Patent ID: 7608947

Claim:
A back-up power supply system, comprising: a threshold detector circuit; a first switching circuit for enabling access to a back-up power source, the first switching circuit comprising at least one transistor; and an inverting switch coupled between the first switching circuit and the threshold detector circuit, the inverting switch comprising an input and an output, the inverting switch configured to receive a signal at the input, invert the signal, and provide the inverted signal from the output to the first switching circuit, the inverting switch further comprising a delay circuit, the delay circuit configured to provide an immediate high-output during an off-to-on transition at the output of the inverting switch and a switching delay during an on-to-off transition at the output of the inverting switch, the inverting switch comprising a first branch in parallel to a second branch comprising the delay circuit, the first branch and the second branch connected to respective gates of a first and a second transistor, the first branch comprising a first resistor and the delay circuit comprising a second resistor and a capacitor, wherein the threshold detector circuit is configured to cause the first switching circuit to enable access to the back-up power source responsive to a voltage provided by a primary power source dropping below a predetermined threshold.