The graphs here show the differences in switching frequency vs. output load using Diodes’ AP62600 synchronous Buck converter as an example. The AP62600 can select its mode of operation to be in PFM, USM, or PWM only (sometimes called Forced PWM). In heavy load conditions, the switching frequency, regardless of operation mode, switches at its set maximum frequency. In PWM Only operation, the device’s switching frequency is constant regardless of the output load. The switching frequency is the same at light load as well as at full load conditions. In PFM or USM, as the load decreases, the average switching frequency also decreases. As mentioned in the previous slide, PFM operation allows the device’s average switching frequency to decrease as low as it needs to go. However, USM will clamp the frequency above the audible range, as shown in the graph on the right.