Voltage regulator

A voltage regulator converts a voltage on a source rail 104 to a respective output voltage on each of plural output voltage rails 106 109. An output rail is selected, its output voltage is compared to a reference voltage for that rail and increased if it is less than the reference voltage. The frequency at which an output voltage rail is selected depends upon the rate at which the regulator previously increased its output voltage and may increase if the previous rate of increase increases or decrease if the previous rate of increase decreases. Further rails may be selected to form a sequence of selection in which voltage rails having a larger previous increase rate are earlier in the sequence. The frequency of selection may be increased or decreased depending on whether the rate at which a comparing step leads to a controlling step exceeds a threshold. The controller may operate in buck or buck-boost mode depending on the level of the source voltage, controlling a switch to apply current to the selected rail.