A fixed-frequency current mode converter comprises a power stage to produce an inductor current and an output voltage, an error amplifier to generate an error signal from the difference between the output voltage and a reference voltage varied with the inductor current, a comparator to compare the error signal with a ramp signal varied with the inductor current to generate a comparison signal, and a PWM generator to generate a PWM signal in response to a fixed-frequency clock and the comparison signal to drive the power stage. A second comparator is further comprised to compare the error signal with a second reference voltage varied with the inductor current, and generates a second comparison signal to reset the clock when the error signal is lower than the second reference voltage.