Constant on time control when the switching power supply through a control circuit and a control method and

Discloses a switching power supply controlled by a constant length of the conduction control circuit and its control method. The switching power supply circuit comprises a switch having a first switch transistor, the switching circuit controls the first switch is turned on and off an input voltage into an output voltage. When the switching power supply switching frequency close to the audio range, a first switch switching circuit is turned on, the lowest switching frequency of the switching circuit is limited so as to avoid generating audible noise.