A digital constant on-time controller adaptable to a direct-current (DC)-to-DC converter includes a current sensing circuit that senses stored energy of the DC-to-DC converter, thereby generating a sense voltage; a semi-amplitude detector that detects half of a peak-to-peak amplitude of the sense voltage, thereby generating a semi-amplitude voltage; a DC voltage detector that detects a DC voltage across an effective series resistor of an energy storage circuit that provides the stored energy of the DC-to-DC converter, thereby generating a DC voltage; an arithmetic device that adds the sense voltage and the semi-amplitude voltage, from which the DC voltage and a predetermined reference signal are subtracted; and a pulse-width modulation (PWM) generator that generates a switch control signal according to a result of the arithmetic device.