RobG
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Since your input is greater than the power supply you can't have that diode to 3.3V unless... 1) you put a series resistance before the ESD to limit the current (probably not desirable since you will be injecting current into the substrate every time the vdd diode turns on). 2) you use a voltage divider before the ESD to attenuate the input to 3.3V max. This is probably the easiest if you can tolerate a resistive input. You could then AC couple to an inverter biased mid-rail that will gain the signal back up to rail-to-rail levels when the input is 3.3V. 3) you put a resistor from input to a virtual ground. This would look like an inverting opamp circuit, except you would have a really simple opamp. Again, this would require that the input drive a resistive load.
Otherwise you will have to use some snapback device or an AC coupled clamp or devices based on a vbe multiplier, etc. Sorry, I let someone else do the esd and I can't find any schematics on the web to illustrate what I'm talking about!
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