
from 4-cell batteries and 5V logic rails to unregulated wall
transformers, lead acid batteries and distributed-power
supplies. A current mode PWM architecture provides fast
transient response with simple compensation compo-
nents and cycle-by-cycle current limiting. Frequency
foldback and thermal shutdown provide additional protec-
tion.
FEATURES
s
s
s
s
s
s
s
s
Wide Input Voltage Range: 3.6V to 25V
Two 1.4A Output Switching Regulators with
Internal Power Switches
Small 16-Lead TSSOP Surface Mount Package
Constant 1.1MHz Switching Frequency
Anti-Phase Switching Reduces Ripple
Independent Shutdown/Soft-Start Pins
Independent Power Good Indicators Ease
Supply Sequencing
Uses Small Inductors and Ceramic Capacitors
APPLICATIO S
s
s
s
s
s
s
Disk Drives
DSP Power Supplies
Wall Transformer Regulation
Distributed Power Regulation
DSL Modems
Cable Modems
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
V
IN
7V TO 25V
4.7µF
CMDSH-3
BOOST1
0.1µF
OUT1
3.3V
1.4A
3.3µH
SW1
UPS140
16.5k
FB1
V
C1
10.0k
10µF
15k
330pF
1nF
RUN/SS1
RUN/SS2
GND
FB2
V
C2
PG1
PG2
SW2
V
IN
BOOST2
Efficiency vs Load Current
CMDSH-3
100
OUT2
5V
1.4A
V
IN
= 12V
LT1940
0.1µF
4.7µH
UPS140
30.1k
90
EFFICIENCY (%)
V
OUT
= 5V
80
15k
10.0k
10µF
70
330pF
100k
60
1940 F01
POWER
GOOD
0
Figure 1. 3.3V and 5V Dual Output Step-Down
Converter with Output Sequencing
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
U
U
V
OUT
= 3.3V
0.5
1.0
LOAD CURRENT (A)
1.5
1940 F01b
1940i
1
LINER [ LINEAR TECHNOLOGY ]
