Patent ID: 7834481

Claim:
A threshold detection circuit for enabling access to a back-up power source, comprising: a comparator having a first input connection directly connected to a first resistor that is directly connected to a backup voltage provided by a backup power source, said first input connection directly connected to a second resistor, so that said first and second resistors provide a voltage divider network between said backup voltage and said first input connection, a second input connection directly connected to a third resistor directly connected to a primary voltage provided by a primary power source, and an output connection for providing a comparator output voltage; a fourth resistor directly connected to said first input connection and directly coupled to a first capacitor, said first capacitor directly coupled to said comparator output connection; a fifth resistor that is directly connected to said first input connection and said comparator output connection, so that said fifth resistor is coupled in parallel to the series of said fourth resistor and the first capacitor, said fourth resistor, said first capacitor and said fifth resistor forming a feedback network; wherein an output voltage of said threshold detection circuit is low when said primary voltage is greater than a predetermined fraction of said backup voltage.