UC3845BN

The UC384xB 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 0.5 mA, 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 UC3842B and UC3844B have UVLO thresholds of 16V (on) and 10V (off), ideally suited off-line applications. The corresponding thresholds for the UC3843B and UC3845B are 8.5V and 7.9V. 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.

Key features

• TRIMMED OSCILLATOR FOR PRECISE FREQUENCY CONTROL
• OSCILLATOR FREQUENCY GUARANTEED AT 250kHz
• CURRENT MODE OPERATION TO 500KHz
• AUTOMATIC FEEDFORWARD 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 AND OPERATING CURRENT