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A new multifrequency small-signal control model for high-bandwidth dc buck regulator is proposed. The describing function approach is taken to model the duty-cycle modulator, incorporating the effects of not only the modulating frequency , the switching frequency , and the two sideband frequencies and . This model is significantly more accurate than the conventional average model and the recently proposed two-frequency model for applications with high control bandwidth-to-switching frequency ratio and high input-to-output voltage step down ratio, such as the v-core voltage regulators. The model is verified by simulations and experiments.