Patent ID: 7133269

Claim:
An overvoltage protection circuit for a receiver, the receiver having an input for detecting the state of a high voltage level input signal using a logic switching threshold based on a low voltage level source voltage and having a maximum voltage limit between the low and high voltage levels, said overvoltage protection circuit comprising: a first pass device having a control terminal receiving the source voltage and a current path for passing the input signal to the input of the receiver up to a first voltage level below the source voltage; a second pass device having a control terminal and having a current path coupled in parallel with said current path of said first pass device; and a protection control circuit, coupled to said control terminal of said second pass device, that controls said second pass device to allow the input signal to rise above said first voltage level up to a threshold voltage below the maximum voltage limit and above the source voltage sufficient to meet the logic switching threshold; wherein said second pass device comprises a first P-channel device having a gate coupled to said protection control circuit, a source for receiving the input signal, and a drain for coupling to the receiver; and wherein the receiver has an output for providing an output signal indicative of the input signal, and wherein said protection control circuit comprises: an N-channel circuit, coupled to said gate of said first P-channel device and receiving the output signal, holds said first P-channel device on while the input signal increases until the input circuit changes the state of the output signal; and a P-channel circuit, coupled to said source and gate of said first P-channel device and receiving the input signal and the source voltage, that turns off said first P-channel device when the input signal reaches said threshold voltage.