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Pulse frequency modulation (PFM) converters are fast in response and simple in design and implementation because compensation is not required. However, the large variation of switching frequency during operation introduces electromagnetic interference (EMI) problems. This article proposes a mixed analog and digital adaptive on-time (AOT) controller for PFM converters so that the switching frequency can be maintained constant adaptively, while the transient response is not degraded. The transient response is handled by an analog controller, which is fast. The switching frequency regulation is implemented by a digital microcontroller.