1 Features

• Optimized for off-line and DC-to-DC converters

• Low start-up current (< 1mA)

• Automatic feedforward compensation

• Pulse-by-pulse current limiting

• Enhanced load-response characteristics

• Undervoltage lockout with hysteresis

• Double-pulse suppression

• High-current totem-pole output

• Internally trimmed bandgap reference

• Up to 500kHz operation

• Error amplifier with low output resistance

2 Applications

• Switching regulators of any polarity

• Transformer-coupled DC-DC converters

3 Description

The UCx84x series of control integrated circuits

provide the features that are necessary to implement

off-line or DC-to-DC fixed-frequency current-mode

control schemes, with a minimum number of external

components. The internally implemented circuits

include an undervoltage lockout (UVLO), featuring a

start-up current of less than 1mA, and a precision

reference trimmed for accuracy at the error amplifier

input. Other internal circuits include logic to provide

latched operation, a pulse-width modulation (PWM)

comparator that also provides current-limit control,

and a totem-pole output stage that is designed to

source or sink high-peak current. The output stage,

designed for driving N-channel MOSFETs, is low

when the output stage is in the off state.

The UCx84x family offers a variety of package

options, temperature range options, choice of

maximum duty cycle, and choice of turnon and turnoff

thresholds and hysteresis ranges. Devices with higher

turnon or turnoff hysteresis are good choices for

off-line power supplies, while the devices with a

narrower hysteresis range are designed for DC-DC

applications. The UC184x devices are specified for

operation from –55°C to 125°C, the UC284x series is

specified for operation from –40°C to 85°C, and the

UC384x series is specified for operation from 0°C to

70°C.