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The UC284xB family of control chips provides the necessary functionality to implement off-line or DC-to-DC fixed-frequency current-mode control schemes with minimal external parts count. Internally implemented circuits include: a trimmed oscillator for precise duty cycle control under voltage lock with less than 0.5mA start-up current; a precision reference accurately calibrated at the error amplifier input; logic to ensure latch operation; also provides current A limit-controlled PWM comparator; and a totem pole output stage designed to source or sink high peak currents. Low output stage, suitable for driving N-channel MOSFETs.

The differences between members of this family are the low voltage lockout threshold and maximum duty cycle range. The UC2842B and UC2844B have UVLO thresholds of 16V(ON) and 10V(OFF), ideal for offline applications, and the corresponding UC2843B and UC2845B thresholds are 8.5V, respectively and 7.9V. The duty cycle of the UC2842B and UC2843B is close to 100%. The UC2844B and UC2845B achieve a zero to <50% range by adding an internal toggle flip-flop that blanks the output every other clock cycle.

key features

Low start-up and low run current

Hysteretic undervoltage lockout

Inscribed reference with undervoltage lockout

Limiting

Fine-tuning oscillators for precision frequency control

Current Mode Operation to 500 kHz

Oscillator frequency guaranteed at 250 kHz

PLL PWM

High current totem pole output

Automatic feedforward compensation