LT1940

6

1940i

The LT1940 is a dual, constant frequency, current mode

buck regulator with internal 2A power switches. The two

regulators share common circuitry including input source,

voltage reference and oscillator, but are otherwise inde-

pendent. This section describes the operation of the

LT1940; refer to the Block Diagram.

If the RUN/SS (run/soft-start) pins are both tied to ground,

the LT1940 is shut down and draws 30

µA from the input

BLOCK DIAGRA

–

+

R

SQ

SLAVE

OSC

∑

INT REG

AND REF

MASTER

OSC

RUN/SS2

RUN/SS1

2

µA

2

µA

CLK1

CLK2

1.25V

125mV

CLAMP

RUN/SS

PG

GND

ERROR

AMP

SLOPE

0.9V

0.5V

CLK

R1

C1

SW

FB

BOOST

IN

D2

C3

L1

D1

C1

R2

OUT

1940 F02

external soft-start capacitors, generating voltage ramps at

these pins. If either RUN/SS pin exceeds 0.5V, the internal

bias circuits turn on, including the internal regulator,

1.25V reference and 1.1MHz master oscillator. In this

both RUN/SS pins are high. Neither switching regulator

will begin to operate until its RUN/SS pin reaches ~0.8V.

Figure 2. Block Diagram of the LT1940 with Associated External Components (One of Two Switching Regulators Shown)