A control circuit controls a switching power factor corrector based on switch off-time modulation by controlling the input electric charge during on-time. The circuit includes a charge current generator that generates charge current as a replica of a current sense signal amplified with a gain corresponding to the square of peak value of a rectified input voltage, a loop capacitor charged with the charge current during on-time intervals and discharged with a discharge current during off-time intervals, a discharge current generator that generates the discharge current proportional to a product of a comparison voltage and a difference between a regulated output voltage and the rectified input voltage, and a PWM modulator that senses a charge voltage of the loop capacitor, turns on the switch for an on-time duration in response to detecting that the charge voltage nullifies, and turns off the switch when the on-time duration has elapsed.