Patent ID: 7298181

Claim:
A power supply monitoring circuit for monitoring and delivering the highest voltage power supply to an IC system including: voltage comparator means having inputs connected to two different power supplies and generating a first signal fed to the gate of a first switching transistor connected between a first of said power supplies and an system power supply output node; inverter means connected to said first signal and outputting a second signal fed to the gate of a second switching transistor connected between a second of said power supplies and said system power supply output node; said voltage comparator means switching said first signal to a first state when said first supply voltage exceeds said second supply voltage to turn on said first transistor and connect the first supply to the system output node, and said second signal turns off the second transistor; said voltage comparator means switching said first signal to a second state when said second supply voltage exceeds said first supply voltage to turn off said first transistor and disconnect the first supply from the system output node, and said second signal turns on the second transistor to connect the second supply to the system output node; further including hysteresis means for delaying the switching of said first signal by said voltage comparator means; said voltage comparator means including a first pair of transistors having their source-drain circuits connected in series between said first power supply and ground; said voltage comparator means further including a second pair of transistors having their source-drain circuits connected between said second power supply and ground.