An apparatus supplies power to at least one discharge lamp. The apparatus includes a buck converter for regulating power supplied to at least one discharge lamp having a switching element driven by PWM pulses, and a control unit for supplying PWM pulses to the buck converter. The control unit includes a function for controlling an output power of the buck converter so as to be constant using PWM pulses of a first frequency; and a function for controlling an output voltage of the buck converter so as to be maximum using PWM pulses of a second frequency that is lower than the first frequency. The buck converter is capable of operating in discontinuous mode based on the PWM pulses of the second frequency.