UCC1884

UCC2884

UCC3884

PRELIMINARY

SLUS160A - AUGUST 1999

FEATURES

• 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 > 2.5MHz)

• Current Mode Operation

• Precision 5V Reference

Frequency Foldback Current Mode PWM Controller

BLOCK DIAGRAM

UDG-96026-1

DESCRIPTION

The UCC3884 is a high performance current mode PWM controller in-

tended 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 de-

creases the average output current sourced into a low impedance load

which can occur during an output short circuit or overload condition. Ex-

cessive short circuit current is more prevalent in high frequency converters

where the propagation delay and switch turn-off time forces a minimum at-

tainable 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.

application

INFO

available