Dual-Output Power-Supply

Controller for Notebook Computers

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9

A synchronous rectifier at LX3 keeps efficiency high by

clamping the voltage across the rectifier diode.

Maximum current limit is set by an external low-value

sense resistor, which prevents excessive inductor cur-

rent during start-up or under short-circuit conditions.

Programmable soft-start is set by an optional external

capacitor; this reduces in-rush surge currents upon

start-up and provides adjustable power-up times for

power-supply sequencing purposes.

P

+5V LDO

LINEAR

REGULATOR

V+

VL

REF

SHDN

GND

+3.3V

REFERENCE

ON

3.3V

5V

4V

2.8V

SYNC

300kHz/200kHz

OSCILLATOR ON

STANDBY

4.5V

FAULT

ON

3.3V

PWM

CONTROLLER

(SEE FIG. 3)

FB3

CS3

BST3

DH3

LX3

DL3

SS3

FB5

CS5

BST5

DH5

LX5

DL5

SS5

PGND

ON3

5V

PWM

CONTROLLER

(SEE FIG. 3)

ON

D1

D2

1.65V

1.65V

ON5

VH

Q1

Q2

Figure 2. MAX786 Block Diagram