A DC-DC converter including a Pulse Width Modulation (PWM) controller for converting an input voltage into an output voltage is provided. The PWM controller includes: an error amplifier, receiving a reference voltage and a feedback voltage and provides an error signal; a compensation unit coupled to an output of the error amplifier, compensating the error signal and comprising a first resister and a first capacitor; a ramp generator, generating a ramp signal according to a constant on time PWM signal; a first comparator coupled to the compensation unit and the ramp generator, comparing the compensated error signal with the ramp signal to generate a trigger signal; and a PWM generator coupled to the first comparator, providing the constant on time PWM signal according to the trigger signal, an input voltage of the DC-DC converter and the output voltage of the DC-DC converter.