This application note introduces the low-cost UC3842 current-mode controller. UC3842 can achieve high performance and low cost.
The document provides a detailed functional description of the UC3842 integrated pulse width modulator (PWM) designed for low-cost current-mode control in power supplies. It explains how the IC achieves good electrical performance and efficiency, particularly for off-line converters driving POWERMOS devices.
Key topics covered include the principles of current-mode control versus conventional schemes, the internal block diagram including the oscillator, error amplifier, and latch, and specific circuit configurations for under-voltage lockout (UVLO) and power sequencing. The note also details oscillator timing calculations, duty cycle limiting methods, synchronization techniques, and compensation strategies for various converter topologies.
The IC operates from a low impedance DC source of 10 V to 30 V.
The UVLO turn-on and turn-off thresholds are fixed internally at 16 V and 10 V respectively.
Start-up current is less than 1 mA for efficient bootstrapping from the rectified input of an off-line converter.