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UC2845BD Datasheet(PDF) 1 Page - STMicroelectronics |
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UC2845BD Datasheet(HTML) 1 Page - STMicroelectronics |
1 / 15 page ![]() UC2842B/3B/4B/5B UC3842B/3B/4B/5B March 1999 HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER . TRIMMED OSCILLATOR FOR PRECISE FRE- QUENCY CONTROL . OSCILLATOR FREQUENCY GUARANTEED AT 250kHz . 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 AND OPERATING CURRENT DESCRIPTION The UC384xB 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 UC3842B and UC3844B have UVLO thresholds of 16V (on) and 10V (off), ideally suitedoff-lineapplicationsThecorrespondingthresh- oldsforthe UC3843BandUC3845Bare8.5 V and7.9 V. The UC3842B and UC3843B can operate to duty cycles approaching 100%. A range of the zero to < 50 % is obtained by the UC3844B and UC3845B by 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 UC3844B and UC3845B) 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 UC3842B 1/15 |
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