LT1940 Datasheet, PDF (1/20 Pages) Linear Technology – Dual Monolithic 1.4A, 1.1MHz Step-Down Switching Regulator

Final Electrical Specifications
LT1940
Dual Monolithic 1.4A,
1.1MHz Step-Down Switching Regulator
FEATURES
DESCRIPTIO
August 2002
s _{Wide Input Voltage Range: 3.6V to 25V}
s _{Two 1.4A Output Switching Regulators with}
Internal Power Switches
s _{Small 16-Lead TSSOP Surface Mount Package}
s _{Constant 1.1MHz Switching Frequency}
s _{Anti-Phase Switching Reduces Ripple}
s _{Independent Shutdown/Soft-Start Pins}
s _{Independent Power Good Indicators Ease}
Supply Sequencing
s _{Uses Small Inductors and Ceramic Capacitors}
U
APPLICATIO S
s _{Disk Drives}
s _{DSP Power Supplies}
s _{Wall Transformer Regulation}
s _{Distributed Power Regulation}
s _{DSL Modems}
s _{Cable Modems}
, LTC and LT are registered trademarks of Linear Technology Corporation.
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.
TYPICAL APPLICATIO
V_IN
7V TO 25V
4.7µF
CMDSH-3
OUT1
3.3V
1.4A
0.1µF
3.3µH
UPS140
16.5k
V_IN
BOOST1 BOOST2
LT1940
SW1
SW2
FB1
FB2
CMDSH-3
0.1µF
4.7µH
UPS140
30.1k
OUT2
5V
1.4A
10.0k
15k
V_C1
V_C2
RUN/SS1
PG1
15k
10.0k
10µF
10µF
330pF
1nF
RUN/SS2
PG2
GND
330pF
100k
POWER
1940 F01 GOOD
Figure 1. 3.3V and 5V Dual Output Step-Down
Converter with Output Sequencing
Efficiency vs Load Current
100
V_IN = 12V
90
V_OUT = 5V
80
V_OUT = 3.3V
70
60
0
0.5
1.0
LOAD CURRENT (A)
1.5
1940 F01b
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.
1940i
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