A control circuit includes: a comparing circuit, having a first input terminal and second input terminal, configured to operably generate a comparison signal according signals received by the first and second input terminals, wherein the first input terminal is utilized for coupling with a reference signal and the second input terminal is utilized for coupling with a feedback signal; a periodic signal generating circuit configured to operably generate a periodic signal and apply the periodic signal to the first input terminal or the second input terminal of the comparing circuit; and a control signal generating circuit for controlling an on time of a power switch according to the comparison signal. The periodic signal generating circuit clamps a limit of the periodic signal to a predetermined value, but does not configure the slope of the periodic signal to be zero when there is no current passing through the inductor.