The NCP1901 is a combination of PFC and half-bridge resonant controllers optimized for off-line adapter applications. This device includes all the features needed to implement a highly efficient and small form factor adapter. It integrates a critical conduction mode (CrM) power factor correction (PFC) controller and a half-bridge controller with a built-in 600 V driver. The half-bridge stage operates at a fixed frequency. Regulation is achieved by adjusting the PFC stage output voltage. This device includes an enable input on the PFC feedback pin, open feedback loop protection and PFC overvoltage and undervoltage detectors. Other features included in the NCP1901 are a 600 V startup circuit and an adjustable frequency oscillator. The controllers are properly sequenced, simplifying system design.