A method and apparatus for digitally controlling the average input current of a switch mode power supply are disclosed. In preferred embodiments, the switch mode power supplies are DC-DC converters. One aspect of the invention relates to a method for controlling the switch of a switch mode power supply by digitally sampling the input current once during each power supply switching cycle at a time that is synchronized with the switching cycle. The predetermined time at which the input current is sampled may be midway through the time that the switch is in the "ON" state. This midway time may also be delayed slightly to compensate for delays associated with the switch.