A circuit for correcting a power factor for an AC direct lighting apparatus that includes a valley signal generating unit configured to receive a full-wave rectified AC input voltage signal and configured to compare an internal reference voltage signal and the AC input voltage signal to generate a valley signal, a reference voltage control unit configured to receive the generated valley signal and count clock cycles of an internal clock to detect a frequency of the AC input voltage signal and configured to determine a frequency of a drive current to control a reference voltage signal based on the determined frequency of the drive current and a reference voltage control clock generating unit configured to generate a pulse width modulation signal associated with a pulse width of the reference voltage signal and configured to generate the reference voltage control clock signal based on the generated pulse width modulation signal and the controlled reference signal. Thus, the circuit may control a phase of a drive current and adjust a variation of the reference voltage signal to improve the power factor of the circuit.