The 6-pin PFC controller NCP1622 is designed to drive PFC boost stages. It is based on an innovative Valley Synchronized Frequency Fold-back (VSFF) method. In this mode, the circuit classically operates in Critical conduction Mode (CrM) when control voltage exceeds a programmable value Vctrl,FF. When Vcontrol is below this preset level Vctrl,FF, the NCP1622 (versions [B**] and [D**]) linearly decays the frequency down to about 30 kHz until Vcontrol reaches the SKIP mode threshold. VSFF maximizes the efficiency at both nominal and light load. In particular, the stand-by losses are reduced to a minimum. Like in FCCrM controllers, internal circuitry allows near-unity power factor even when the switching frequency is reduced. Housed in a TSOP6 package, the circuit also incorporates the features necessary for robust and compact PFC stages, with few external components.