Abstract: A power supply monitoring circuit that monitors and delivers the highest voltage power supply to an IC system includes a voltage comparator that receives two different power supply voltages, and outputs a first signal to the gate of a first switching transistor connected between a first power supply and an system power supply output node. The comparator output is also input to an inverter, the output of which comprises a second signal connected to the gate of a second switching transistor connected between a second power supply and the system power supply output node. When the first supply voltage exceeds the second supply voltage, the first transistor is switched on to connect the first supply to the system output node, and the second transistor is switched off; and vice versa. The comparator includes designed-in hysteresis to prevent simultaneous switching of the two transistors.