HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

FEATURES

■ TRIMMED OSCILLATOR DISCHARGE CURRENT

■ CURRENT MODE OPERATION TO 500kHz AUTOMATIC FEED FORWARD COMPENSATION

■ LATCHING PWM FOR CYCLE-BY-CYCLE CURRENT LIMITING

■ INTERNALLY TRIMMED REFERENCE WITH UNDERVOLTAGE LOCKOUT

■ HIGH CURRENT TOTEM POLE OUTPUT UNDERVOLTAGE LOCKOUT WITH HYSTERESIS

■ LOW START-UP CURRENT (< 0.5mA) DOUBLE PULSE SUPPRESSION

Figure 1. Package Table 1. Order Codes

DESCRIPTION

The UC384xA family 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 off-state.

Differences between members of this family are the under-voltage lockout thresholds and maximum duty cycle ranges. The UC3842A and UC3844A have UVLO thresholds of 16V (on) and 10V (off), ideally suited off-line applications The corresponding thresholds for the UC3843A and UC3845A are 8.5 V and 7.9V. The UC3842A and UC3843A can operate to duty cycles approaching 100%. A range of the zero to < 50 % is obtained by the UC3844A and UC3845A by the addition of an internal toggle flip flop which blanks the output off every other clock cycle.

Figure 2. Block Diagram (toggle flip flop used only in UC3844A and UC3845A)

Table 2. Absolute Maximum Ratings

* All voltages are with respect to pin 5, all currents are positive into the specified terminal.

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