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In Forced PWM operation, the switching frequency is constant since the power MOSFETs are always switching regardless of the load. If the load is very light, the device’s own power consumption dominates the total system power dissipation, which leads to poor power conversion efficiency. This can be seen in the green signal in the efficiency chart here. PFM operation addresses the Forced PWM operation’s light load efficiency issue by decreasing the amount of switching the device does at light loads. The end result is a higher light load power conversion efficiency as seen in the blue signal in the chart. Lastly, USM is somewhere in the middle between PFM and Forced PWM. As the load decreases, the switching frequency in USM also decreases. However, since the device also needs to ensure that the switching frequency stays above the audible range, it is forced to switch more often than if it were purely in PFM mode. This extra switching required to stay above the audible range at light loads results in a trade-off in efficiency that is lower than PFM’s efficiency but higher than Forced PWM’s efficiency. USM’s power conversion efficiency is shown as the red signal in the chart.
