DESCRIPTION:

TheUC384xBfamilyofcontrolICsprovidesthenecessary features to implement off-line or DC to DCfixed frequencycurrent mode control schemes witha minimal external parts count. Internally implemented circuits includea trimmed oscillator for precise DUTY CYCLE CONTROL under voltage lockoutfeaturingstart-upcurrentless than0.5mA,a precision reference trimmed for accuracy at the erroramp input,logicto insurelatchedoperation,a PWM comparatorwhich alsoprovidescurrentlimit control,and a totem pole output stage designed to sourceor sink high peakcurrent.Theoutputstage,suitablefor driving N-Channel MOSFETs, is low in the offstate.Differences between members of this family are theunder-voltagelockout thresholdsand maximum dutycycle ranges. The UC3842B and UC3844B haveUVLO thresholds of 16V (on) and 10V (off), ideallysuitedoff-lineapplicationsThecorrespondingthresholdsfortheUC3843BandUC3845Bare8.5Vand7.9V. The UC3842B and UC3843B can operateto dutycycles approaching 100%. A range of the zero to <50 % is obtainedby the UC3844Band UC3845Bbytheadditionofan internaltoggleflip flopwhich blanksthe outputoff everyotherclock cycle.

Features：

. TRIMMED OSCILLATOR FOR PRECISE FREQUENCY CONTROL

. OSCILLATOR FREQUENCY GUARANTEEDAT 250kHz

. CURRENT MODE OPERATION TO 500kHz

. AUTOMATIC FEED FORWARD COMPENSATION

. LATCHING PWM FOR CYCLE-BY-CYCLECURRENT LIMITING

.INTERNALLY TRIMMED REFERENCE WITHUNDERVOLTAGE LOCKOUT

. HIGH CURRENT TOTEM POLE OUTPUT

. UNDERVOLTAGE LOCKOUT WITH HYSTERESIS

. LOW START-UP AND OPERATING CURRENT