A passive and hardware efficient output capacitor time-constant estimator for digitally controlled dc-dc converters is introduced. The estimator emulates the equivalent RC circuit of the output capacitor with a much smaller version, placed in parallel, and adjusts its own resistance until the two circuits have the same time-constant. The adjustment is based on a very simple zero voltage crossing detection and synchronization with the digital pulse-width modulator operation.The effectiveness of the proposed estimator is illustrated using a 5V-to-1V/5A, 500kHz buck converter demonstrating accuracy within a few tens of ns in the detection of capacitor zero current crossing points.