38021

·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

The 38021 of control ICs provides the
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 lockout featuring start-up current less than 0.5mA, a precision reference trimmed for accuracy at the error amp input, logic to insure latched operation, a PWM comparator which 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 in the offstate.

Differences between members of this family are the under-voltage lockout thresholds and maximum duty cycle ranges. The 38021and UC3844T have UVLO thresholds of 16V (on) and 10V (off), ideally suited to off-line applications The corresponding thresholds for the 38021 and UC3845T are 8.5 V and 7.9 V. The UC3842T and UC3843T can operate to duty cycles approaching 100%. A range of zero to < 50 % is obtained by the UC3844T and UC3845T by the addition of an internal toggle flip flop which blanks the output off every other clock cycle.