This paper provides a simple analytical model for the closed-loop output impedance of a direct-duty-ratio-controlled buck converter, which can be used to predict the behavior of the output voltage during a load-current transient. The modeling method utilizes standard control theory to obtain the model for the sensitivity function based on the crossover frequency and phase margin of the output-voltage feedback loop as well as on a clever estimate for the corresponding open-loop output impedance. The modeling method is validated by means of simulations and experimental tests.