A DC-DC power converter comprising: a switch capacitor power converter coupled to a power converter input, a power converter output and comprising at least one switchable capacitor; a controller coupled to the switch capacitor power converter; wherein the controller is configured to generate at least two non-overlapping clock signals (260, 272) and to modulate the non-overlapping time duration of the at least two non-overlapping clock signals (260, 272) dependent on the voltage and/or current value of at least one of the power converter output, a reference value, and the power converter input and further configured to decrease the non-overlapping time duration in response to at least one of an increase in the output current supplied by the DC-DC power converter, an increase in the reference value, and a decrease in the input voltage of the DC-DC power converter characterised in that the controller is coupled to the power converter input, the power converter output and a reference input arranged to receive the reference value, and wherein the switch capacitor power converter is configured to be controlled by the at least two non-overlapping clock signals (260, 272); and wherein the controller comprises an error detector, a non-overlapping clock generator and a control module, wherein the error detector comprises a first input for receiving the reference value, a second input coupled to the power converter output and an output coupled to the non-overlapping clock generator and the control module and wherein the control module is configured to generate a modulation control signal for the non-overlapping clock generator and the non-overlapping clock generator is configured to modulate the clock frequency dependent on the difference between the output voltage and the reference value. a switch capacitor power converter coupled to a power converter input, a power converter output and comprising at least one switchable capacitor; a controller coupled to the switch capacitor power converter; wherein the controller is configured to generate at least two non-overlapping clock signals (260, 272) and to modulate the non-overlapping time duration of the at least two non-overlapping clock signals (260, 272) dependent on the voltage and/or current value of at least one of the power converter output, a reference value, and the power converter input and further configured to decrease the non-overlapping time duration in response to at least one of an increase in the output current supplied by the DC-DC power converter, an increase in the reference value, and a decrease in the input voltage of the DC-DC power converter characterised in that the controller is coupled to the power converter input, the power converter output and a reference input arranged to receive the reference value, and wherein the switch capacitor power converter is configured to be controlled by the at least two non-overlapping clock signals (260, 272); and wherein the controller comprises an error detector, a non-overlapping clock generator and a control module, wherein the error detector comprises a first input for receiving the reference value, a second input coupled to the power converter output and an output coupled to the non-overlapping clock generator and the control module and wherein the control module is configured to generate a modulation control signal for the non-overlapping clock generator and the non-overlapping clock generator is configured to modulate the clock frequency dependent on the difference between the output voltage and the reference value.