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 and to modulate the non-overlapping time duration of the at least two non-overlapping clock signals 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 ; 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 and to modulate the non-overlapping time duration of the at least two non-overlapping clock signals 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 ; 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. <EOS>