In one embodiment, a method of controlling a switching power supply can include: (i) generating an ideal on time signal according to an input voltage and an output voltage of the switching power supply; (ii) generating a ripple voltage signal having a predetermined constant value when a power switch of the switching power supply is on, and a linearly decreasing value when the power switch is off; (iii) generating a regulating voltage signal according to an output voltage sense signal and the ripple voltage signal; (iv) generating a regulating control signal by comparing the regulating voltage signal against a first reference signal; (v) generating an on time control signal according to the regulating control signal and the ideal on time signal; and (vi) generating a driving signal according to the on time control signal for driving the power switch.