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I have designed a small MCU circuit which is to be supplied by a 3.3V DC supply.

Since this is a small subcircuit which will be mounted upon other PCB's, I need to consider ESD (at least on the power supply).

The circuit input power pin is connected to a PI filter, two capacitors and a ferrit bead, mainly to suppress noise, but will this be sufficient to protect against ESD?

I have been looking at different TVS diodes and application notes, but they all seem not to mention power lines, is this because the capacitance on power net is normally enough to shunt any ESD?

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Normally the capacitance alone is enough to protect the power lines. Even a 0.1 uF cap will absorb most of the zap. If you want to make extra sure, use the PI filter. Unfortunately, odds are high that you'll have issues on your other lines long before you have issues with ESD getting on your power. Start by filtering/protecting your MCU's reset line, even if it doesn't go over a cable.

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