The NCP1601 is a controller designed for Power Factor Correction (PFC) boost circuits. The device operates in fixed-frequency Discontinuous Conduction Mode (DCM) and variable-frequency Critical Conduction Mode (CRM) and takes advantages from both operating modes. DCM limits the maximum switching frequency. It simplifies the front-ended EMI filter design. CRM limits the maximum currents of the boost stage diode, MOSFET and inductor. It reduces the costs and improves the reliability of the circuit. This device substantially exhibits unity power factor while operating in DCM and CRM. The NCP1601 minimizes the required number of external components. It incorporates high safety protection features that make the NCP1601 suitable for robust and compact PFC stages.