MCU pins ESD protection - Page 1
Forum section: electronics / beginners

author: Miha
author_rank: Contributor
timestamp: July 17, 2023, 11:50:32 am
content: Hello.I need to provide the ISO 10605 (8kV Contact / 15kV Air Gap) protection for a MCU pins. Usual pins protection for MCU ADC inputs included 100k/10k resistive devider and 0.1uF ceramic capacitor and 100k/100k and 100pF for discrete inputs.For reach the ISO10605 level I suggest to add a extra ESD diode, like ESD5Z3.3T1G (IEC61000−4−2 Level 4) and some resistive current limiters, see pictures. Is this protection good enough?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 17, 2023, 12:35:29 pm
content: I had something very similar (R-C-R, with TVS in parallel with middle C), but much smaller series resistor values, like 100R or so, pass EN 50491-5-2; EN 301 489-17 (4kV contact, 8kV air gap). It would have passed with A criteria (said protected inputs were converting ADC values, with some filtration, and the thing remained functional) even though just B was required.

author: Miha
author_rank: Contributor
timestamp: July 17, 2023, 01:27:17 pm
content: Thanks. I need just "C".

author: Gyro
author_rank: Super Contributor
timestamp: July 17, 2023, 09:34:34 pm
content: Be careful of the voltage rating of the input series 100k resistor, if this flashes over (which it is almost certain to do), you will inject a significant current spike into the MCU ground reference, potentially causing a crash. You might want to consider spitting it into a resistor chain (probably only practical for a few inputs). Certainly the ground returns of everything to the left of the series 10k resistor need to be very closely bonded to the case metalwork.

author: floobydust
author_rank: Super Contributor
timestamp: July 18, 2023, 12:14:30 am
content: I've found R-C is a guaranteed fail when it comes to ESD. At best it arcs across the resistor's element/pads and instead usually arcs to an adjacent PCB trace or connector pin on that run adhering to Murphy's Law.I use a 2-stage approach- first an input capacitor 4.7-10nF which provides capacitive voltage-divider action and greatly lowers the impulse voltage. See graph.Stage 2 can be the R-C (C-R-C) or TVS etc. depending on the MCU's pin injection current rating.Regardless of your approach, a discharge path is expected to be a low impedance ground of else things will crash and reboot or worse.

author: Miha
author_rank: Contributor
timestamp: July 18, 2023, 12:44:53 pm
content: Thanks ALL.I found a good doc for the theme:https://www.st.com/resource/en/application_note/an5612-esd-protection-of-stm32-mcus-and-mpus-stmicroelectronics.pdf

author: mvs
author_rank: Frequent Contributor
timestamp: July 18, 2023, 12:59:37 pm
content: Quote from: Miha on July 17, 2023, 01:27:17 pmYes, but RC is not that expensive and usually you want to have also a basic short circuit protection (low voltage, low impedance) on board external inputs in automotive applications.Quote from: Miha on July 17, 2023, 11:50:32 am9K input impedance is quite high, it may already affect performance/precision of some SAR ADC's.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2023, 01:18:59 pm
content: Some relevant information:https://www.ti.com/lit/an/slva954b/slva954b.pdf?ts=1689633443439Yeah, something to shunt the input may be a good idea. At higher voltages, consider MOVs: they're available in small SMT chips. Small zeners above, oh, 30V or so, don't have enough capacity to withstand typical ESD levels, and you'll need more like an SMA size TVS to handle it. But even if you have something to knock it down just to some kV (in the case you can't tolerate a lower-voltage zener or MOV, say for withstanding overvoltage for surge or cross-wiring-proof purposes), that's a big help. In which case, you'll definitely need more than a chip resistor there, but a small THT resistor rated for surge and/or high voltages will fit the bill. (Generic chip resistors blow out above a few hundred volts. You usually see three or four in series across mains (1.5-2.5kV surge), for example.)A capacitor in front, helps with ESD as said, but note that, although it helps to filter high-impedance sources, it does little for low-impedance sources: you still need the cap(s) after the divider. It feels a bit redundant. It's also prone to resonate: the low impedance provides a return path for reflections and the wire/cable can resonate, or the cap resonates with cable inductance. A ferrite bead in front might be used to dampen this (and provide a little RF filtering).The series resistor on the MCU side is beneficial for reducing peak current into the MCU, while the TVS is at peak clamping voltage, but there is still some rationale to using little or none of it. Typically, logic/IO pins have some internal resistance already. It's not good to depend upon this, of course, but it often turns out that device survival greatly increases with an external TVS directly in parallel with the pin -- even if the pin itself still experiences significant current during the event. Think of it as knocking down 15kV ESD to more like 1.5kV (if the current ratio is 10:1 TVS:pin), and devices are typically rated for 1 or 2kV so it's likely to surve.The series resistor is of course preferable, and just 10s of ohms can make the TVS's clamping of say 20V look like an equivalent ESD strike of a few hundred volts -- still enough that internal state may be upset, or CMOS latchup triggered, but well within ESD ratings.So, if you really need to save the components / layout area, kind of thing, and can tolerate a less confident design (maybe in testing, you find it's not enough and need to re-spin with the series resistor, and there's enough time to spare on the project to accommodate that), that sort of thing.Tim