4

Rev. 08/19/05

SP6128A Low Voltage, Synchronous Step Down PWM Controller

© Copyright 2005 Sipex Corporation

General Overview

The SP6128A is a constant frequency, voltage

mode, synchronous PWM controller designed

for low voltage, DC/DC step down converters.

It is intended to provide complete control for a

high power, high efficiency, precisely regulated

output voltage from a highly integrated 14-pin

solution.

The internal free-running oscillator accurately

sets the PWM frequency at 300kHz without

requiring any external elements and allows the

use of physically small, low value external com-

ponents without compromising performance. A

transconductance amplifier is used for the error

amplifier, which compares an attenuated sample

of the output voltage with a precision, 0.8V

reference voltage. The output of the error ampli-

fier (COMP), is compared to a 0.75V peak-to-

peak ramp waveform to provide PWM control.

The COMP pin provides access to the output of

the error amplifier and allows the use of external

components to stabilize the voltage loop.

High efficiency is obtained through the use of

synchronous rectification. Synchronous regula-

tors replace the catch diode in the standard buck

MOSFET switch allowing for significant ef-

ficiency improvements. The SP6128A in-

cludes two fast MOSFET drivers with inter-

nal non-overlap circuitry and drives a pair of

N-channel power transistors. The SP6128A

includes an internal 0.27V/ms soft-start cir-

cuit that provides controlled ramp up of the

output voltage, preventing overshoot and in-

rush current at power up.

Current limiting is implemented by monitoring

side N-channel MOSFET while it is conducting,

thereby eliminating the need for an external

sense resistor. The overcurrent threshold can be

programmed by a single resistor.

FUNCTIONAL DIAGRAM

+

-

-

+

-

+

Synchronous

Driver

PWM

Logic

S

Q

R

Reset

Dominant

R

Q

S

SWN

Reference

6

10

0.8V

UVLO

FAULT

SWN

12

1 GL

13

DRIVER ENABLE

RESET

Dominant

PWM COMP

FAULT

GH

GH

5µA

350mV

SHUTDOWN

GM

ERROR

AMP

Over Current

2.8V ON

2.7V OFF

COMP

SHUTDOWN

F = 300kHz

750mV RAMP

0.27V/ms

SOFTSTART

COMP

3

-

+

-

+

1V

+

-

GND

5

14 BST

15µA

2

11

PGND

4

OPERATION