Current Mode PWM Controller

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.