A control unit for an LED assembly comprising a first and second LED unit, said LED units being serial connected is described. The LED assembly, in use, is powered by a switched mode power supply. The control unit being arranged to receive an input signal representing a desired output characteristic of the LED assembly, determine a first and second duty cycle for respective LED units associated with a nominal current of the switched mode power supply, for providing the desired output characteristic, determine the largest of the first and second duty cycles for respective LED units, determine a reduced current based on at least the largest of the duty cycles, adjust the first and second duty cycle for respective LED units based on the reduced current, provide an output signal for the LED assembly and the switched mode power supply based on the adjusted first and second duty cycles and the reduced current for obtaining the desired characteristic.