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This paper describes a possible control method for the operation of a variable switching frequency buck converter. Starting from the ideal model for a Pulse Frequency Modulation (PFM) mode dc-dc converter , a simple control system, involving only two comparators and a latch is presented here. A mathematical model for this frequency modulated control system is developed and demonstrated. To prove the theoretical behavior of this model, an alternative to the digital modulator, proposed by F.C. Lee , is introduced in order to open the loop and to evaluate the gain and the phase of the system. Results confirm a perfect agreement between the mathematical model and simulations at low frequency. Finally the stability of the converter circuit is demonstrated by forcing a load current step and analyzing the response of the control circuit.