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 1 mA, and a precision reference trimmed for
accuracy at the error amplifier input. Other internal circuits include logic to ensure 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,
suitable for driving N-channel MOSFETs, is low when it 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 ideal choices for off-line power supplies, while the
devices with a narrower hysteresis range are suited 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.