ESD Protection for SWD interface

[Sikanda]

I have a problem to select the right ESD Diode. Let’s assume I want to choose ESD protection for an SWD interface using TVS diodes. If the microcontroller operates at a voltage of 5V, the V_RM should be at 5V, right? But the V_BR and V_CL are already above the voltage where the internal protection diodes of the microcontroller would activate. How do I choose the right diode?

Here is an example from ST: https://www.st.com/resource/en/application_note/an5612-esd-protection-of-stm32-mcus-and-mpus-stmicroelectronics.pdf Page 25

The ESDALC6V1W is designed for a 3V operating voltage, but even here, the V_BR and V_CL are much higher than the voltages at which the protection diodes activate.

[ataradov]

ESD diodes are not supposed to protect against continuous high voltage, they are for fast transients. The diode just takes the brunt of the spike and the MCU itself will be fine with a short leftover spike.

If you suspect a possibility of continuous high voltage, you will need a better protection method.

[thm_w]

You can read a bit here:
https://toshiba.semicon-storage.com/eu/semiconductor/knowledge/e-learning/basics-of-tvs-diodes/chap3/chap3-3-2.html
https://fedevel.com/forum/other/17085-esd-protection-guides

Essentially what ataradov said.
The internal protection diodes will eventually activate, and eventually charge up the rail caps, etc. but its probably not nearly as fast as the esd diode.

[PGPG]

V_CL are already above the voltage where the internal protection diodes of the microcontroller would activate.

There is nothing wrong in activating internal protection diodes.
What can be wrong is to allowing the current going through these diodes to be higher than value that can lead to latch-up.
I think typical value for this current limit is 20mA (probably can be found in datasheet).
If V_CL is for example 7.7V then (7.7-5.7)/20m = 100Ω between ESD protection and IC pin will limit internal diode current to safe value.

I am not using any protection at PDI interface (ATXmega 2 wire In System programming). I put at PCB 3 holes from 2x2 1.27mm pin-header near the IC. It takes not a lot of area. I shifted holes a little to make pin header (with one pin removed) used to connect during programming not be too loose in these holes.
In production interface is used only once for few seconds and then it is not exposed to ESD. We never noticed any problems with this.
You can see it in use at PCB here:
https://forum.kicad.info/t/approaching-pcb-track-routing-for-a-newbie/36302/8