4

Rev J: 3/4/07

SP765 Wide Input Voltage Range 3A, 900kHz, Buck Regulator

© Copyright 2007 Sipex Corporation

General Overview

The SP7651 is a fixed frequency, voltage

mode, synchronous PWM regulator opti-

mized 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 20V for step down

conversion.

The heart of the SP765 is a wide bandwidth

transconductance amplifier designed to ac-

commodate Type II and Type III compensa-

tion schemes. A precision 0.8V reference,

present on the positive terminal of the error

amplifier, permits the programming of the

The output of the error amplifier, COMP,

which is compared to a .V peak-to-peak

ramp, is responsible for trailing edge PWM

control.This voltage ramp, and PWM control

logic are governed by the internal oscillator

THEORY OF OPERATION

that accurately sets the PWM frequency to

900kHz.

The SP765 contains two unique control

features that are very powerful in distributed

applications.First,asynchronousdrivercon-

trol is enabled during startup, to prohibit the

low side NFET from pulling down the output

until the high side NFET has attempted to

turn on. Second, a 100% duty cycle timeout

ensuresthatthelowsideNFETisperiodically

enhanced during extended periods at 100%

dutycycle.Thisguaranteesthesynchronized

refreshing of the BST capacitor during very

large duty ratios.

The SP765 also contains a number of

valuableprotectionfeatures.Programmable

value at which the conversion voltage can

PIN DESCRIPTION

Pin #

Pin Name Description

-3

Ground connection for the synchronous rectifier

4,8,9-2

GND

Ground Pin. The control circuitry of the IC and lower power driver are refer-

enced to this pin. Return separately from other ground traces to the (-) terminal

of Cout.

5

Feedback Voltage and Short Circuit Detection pin. It is the inverting input of the

Error Amplifier and serves as the output voltage feedback point for the Buck

Converter. The output voltage is sensed and can be adjusted through an exter-

short circuit fault is detected and the IC enters hiccup mode.

6

COMP

Output of the Error Amplifier. It is internally connected to the inverting input of

the PWM comparator. An optimal filter combination is chosen and connected

7

UVIN

set minimum operating voltage.

9

SS

Soft Start. Connect an external capacitor between SS and GND to set the soft

start rate based on the 0µA source current. The SS pin is held low via a mA

(min) current during all fault conditions.

0-3

Input connection to the high side N-channel MOSFET. Place a decoupling

4-6,23-26

LX

7

NC

No Connect

8

BST

High side driver supply pin. Connect BST to the external boost diode and ca-

pacitor as shown in the Typical Application Circuit on page . High side driver

is connected between BST pin and SWN pin.

22

Vcc

Input for external 5V bias supply