SC1402 datasheet(10/18 Pages) SEMTECH

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SC1402

652 MITCHELL ROAD

NEWBURY PARK CA 91320

Multi-Output, Low-Noise Power Supply

Controller for Notebook Computers

December 15, 2000

SEQ

RUN/ON3

TIME/ON5

RESET

DESCRIPTION

REF

LOW

Follows 3.3V SMPS.

Independent start control mode.

Both SMPSs off.

REF

LOW

HIGH

Low.

5V SMPS ON, 3.3V SMPS OFF.

REF

HIGH

LOW

Follows 3.3V SMPS.

3.3V SMPS ON, 5V SMPS OFF.

REF

HIGH

Follows 3.3V SMPS.

Both SMPSs on.

GND

LOW

TIMING CAP

Low.

Both SMPS off.

GND

HIGH

TIMING CAP

High after both outputs

are in regulation.

5V starts on rising edge of RUN/ON3, 3V

starts when TIME/ON5 > REF.

LOW

TIMING CAP

Low.

Both SMPS off.

HIGH

TIMING CAP

High after both outputs

are in regulation.

3.3V starts on rising edge of RUN/ON3,

5V starts when TIME/ON5 > REF.

inductor.

A heavy load on the 12OUT regulator when the 5V

SMPS is in PSAVE will cause the VDD input to drop,

browning out the regulator. If the output drops 0.8%

from its nominal value, the 5V SMPS is forced out of

PSAVE mode and into continuous conduction mode for

8 cycles. This recharges the bulk input capacitor on

VDD. The 12OUT linear regulator has a current limit to

prevent damage under short circuit conditions.

Over-voltage protection is provided on the VDD input.

If the VDD input is above 19V, an over-voltage is de-

tected and a 10mA current shunt load is applied to

VDD. The over-voltage threshold has 0.5V of hystere-

sis.

Power up Controls and Soft Start

The user has control of the SC1402 startup sequence

by setting the SEQ, RUN/ON3 and TIME/ON5 pins as

described in the following table.

Each SMPS contains its own counter and DAC to grad-

ually increase the current limit at startup to prevent in-

put surge currents. The current limit is increased from

0, 20%, 40%, 60%, 80%, to 100% linearly over the

course of 512 switching cycles.

A RESET output is also generated at startup. The

RESET pin is held low for 32K switching cycles. An-

other timer is used to enable the undervoltage protec-

tion. The undervoltage protection circuitry is enabled

after 6144 switching cycles at which time the SMPS

should be in regulation.

When SEQ is set to REF, the RESET only monitors

the 3.3V SMPS in regulation. The 5V SMPS is ignored.

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