3
\$\begingroup\$

I'm trying to protect a GPIO pin on an nRF52840 I'm using a push button input from ESD. The standard we are testing against is IEC 61000-4-2 level 4 which is 6kV contact at 22.5A. The GPIO is using the internal pull-up on the nRF52 which is ranges from 11k to 16k ohms. The absolute maximum input voltage of 3.6V on the GPIO pin with a VDD of 3.3V.

I see it's hard to find ESD TVS that clamp at this voltage due to need to have a working voltage of 3.3V. I've simulated this ESD discharge into the micro pin with the clamping diodes and pull-up in LTSpice without a TVS the input is exposed to ~17V. When I put a 3.3V bidirectional TVS the voltage is clamped to ~5.4V which exceeds the input absolute maximum. My research has led me to believe the best way to lower voltage seen at the input further is to add a series resistor after the TVS.

Am I on the right path? Is there another method I should consider to protect this input?

Thanks

\$\endgroup\$
4
  • \$\begingroup\$ The sentence "I'm using for a push button input from ESD." is unclear. Can you please provide a clarification? \$\endgroup\$ Commented Jan 8 at 19:45
  • \$\begingroup\$ Sorry for the confusion, the microcontroller in my design is an nRF52840. I'm using an input on the nRF52840 connected to an external push button and I need to protect the input from ESD, specifically a 6kV contact discharge. \$\endgroup\$ Commented Jan 9 at 0:30
  • \$\begingroup\$ Are you power constrained? Or must it be a passive only solution? \$\endgroup\$ Commented Jan 9 at 16:12
  • \$\begingroup\$ You might want to consider something like the circuit shown in Well protected digital input with long wires. I've seen circuits like this on low speed inputs of military hardware. The TVS diode (U3) clips the spike, and the series resistor (R7) limits current. The Zener diode (D3) acts as a clamp, and the series resistor (R8) further limits current to the optocoupler. A logic buffer or comparator also works if optical isolation isn't an option. \$\endgroup\$ Commented Jan 9 at 16:28

1 Answer 1

Reset to default
2
\$\begingroup\$ 
      Am I on the right path?

Yes but you need series R's with enough length to add 6kV protection to attenuate before the clamp TVS. Using multiple small SMT's increases the insulation. < 1kV/mm depending on dust levels for DC and rated higher for microseconds of duration. Perhaps a 1210 or a couple 603's in series with a suitable R value will do the trick. Be careful the signal path does not mutually couple into other signals or cause ground shift. Adding input C shunt is another method to attenuate with low ESR C0G cap after the R.

\$\endgroup\$
2
  • \$\begingroup\$ Tony, shouldn't the resistor go after the TVS? \$\endgroup\$ Commented Jan 8 at 21:42
  • \$\begingroup\$ You may put one after but I was thinking of 2 resistors that block 6kV or an RV25 1Meg . How much voltage at 6mA? \$\endgroup\$ Commented Jan 9 at 3:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.