The UCx84x series of control integrated circuits provide the features necessary to implement AC-DC or DC-to-DC fixed-frequency current-mode control schemes with a minimum number of external components. Protection circuitry includes undervoltage lockout (UVLO) and current limiting. Internally implemented circuits include a start-up current of less than 1 mA, a precision reference trimmed for accuracy at the error amplifier input, logic to ensure latched operation, a pulse-width modulation (PWM) comparator that also provides current-limit control, and a totem-pole output stage 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.
Major differences between members of these series are the UVLO thresholds, acceptable ambient temperature range, and maximum duty-cycle. Typical UVLO thresholds of 16 V (ON) and 10 V (OFF) on the UCx842 and UCx844 devices make them ideally suited to off-line AC-DC applications. The corresponding typical thresholds for the UCx843 and UCx845 devices are 8.4 V (ON) and 7.6 V (OFF), making them ideal for use with regulated input voltages used in DC-DC applications. The UCx842 and UCx843 devices operate to duty cycles approaching 100%. The UCx844 and UCx845 obtain a duty-cycle range of 0% to 50% by the addition of an internal toggle flip-flop, which blanks the output off every other clock cycle.
The UC184x-series devices are characterized for operation from –55°C to 125°C. UC284x-series devices are characterized for operation from −40°C to 85°C. The UC384x devices are characterized for operation from 0°C to 70°C.