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.