LT1940

1

1940i

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.

Final Electrical Specifications

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Disk Drives

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DSP Power Supplies

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Wall Transformer Regulation

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Distributed Power Regulation

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DSL Modems

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Cable Modems

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Wide Input Voltage Range: 3.6V to 25V

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Two 1.4A Output Switching Regulators with

Internal Power Switches

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Small 16-Lead TSSOP Surface Mount Package

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Constant 1.1MHz Switching Frequency

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Anti-Phase Switching Reduces Ripple

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Independent Shutdown/Soft-Start Pins

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Independent Power Good Indicators Ease

Supply Sequencing

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Uses Small Inductors and Ceramic Capacitors

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

Dual Monolithic 1.4A,

1.1MHz Step-Down Switching Regulator

August 2002

The LT

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.

, LTC and LT are registered trademarks of Linear Technology Corporation.

Figure 1. 3.3V and 5V Dual Output Step-Down

Converter with Output Sequencing

Efficiency vs Load Current

7V TO 25V

BOOST1

SW1

FB1

RUN/SS1

RUN/SS2

BOOST2

SW2

FB2

PG1

PG2

LT1940

GND

1940 F01

330pF

10

µF

10

µF

4.7

µF

0.1

µF

0.1

µF

15k

330pF

15k

10.0k

16.5k

100k

30.1k

10.0k

4.7

µH

3.3

µH

POWER

GOOD

OUT2

5V

1.4A

OUT1

3.3V

1.4A

UPS140

UPS140

CMDSH-3

1nF

CMDSH-3

LOAD CURRENT (A)

0

100

90

80

70

60

0.5

1.0

1940 F01b

1.5