1FEATURES

· Frequency Foldback Reduces Operating

Frequency Under Fault Conditions

· Accurate Programmable Volt-Second Clamp

· Programmable Maximum Duty Cycle Clamp

· Oscillator Synchronization

· Overcurrent Protection

· Shutdown With Full Soft Start

· Wide Gain Bandwidth Amplifier (GBW > 2.5

MHz)

· Current Mode Operation

· Precision 5-V Reference

DESCRIPTION

The UCC3884 is a high performance current mode

PWM controller intended for single ended switch

mode power supplies. The chip implements a frequency foldback scheme that decreases the

oscillator frequency as the output voltage falls below

a programmed value. This technique decreases the

average output current sourced into a low impedance

load which can occur during an output short circuit or

overload condition. Excessive short circuit current is

more prevalent in high frequency converters where

the propagation delay and switch turn-off time forces

a minimum attainable duty cycle. An accurate

volt-second clamp limits the duty cycle during line or

load transient conditions which could otherwise

saturate the transformer. The volt-second clamp may

also be used with an external overvoltage protection

circuit to handle fault conditions such as current

sense disconnect or current transformer saturation.

The frequency foldback, volt-second clamp,

cycle-by-cycle current limit, and overcurrent shutdown

provide a rich set of protection features for use in

peak current-mode pulse width modulators.