A charge pump controller for controlling a charge pump adapted to convert an input voltage into an output voltage with a conversion ratio is presented. The charge pump is operable in a plurality of modes corresponding to different conversion ratios. The controller includes a first selector for selecting a mode of operation of the charge pump. The first selector comprises a first input for coupling to a voltage supply; and a second input for coupling to a source signal. The first selector identifies a target value of the output voltage. The selector calculates a product of the conversion ratio and the input voltage. The selector compares the product with the target value and selects a mode of operation of the charge pump by increasing or decreasing the conversion ratio based on the comparison. The selector maintains the conversion ratio for a length of time before decreasing the conversion ratio.