LM2631

Synchronous Step-Down Power Supply Controller

General Description

The LM2631 controller provides all the active functions for

step-down (buck) switching converters. These dc-to-dc con-

verters provide core CPU power in battery-operated sys-

tems.

High efficiency is achieved by using synchronous rectifica-

tion and pulse-skipping mode operation at light load. Inex-

pensive N-channel MOSFETs are used to reduce system

cost. Bootstrap circuit is used to drive the high-side

N-channel MOSFET.

Current mode control scheme is used to improve line regula-

tion and transient response, also provides cycle-by-cycle

current limiting.

The operating frequency is adjustable between 200 kHz and

400 kHz. An external shutdown pin can be used to disable

the device and reduce the quiescent current to 0.1 µA. In low

noise applications, bringing the FPWM pin high can force the

device to operate in constant frequency mode. Other fea-

tures include the external synchronization pin, and the

PGOOD pin to indicate the state of the output voltage.

Protection circuitry includes thermal shutdown, undervoltage

and overvoltage shutdown protection, soft-start capability,

and two levels of current limits: The first level simply limits

the load current directly; at the second level, if the load pulls

the output voltage down below 80% of the regulated value,

the chip will shut down. This operation is disabled during

startup, but an internal timer will enable it if the output does

not come up in the preset time.

Features

n 4.5V to 30V input range

n Adjustable output (1.5V to 7V)

n 200 kHz to 400 kHz adjustable operating frequency

n Externally synchronizable

n On-board power good function

n Precision 1.24V reference output

n 0.8 mA typical quiescent current

n 0.1 µA shutdown current

n Thermal shutdown

n Direct current limit protection

n Input undervoltage lockout

n Output undervoltage shutdown protection

n Output overvoltage shutdown protection

n Programmable soft-start function

n Tiny TSSOP package

Applications

n Notebook and subnotebook computers

n Cellular phones

n Portable instruments

n Battery-powered digital devices

Typical Application Circuit

DS100937-1

April 1999

© 1999 National Semiconductor Corporation

DS100937

www.national.com