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The derivation of a nonlinear output capacitor current programming technique applicable for a buck converter operating in continuous and discontinuous conduction modes is presented. The proposed control method can make the converter system exhibit fast transient response to large-signal disturbances. Its principle of operation is based on using the output capacitor current and instantaneous output voltage to predict the output voltage at the end or middle of a switching cycle to dictate the state of the switch. With the inclusion of the effects of the equivalent series resistance (ESR) of the output capacitor, the system stability, steady-state characteristics, and output sensitivity to circuit component variation will be discussed. The proposed method has been verified by comparing the experimental results of a 25 W, 12V/5V prototype with theoretical predictions.