DC/DC Converter Application Note

The general PWM control DC/DC converters may cause ringing in the Lx voltage because there is a zero current period to prevent a reverse coil current in low load condition. However the forced PWM control DC/DC converters do not cause ringing by forcing them into PWM control in low load condition.
Although the reverse coil current is present, the reverse current is not wasted because it flows to the output in the next cycle.

Ricoh forced PWM control DC/DC converters are able to switch automatically between PWM and VFM mode. If one prefers that the supply current and efficiency are most important, please select automatic PWM/VFM switching mode. In that case the DC/DC converter operates in VFM mode during low load conditions. If one prefers that no ringing in low load condition should appear and operation with a constant frequency, please select fixed forced PWM control mode.