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A method for estimating the phase current unbalance in a multiphase buck converter is presented. The method uses the information contained in the voltage drop at the input capacitor's effective series resistance (ESR) to estimate the average current in each phase. Although the absolute estimation of the currents depends on the value of the ESR and is therefore not absolutely accurate, the relative estimates of the currents with respect to one other are shown to be very accurate. The method can be implemented with a low-rate down-sampling A/D converter and is not computationally intensive. Experimental results are presented, showing good agreement between the estimates and the measured values.