Drive circuit for driving a load with constant current

abA drive circuit for driving a load comprises: a switched mode power supply for supplying at the output a switched output current (IL); a controller for controlling the power supply; a current sensor for generating a current sense signal (Vi ) representing the output current (IL); a voltage sensor for generating a voltage sense signal (Sy) representing the output voltage (Vp; Vp+Vis) of the circuit. The controller receives the current sense signal, and generates a switching time control signal (Sc) for the switched mode power supply on the basis of the current sense signal. The controller further receives the voltage sense signal. In response to a change in the voltage sense signal, the controller changes the switching time control signal such as to effectively compensate an effect of the output voltage change on the average value of the output current.