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Consider a circuit like this...

schematic

simulate this circuit – Schematic created using CircuitLab

What happens to the voltage seen by the MCU between VCC and GND when this circuit takes a positive ESD hit to the bottom terminal of SW1?

The circuit ground is not connected to earth ground. L1 is parasitic inductance at the very high dv/dt that results from the ESD strike.

  • Does the MCU blackout?
  • What is the best defense for this failure mode?
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  • \$\begingroup\$ The answer depends a lot on the actual physical arrangement of the wiring, and also the parasitic capacitance between various parts of the circuit and surrounding objects. \$\endgroup\$ – The Photon Feb 4 '19 at 5:40
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What happens to the voltage seen by the MCU between VCC and GND when this circuit takes a positive ESD hit to the bottom terminal of SW1?

Not a really good way to tell without knowing more of the physical construction of the circuit, but it looks something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Air is in the giga-ohms+ range depending on the humidity and has a few pF's of capacitance to fF's depending on how far the metal conductors are apart. Because the capacitance matters a lot the air and the physical construction will determine the return current path.

My intuition says: odds are having the diode there will sink the current to the ground plane which will be more inclined to take the current in most cases. But if not, you just burned out your MCU port.

A better way would be to have some kind of chassis ground and a shielded enclosure.

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In complement of laptop2d answer, the usual way to protect is using TVS diodes connected to plane ground. Sometimes RC circuit, with both resistor and capacitor rated for ESD is added between the ESD port and plane ground, right after the TVS.

An old school method is to have a big capacitor (>480uF) with rated voltage connecting the PCB to mechanical grounded metal enclosure. In harsh ESD enviroment or high speed signal paths, gas discharge tubes (GDT) are preferred rather than TVS. (They do exist in SMD format).

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