LM2638

Motherboard Power Supply Solution with a 5-Bit

Programmable Switching Controller and Two Linear

Regulator Controllers

General Description

The LM2638 provides a comprehensive embedded power

supply solution for motherboards hosting high performance

high performance MPUs. The LM2638 incorporates a 5-bit

programmable, synchronous buck switching controller and

two high-speed linear regulator controllers in a 24-pin SO

package. In a typical application, the switching controller

supplies the MPU core, and the linear regulator controllers

supply the GTL+ bus and the clock or graphics chip core. A

charge pump pin helps provide the necessary voltage to

power the linear sections when 12V is shut off during system

standby such as STR mode.

Switching Section — The switching regulator controller fea-

tures an Intel-compatible, 5-bit programmable output volt-

age, over-current and over-voltage protection, a power good

signal, and a logic-controlled output enable. There are two

user-selectable over-current protection methods. One pro-

vides accurate over-current protection with the use of an ex-

ternal sense resistor. The other saves cost by taking advan-

tage of the r

over voltage, the controller turns off the high side FET and

turns on the low side.

Linear Section — The two linear regulator controllers fea-

ture wide control bandwidth, N-FET and NPN transistor driv-

ing capability and an adjustable output. The wide control

bandwidth makes meeting the GTL+ bus transient response

requirement an easy job. In minimum configuration, the two

controllers default to 1.5V and 1.25V respectively.

Both linear controllers have under voltage latch-off.

Features

n Provides 3 regulated voltages

n Power Good flag and output enable

n Charge pump pin

Switching Section

n Synchronous rectification

n 5-bit DAC programmable down to 1.3V

n Switching frequency: 50 kHz to 1 MHz

n Over-voltage protection

n Two methods of over-current protection

n Adaptive non-overlapping FET gate drives

n Soft start without external capacitor

Linear Section

n N-FET and NPN drive capability

n Ultra fast response speed

n Under voltage latch-off at 0.63V

n Output voltages default to 1.5V and 2.5V yet adjustable

Applications

n Embedded power supplies for motherboards

n Triple DC/DC power supplies

n Programmable high current DC/DC power supply

Pin Configuration

24-Lead SOIC

DS101034-1

Top View

Order Number LM2638M

See NS Package Number M24B

May 1999

© 1999 National Semiconductor Corporation

DS101034

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