LT1940 datasheet(5/20 Pages) LINER

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LT1940

1940i

PI FU CTIO S

BOOST1, BOOST2 (Pins 1, 8): The BOOST pins are used

to provide drive voltages, higher than the input voltage, to

the internal bipolar NPN power switches.Tie through a

diode from VOUT or from VIN.

SW1, SW2 (Pins 2, 7): The SW pins are the outputs of the

internal power switches. Connect these pins to the induc-

tors, catch diodes and boost capacitors.

VIN (Pins 3, 4, 5, 6): The VIN pins supply current to the

LT1940’s internal regulator and to the internal power

switches. These pins must be tied to the same source, and

must be locally bypassed.

FB1, FB2 (Pins 9, 16): The LT1940 regulates each feed-

back pin to 1.25V. Connect the feedback resistor divider

taps to these pins.

VC1, VC2 (Pins 10, 15): The VC pins are the outputs of the

internal error amps. The voltages on these pins control the

peak switch currents. These pins are normally used to

compensate the control loops, but can also be used to

override the loops. Pull these pins to ground with an open

drain to shut down each switching regulator.

PG1, PG2 (Pins 11, 14): The Power Good pins are the

open collector outputs of an internal comparator. PG

remains low until the FB pin is within 10% of the final

regulation voltage. As well as indicating output regulation,

the PG pins can be used to sequence the two switching

regulators. These pins can be left unconnected. The PG

outputs are valid when VIN is greater than 2.4V and either

of the RUN/SS pins is high. The PG comparators are

disabled in shutdown.

RUN/SS1, RUN/SS2 (Pins 12, 13): The RUN/SS pins are

use to shut down the individual switching regulators and

the internal bias circuits. They also provide a soft-start

function. To shut down either regulator, pull the RUN/SS

pin to ground with an open drain or collector. Tie a

capacitor from these pins to ground to limit switch current

during start-up. If neither feature is used, leave these pins

unconnected.

GND (Underside Metal): The underside exposed pad

metal of the package provides both electrical contact to

ground and good thermal contact to the printed circuit

board. The underside must be soldered to the circuit

board for proper operation.

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