LM2635 5-Bit Programmable

Synchronous Buck Regulator Controller

General Description

The LM2635 is a high speed controller designed specifically

for use in synchronous DC/DC buck converters for the

cepts the VID code directly from the CPU and adjusts the

output voltage from 1.8V to 3.5V. It provides the power good,

over-voltage protection, and output enable features as re-

quired by Intel VRM specifications. Current limiting is

achieved by monitoring the voltage drop across the r

DS_ON

of the high side MOSFET, which eliminates an expensive

current sense resistor.

The LM2635 employs a fixed-frequency voltage mode PWM

architecture. To provide a faster response to a large and fast

load transient, two ultra-fast comparators are built in to moni-

tor the output voltage and override the primary control loop

when necessary. The PWM frequency is adjustable from 50

kHz to 1 MHz through an external resistor. The wide range of

PWM frequency gives the power supply designer the flexibil-

ity to make trade-offs between load transient response per-

formance, MOSFET cost and the overall efficiency. The

adaptive non-overlapping MOSFET gate drivers help avoid

any potential shoot-through problem while maintaining high

efficiency. BiCMOS gate drivers with rail-to-rail swing ensure

that no spurious turn-on occur. When only 5V is available, a

bootstrap structure can be employed to accommodate an

NMOS high side switch. The precision reference trimmed to

2% over temperature is available externally for use by other

regulators. Dynamic positioning of load voltage, which helps

cut the number of output capacitors, can also be imple-

mented easily.

Features

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1.8V to 3.5V 5-bit programmable output voltage

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Synchronous rectification

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Power Good flag and output enable

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Over-voltage protection

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Initial Output Accuracy: 2% over temperature

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Current limit without external sense resistor

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Adaptive non-overlapping MOSFET gate drives

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Adjustable switching frequency: 50 kHz to 1 MHz

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Dynamic output voltage positioning

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1.256V reference voltage available externally

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Plastic SO-20 package

Applications

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Motherboard power supply/VRM for Cyrix Gxm, Cyrix

Gxi, Cyrix MII, Pentium II, Deschutes, Pentium Pro,

6x86 and K6 processors

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5V to 1.8V–3.5V high current power supplies

Typical Application

DS100119-1

FIGURE 1. 5V to 1.8V–3.5V, 14A Power Supply

May 2000

© 2000 National Semiconductor Corporation

DS100119

www.national.com