A circuit includes a charge pump, a first level shifter, a second level shifter, a voltage follower and a current mirror. The charge pump is configured to generate a voltage difference between the input node and the output node. The first level shifter is coupled to the charge pump output and configured to apply a first voltage variation to the charge pump output in response to a bias current. The second level shifter is coupled to the input node and configured to apply a second voltage variation to the charge pump input. The voltage follower is configured to equalize outputs from the first and second level shifters and provide a difference current which is multiplied by the current multiplier to generate a charging current applied to the charge pump.