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

DESCRIPTION

The UCC3581 voltage mode pulse width modulator is designed

to control low power isolated DC - DC converters

in applications such as Subscriber Line Power (ISDN

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

flyback converters, the UCC3581 features BiCMOS circuitry

for low startup and operating current, while maintaining

the ability to drive power MOSFETs at

frequencies up to 100kHz. The UCC3581 oscillator allows

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 control

chip VDD during startup, and an on-board 4.0V logic

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 external

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 minimum

duty cycle clamp. When a duty cycle below the

minimum is called for, the modulator skips cycles to provide

the correct average duty cycle required for output

regulation. This effectively reduces the switching frequency,

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 specified

for operation from ?40°C to +85°C, and the

UCC3581 is specified for operation from 0°C to +70°C.