DESCRIPTION
TheUC384xB family ofcontrolICs providesthe necessary features to implement off-line or DC to DC fixed frequency current mode control schemes with a minimal external parts count. Internally implemented circuits include a trimmed oscillator for precise DUTY CYCLE CONTROL under voltage lockoutfeaturingstart-up current less than0.5mA,a precision reference trimmed for accuracy at the error amp input, logicto insure latched operation,a PWM comparatorwhich alsoprovidescurrent limit control, and a totem pole output stage designed to source or sink high peakcurrent. The outputstage, suitable for driving N-Channel MOSFETs, is low in the offstate. Differences between members of this family are the under-voltagelockout thresholds and maximum duty cycle ranges. A range of the zero to < 50 % is obtained by the UC3844B and UC3845B by the addition of an internal toggle flip flopwhich blanks the output off every otherclock cycle. BLOCK DIAGRAM (toggle flip flop used only in UC3844B and UC3845B) Vi 7 34V GROUND 5 UVLO S/R 5V REF INTERNAL BIAS VREF GOOD LOGIC RT/CT 4 OSC ERROR AMP.
STMicroelectronics
