A technique relates to a digital phase locked loop (DPLL) including a digitally controlled oscillator (DCO), the DCO having delay elements and a current fill factor corresponding to a proportion of the delay elements in operation. A voltage regulator controller is operable to obtain a result of a comparison between a predefined fill factor and the current fill factor, the voltage regulator controller being operable to adjust voltage supplied to the DCO based on the result, the predefined fill factor indicating a predetermined proportion of the delay elements to be in operation.