A method of operating a voltage regulator including a source voltage rail and a plurality of output voltage rails, the method including: converting a source voltage on the source voltage rail to a respective output voltage on each output voltage rail; selecting an output voltage rail; comparing the output voltage on the selected output voltage rail to a reference voltage for the selected output voltage rail; and if the output voltage of the selected output voltage rail is less than the reference voltage for the selected output voltage rail, controlling the voltage regulator to increase the output voltage on the selected output voltage rail, wherein the frequency at which an output voltage rail is selected is dependent upon the rate at which the voltage regulator has previously increased the output voltage on that output voltage rail.