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Abstract

A switched noise filter circuit for DC-DC converters which use the instantaneous output voltage to establish the converter's duty ratio. The converter cycles the switching element on and off for time intervals Ton and Toff, respectively. A switching control circuit includes a filter capacitance connected between the feedback node and ground, and a comparator which compares a feedback voltage Vfb with a fixed voltage Vref; at least one of Ton and Toff is a 'modulated' interval which is terminated when Vfb crosses Vref due to the discharge of the filter capacitance. A switched noise filter circuit applies an offset voltage to Vfb during at least one of Ton, and Toff, with the offset voltage disconnected from Vfb by the beginning of the modulated interval or shortly thereafter. When the offset voltage is properly applied, the effect of extraneous electromagnetic noise coupled into Vfb is reduced.

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