A method of generating a pumping voltage in an integrated circuit includes receiving an external clock signal from outside of the integrated circuit. The frequency of the received external clock signal is changed according to one or more modulation ratios, resulting in one or more respective modulated external clock signal. The external clock signal or one of the modulated external clock signals is then selected for use as a pump clock signal. The pump clock signal is used for driving the pump capacitance of a pump circuit for generating the pumping voltage.