The paper describes a DC-DC converter for use with low voltage microprocessor loads. The control method is a hysteretic current-mode control in the continuous conduction mode which has fast transient response. At light loads, the converter operates in the discontinuous conduction mode using a peak current control method which causes the switching frequency to be proportional to load current, thus maintaining high e ciency in a very wide range of loads. The control method implementation, transient response and output inductor design equations, and equations for designing an input lter to reduce input current di/dt are provided. An inductor current estimator which provides higher e ciency, good transient response, and current limiting, is presented. Experimental results for a 5.0V input, 3.1V output, 13A DC-DC converter are included to verify the theoretical information.