An example method of controlling a power supply to have a constant current output includes receiving an input current sense signal, an input voltage sense signal, and an output voltage sense signal. A control signal is then generated to control switching of a switch of the power supply to regulate an output current of the power supply. The generating of the control signal includes integrating the input current sense signal during a switching period of the control signal to generate an integrated signal representative of a charge taken from an input voltage source of the power supply. Generating the control signal also includes controlling the switching of the switch such that the integrated signal is proportional to a ratio of the output voltage sense signal to the input voltage sense signal.