During the light load condition, the MPQ4559 runs at pulse skipping mode to improve the light load efficiency. The frequency is reduced in order to reduce the switching loss. The pulse skipping mode threshold is related to the inductance. If higher inductance is used, the inductor current will rise and fall in a slower rate, so the threshold for the device going to pulse skipping mode is lower. You can try to use a higher inductance to lower the current threshold of MPQ4559 entering pulse skipping mode.
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