UC3843BNG

The Texas Instruments UC3843BNG is a high-performance fixed-frequency current-mode PWM controller optimized for off-line and DC-DC converter applications. The UC3843B is the industry-standard controller for flyback and forward converter topologies. It features a trimmed oscillator for precise frequency, a temperature-compensated 5V reference with +-1% accuracy, a high-gain error amplifier, current-sense comparator, and a totem-pole output stage capable of +-1A peak current for direct MOSFET drive. The current-mode architecture provides inherent input voltage feedforward, excellent line regulation, and simplified loop compensation compared to voltage-mode control. Pulse-by-pulse current limiting ensures safe operation during overload and short-circuit conditions.

The UC3843BNG operates as a fixed-frequency current-mode PWM controller: 1) Oscillator: An external resistor (RT) and capacitor (CT) set the switching frequency up to 500kHz. The oscillator generates a clock pulse that sets the output flip-flop at the beginning of each switching cycle. 2) Current-Mode Control: The output switch is turned on at the start of each cycle. When the voltage across the current-sense resistor (sensed at the ISENSE pin) reaches the threshold set by the error amplifier output, the comparator resets the flip-flop, turning off the switch. This creates natural input voltage feedforward and simplifies loop compensation to a single-pole response. 3) Error Amplifier: Compares the output voltage feedback with the 5V reference, adjusting the current-sense threshold to maintain regulation. 4) UVLO: The 8.4V start/7.6V stop threshold prevents operation at insufficient supply voltage, ensuring clean start-up.

• Off-line flyback converter for consumer electronics power adapters
• DC-DC boost converter for battery-powered industrial equipment
• Isolated telecom power supply with current-mode control