In a digital boost or buck-boost converter, a pulse width modulation signal has an on-time and an off-time. A constant off-time period is provided to set the off-time to be constant, and an on-time period to determine the on-time is derived by monitoring the output voltage of the converter. With the constant off-time, the output voltage and the on-time period will have a linear relation, thereby reducing the output ripple when the converter operates with a high duty-ratio.