The STMicroelectronics L4985B is a continuous-conduction-mode (CCM) power-factor-correction (PFC) controller designed for AC-DC power supplies that need to meet harmonic-current limits. It operates at a fixed 130 kHz switching frequency and runs from a 10 V to 24.5 V supply rail, making it a straightforward fit for boost-type PFC stages in offline supplies from a few hundred watts up toward 1 kW and beyond, depending on the external MOSFET and inductor chosen.
Key ratings that drive the design
The 130 kHz switching frequency is a fixed oscillator — no external timing resistor or capacitor, which saves a pin and a couple of passives on the board. At this frequency the inductor is small enough for a compact layout but the switching losses in the MOSFET and diode still need to be budgeted; plan for a fast-recovery diode and a MOSFET with low gate charge. Startup current is 400 µA, which is low enough that a small-value resistor from the rectified DC bus can charge the VCC capacitor without excessive dissipation during startup. Once running, the controller takes its bias from the PFC output or an auxiliary winding. The junction temperature rating goes to 150°C, so this controller can live on the same hot side of the board as the power stage without needing a separate cool island. In a sealed or fanless enclosure that thermal headroom matters.