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UC2845B：Current Mode PWM

The UC284xB 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 UC2842B and UC2844B have UVLO thresholds of 16V (on) and 10V (off), ideally suited off-line applications The corresponding thresholds for the UC2843B and UC2845B are 8.5V and 7.9V. The UC2842B and UC2843B can operate to duty cycles approaching 100%. A range of the zero to < 50 % is obtained by the UC2844B and UC2845B by the addition of an internal toggle flip flop which blanks the output off every other clock cycle.

Key Features

• LOW START-UP AND OPERATING CURRENT
• UNDERVOLTAGE LOCKOUT WITH HYSTERESIS
• INTERNALLY TRIMMED REFERENCE WITH UNDERVOLTAGE LOCKOUT
• CURRENT LIMITING
• TRIMMED OSCILLATOR FOR PRECISE FREQUENCY CONTROL
• CURRENT MODE OPERATION TO 500kHz
• OSCILLATOR FREQUENCY GUARANTEED AT 250kHz
• LATCHING PWM FOR CYCLE-BY-CYCLE
• HIGH CURRENT TOTEM POLE OUTPUT
• AUTOMATIC FEED FORWARD COMPENSATION

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