4

Date: 08/25/04

SP7651 Wide Input Voltage Range 3A, 900kHz, Buck Regulator

© Copyright 2004 Sipex Corporation

General Overview

The SP7651 is a fixed frequency, voltage mode,

synchronous PWM regulator optimized for high

efficiency. The part has been designed to be

especially attractive for split plane applications

utilizing 5V to power the controller and 2.5V to

28V for step down conversion.

The heart of the SP7651 is a wide bandwidth

transconductance amplifier designed to accom-

modate Type II and Type III compensation

schemes. A precision 0.8V reference, present on

the positive terminal of the error amplifier per-

mits the programming of the output voltage

error amplifier, COMP, which is compared to a

1.1V peak-to-peak ramp is responsible for trail-

ing edge PWM control. This voltage ramp, and

PWM control logic are governed by the internal

oscillator that accurately sets the PWM fre-

quency to 900kHz.

THEORY OF OPERATION

The SP7651 contains two unique control fea-

tures that are very powerful in distributed appli-

cations. First, asynchronous driver control is

enabled during start up, to prohibit the low side

NFET from pulling down the output until the

high side NFET has attempted to turn on. Sec-

ond, a 100% duty cycle timeout ensures that the

low side NFET is periodically enhanced during

extended periods at 100% duty cycle. This guar-

antees the synchronized refreshing of the BST

capacitor during very large duty ratios.

The SP7651 also contains a number of valuable

protection features. Programmable UVLO al-

the conversion voltage can safely begin down

that the controller itself has enough voltage to

properly operate. Other protection features in-

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