ESD protection for ESP32 GPIOs

Question

I want some of the ESP32's GPIOs to be accessible via 3.5mm headphone jacks or RJ11 jacks and therefore need suitable ESD protection. I wonder how much is needed for optimal protection.

Conditions/constaints:

• The board is powered by a 12 V wall plug
• The voltage is brought to 3.3 V via a step down converter based on the LM2575
• 2 Layer PCB. The bottom layer is GND
• I want to protect against ESD strikes described in IEC 61000-4-2 (8kV contact, 16kV air-gapped)
• IOs should
  • be working as inputs and outputs
  • be working with a max. frequency of around 250 Hz
  • have an output voltage/current enough to drive a bipolar transistor or MOSFET.

My initial idea

Roughly based on this digikey article and other resources found on the Internet I came up with these circuits.

However, I'm still confused by all the info floating around. Let me try to explain my thinking:

Assuming that I have a TVS diode with a clamp voltage between 8 and 16 V. E.g. the T3V3S5

In the event of an ESD strike, I therefore have to assume that the ESP32 is exposed to a voltage between around 8 and 16 V. Through the 1k resistor I want to limit the current so that the pulse for the internal diode remains manageable. Worst-case the internal circuit would have something around 16V-3.3V = 12.7V and (16V-3.3V)/1000Ohm = 12.7mA to cope with, correct? Is that something the ESP32 would survive?

Can I assume that the circuit on the left is already sufficient to protect the ESP? In this case of course the ESP32's internal protection circuit is used. Or should I rather throw external diodes to it like in the example on the right?

Unfortunately I cannot find any documentation on the internal protective measures of the ESP32 GPIOs. If you have some info please let me know.

Questions

1. Do the circuits make sense? Is one of them suitable to protect the ESP32?
2. Will the step-down regulator be able to cope with the heightened power rail or do I need some more circuitry there?
3. Should I rather use something like this to avoid latching up the PSU rail?
4. What if the board is not powered?

Answer 1

For an input, the TVS is fine for ESD protection, place it close to the input. Most semiconductor have a internal ESD protection with a limit of 10mA. Therefore a suitable series resistor is required to limit the surge current. 1kohm is not sufficient, I would suggest to go with 4.7kohm. The Schottky diode are not necessary for input.

In the case where you would need a low impedance, such as 100 ohm, then the Schottky are required.

Answer 2

I am dealing with a similar problem in a project where I want ESD protection for the UART RX and TX connectors.

Since the esp32 max High-level input voltage is VDD+0.3 i.e. 3.9V, I don't think that "using TVS diode with a reverse standoff voltage of 3.3 volts" would work that well, because in a ESD event the TVS clamping voltage, which can be way higher than 3.9V, would be the one applied to the GPIOs. I think that a TVS with clamping voltage lower than 3.9V is the right choice.

https://assets.nexperia.com/documents/data-sheet/PESD2V0Y1BSF.pdf

This one, for example, its Reverse Standoff is 2V so up to this it won't conduct. Its breakdown voltage is between 3v3 and 3v6 so then it starts to conduct and it clamps at 3V. So for my application I think it suits perfectly and maybe it could help others. Its peak current is 6A so I've put a resistor and a PTC of 120mA which will limit the current flowing through the circuit so it never reaches the peak current of the TVS.

Answer 3

I would instead just use a TVS diode with a reverse standoff voltage of 3.3 volts.

https://www.diodes.com/assets/Datasheets/DESD3V3E1BL.pdf