In a switched mode power supply (SMPS) then selectively operates in
continuous current mode (CCM) and discontinuous current mode (DCM), the
minimum switching frequency of the SMPS is adjusted by setting a minimum
duration for DCM operation. A resistance value applied to an external pin
of the SMPS controls the duration of a timer that is reset in CCM and
activated upon entering DCM. Upon expiration of the variable duration,
the SMPS reverts to CCM for at least one switching cycle. This allows the
SMPS minimum effective switching frequency to be set, for each
application in which the SMPS is deployed, at a level that avoids audible
noise (but which may be lower than an ultrasonic frequency), thus taking
maximum advantage of the efficiencies of DCM operation under light load
conditions.