This paper presents a high accuracy and high bandwidth current sense circuit for digitally controlled DC-DC buck converters. The circuit uses lossless inductor DCR current sensing and a sigma delta modulator ADC to sense the average current in the inductor to high accuracy. These accurate but low bandwidth measurements are unsuitable for use in feedback control loops as their acquisition delay would limit the achievable loop bandwidth. To solve this issue a Kalman Filter estimates the inductor current at the switching frequency of the converter. The output of the estimator is shown through experimental results to achieve 0.3A static current accuracy when used in a buck converter switching at 1MHz and using a power efficient inductor with a DCR of only 165 mu Ohms.