Multi-topology current-mode PWM controller in 16-DIP
The UC3856N is a current-mode PWM controller from Texas Instruments that handles step-up, step-down, and step-up/step-down topologies including buck, boost, flyback, forward converter, full-bridge, half-bridge, and push-pull — giving a single BOM line flexibility across isolated and non-isolated power stages. It switches at up to 200 kHz with a maximum duty cycle of 47%, so the transformer reset time in a forward converter or the off-time in a boost stage is fixed at just over half the period — plan the magnetics around a 53% minimum off-time.
Supply range, temperature grade, and control features
The supply range spans 7 V to 40 V on Vcc/Vdd, which covers standard 12 V and 24 V industrial rails with headroom for startup transients — the undervoltage lockout thresholds are set internally for that band. Control features include enable, frequency control, and soft start — the soft-start pin lets you ramp the duty cycle on power-up to avoid output overshoot and inrush current into the load cap. Synchronous rectifier capability is built in, meaning the controller can drive SR FETs on the secondary side of an isolated converter for higher efficiency at low output voltages.