UCC1581

UCC2581

UCC3581

SLUS295 - MARCH 1999

FEATURES

• Low 85µA Startup Current

• Low 300µA Operating Current

• Automatically Disabled

Startup Preregulator

• Programmable Minimum Duty

Cycle with Cycle Skipping

• Programmable Maximum

Duty Cycle

• Output Current 1A Peak

Source and Sink

• Programmable Soft Start

• Programmable Oscillator

Frequency

• External Oscillator

Synchronization Capability

Micropower Voltage Mode PWM

BLOCK DIAGRAM

DESCRIPTION

The UCC3581 voltage mode pulse width modulator is

designed to control low power isolated DC - DC convert-

ers in applications such as Subscriber Line Power (ISDN

I.430). Primarily used for single switch forward and

flyback converters, the UCC3581 features BiCMOS cir-

cuitry for low startup and operating current, while main-

taining

the

ability

to

drive

power

MOSFETs

at

frequencies up to 100kHz. The UCC3581 oscillator al-

lows the flexibility to program both the frequency and the

maximum duty cycle with two resistors and a capacitor. A

TTL level input is also provided to allow synchronization

to an external frequency source.

The UCC3581 includes programmable soft start circuitry,

overcurrent detection, a 7.5V linear preregulator to con-

supply.

The UCC3581 provides functions to maximize light load

efficiency that are not normally found in PWM controllers.

A linear preregulator driver in conjunction with an exter-

nal depletion mode N-MOSFET provides initial controller

power. Once the bootstrap supply is functional, the

preregulator is shut down to conserve power. During light

load, power is saved by providing a programmable mini-

mum duty cycle clamp. When a duty cycle below the

minimum is called for, the modulator skips cycles to pro-

vide the correct average duty cycle required for output

regulation. This effectively reduces the switching fre-

quency, saving significant gate drive and power stage

losses.

The UCC3581 is available in 14-pin plastic and ceramic

dual-in-line packages and in a 14-pin narrow body small

outline IC package (SOIC). The UCC1581 is specified for

operation from

−55°C to +125°C, the UCC2581 is speci-

fied for operation from

−40°Cto+85°C, and the

UCC3581 is specified for operation from 0

°C to +70°C.

Note: Pin Connection shown for 14-pin Package

UDG-95011-1