A retrofit LED driver is for connecting to a high frequency ballast and for driving a LED load. A switch arrangement is used to couple the driver output power to the LED load or isolate the output power from the LED load. Voltage regulation is used when the output power is isolated from the LED load, and current regulation is used when the output power is coupled to the LED load. The voltage regulation is used so as to increase an output impedance as seen from the high frequency ballast by introducing an additional impedance of non-LED light source, when the output power coupled to the LED light source is less than a threshold, for example in deep dimming mode or standby mode instructed by the user. In this way, the effective impedance of the lamp is increased, which enables switching noise to be reduced, meanwhile the overall LED output power is not increased, as desired by the user.