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.