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UC3844A 데이터시트(PDF) 1 Page - STMicroelectronics |
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1 / 15 page UC2842A/3A/4A/5A UC3842A/3A/4A/5A March 1999 HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER . TRIMMED OSCILLATOR DISCHARGE CUR- RENT . CURRENT MODE OPERATION TO 500kHz . AUTOMATIC FEED FORWARD COMPENSA- TION . LATCHING PWM FOR CYCLE-BY-CYCLE CURRENT LIMITING . INTERNALLY TRIMMED REFERENCE WITH UNDERVOLTAGE LOCKOUT . HIGH CURRENT TOTEM POLE OUTPUT . UNDERVOLTAGE LOCKOUT WITH HYSTER- ESIS . LOW START-UP CURRENT (< 0.5mA) . DOUBLE PULSE SUPPRESSION DESCRIPTION The UC384xA family of control ICs provides the nec- essary features to implement off-line or DC to DC fixed frequency current mode control schemes with a minimal external parts count. Internally imple- mented circuits include a trimmed oscillator for pre- cise DUTY CYCLE CONTROL under voltage lock- outfeaturingstart-up current less than0.5mA,a pre- cision reference trimmed for accuracy at the error amp input, logic to insure latched operation, a PWM comparatorwhich alsoprovidescurrent limit control, and a totem pole output stage designed to source or sink high peakcurrent. The output stage, suitable for driving N-Channel MOSFETs, is low in the off- state. Differences between members of this family are the under-voltagelockout thresholds and maximum duty cycle ranges. The UC3842A and UC3844A have UVLO thresholds of 16V (on) and 10V (off), ideally suitedoff-lineapplicationsThecorrespondingthresh- olds for the UC3843A and UC3845A are 8.5 V and 7.9V. The UC3842A and UC3843A can operate to dutycycles approaching100%.A range of the zeroto < 50 % is obtainedby theUC3844Aand UC3845Aby the addition of an internal toggle flip flop which blanks the output off every other clock cycle. BLOCK DIAGRAM (toggle flip flop used only in UC3844A and UC3845A) UVLO S/R 5V REF 34V INTERNAL BIAS VREF GOOD LOGIC 2.50V T S R OSC R1V CURRENT SENSE COMPARATOR 2R + - PWM LATCH 7 5 4 2 1 3 8 6 ERROR AMP. Vi GROUND RT/CT VFB COMP CURRENT SENSE VREF 5V 50mA OUTPUT D95IN331 Mini dip ® SO8 UC3842A 1/15 |
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