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A sensorless multi-phase average current-program mode controller with an accurate self-tuning current estimator is introduced. Based on the values of the duty ratio, and the input and output voltages, the estimator calculates the average inductor current of each phase in a multi-phase dc-dc converter. The averaged values are calculated over one switching cycle. The estimator employs a phase-by-phase self calibration scheme. During the calibration, regulators of all currents but one are "frozen" and a small load step is intentionally introduced, by a test current sink. Based on the response, the parameters of the active phase are automatically adjusted such that accurate estimation and equal current sharing are achieved. Experiments with a 12 V-to-1.5 V, 60 W, 500 kHz, two-phase buck converter verify that the controller ensures fast estimation and, at the full load, a current sharing error of less than 5%.