A switching power supply that uses the intrinsic series resistance of an output bypass capacitor to sense changes in current flow through a switch that is connects between the input and output of the switching power supply. When the switch runs on, current flows from the input to the output and through a bypass capacitor. The intrinsic series resistance of the bypass capacitor develops a voltage across it as current flows through the capacitor. This voltage is used by a sense circuit to help determine when to shut off the switch. A low-cost regulator develops an output voltage that is divided and compared to a reference to determine if the input voltage is sufficient. If it is not, the power supply is not allowed to operate and the switch is not allowed to turn on.