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It's usual in my electronics design to have external connectors where GPIO signals are routed. This GPIO lines are usually 3,3V and goes directly to the MCU in some cases. In this scenario, I put a TVS in each line to protect it again human body ESD. (IEC 61000-4-2). I i'm not wrong, the discharge model is something like this: enter image description here

So, the 8kV are applied over a 330Ohms resistor to my Dut (this means, my GPIO port directly). If I'm not wrong, the parameters in this case to choose the TVS should be: Vrwm > 3,3V Vclamp < absolute max. voltage for GPIO pin Ipp > 8000/330 = 24,24A

Am I right? or am I missing something? So, in this case, a TVS like PESD3V3Y1BSF should work (but I could not find the Ipp in the datasheet)

Thanks to all for the help!

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    \$\begingroup\$ Always pick a TVS with voltage margin above the expected nominal voltage. You don't want it to conduct during normal conditions, only during transients. \$\endgroup\$
    – Lundin
    Feb 3 at 12:09

2 Answers 2

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You place another resistor after the diode. The point is to have the diode carry most of the current and reduce the strike voltage to something mundane that the second resistor can actually handle.

If the pin allows 10 mA and 3.6 V, and the diode limits to 24 V, then you need at least: $$R_\text{lim}>\frac{(24-3.6)\text{ V}}{10 \text{ mA}}\approx 2\text{ k}\Omega$$

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  • \$\begingroup\$ Thaaanks, that's a very good point! but my worries are: Will de diode die if it's not rated for 24,4A? As it will create a "short" in the line to gnd when the ESD event occur and if the tvs can not handle that amount of current, might die, so the next ESD event might damage the components after the diode, even if the resistor is placed, unless I put a huge resistor value, but a big resistor there can affect the performance of the device... \$\endgroup\$
    – Alber
    Feb 6 at 7:27
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    \$\begingroup\$ The ESD diode is designed to handle ESD events as in 61000-4-2 with the max rating given in its datasheet. @Alber \$\endgroup\$
    – tobalt
    Feb 6 at 7:33
  • \$\begingroup\$ Ok, so, then I assume the Ipp is enough to handle the XX kV according the waveform specified IEC 61000-4-2 or whatever, independently of the Ipp marked in the datasheet? In that case that value is irrelevant, I think (the ipp) because you want to be protected against certain voltage level and waveform... don't care about the current in this case... \$\endgroup\$
    – Alber
    Feb 6 at 7:46
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    \$\begingroup\$ @Alber Yes exactly. If the datasheet indicates suitability for e.g. 8 kV of 61000-4-2, then it will handle it repeatedly, while being directly subject to the strike, i.e. the full un-attenuated waveform strikes the diode it. If the peak current rating differs from the expected value according to the IEC waveform, I would sure raise an eyebrow however and question the competence of the manufacturer... But in general, I don't think ESD diodes have peak current ratings usually, because it is implicit from the 61000-4-2 rating \$\endgroup\$
    – tobalt
    Feb 6 at 8:12
  • \$\begingroup\$ I have seen devices being careful to note how many pulses they're tested for (implied: rated for?), which is... kind of curious. This one does not; I would be inclined to expect it's good for ballpark hundreds to thousands of events before failure. FYI, in event of failure, TVSs usually fail shorted, so it won't expose your device to further ESD but it won't be functional in either case anyway. \$\endgroup\$
    – Tim Williams
    Feb 6 at 11:27
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The PESD3V3Y1BSF is a small device, much smaller (low capacitance) than you need for GPIOs. This isn't a downside; its performance specs are impressive! But it will be challenging to solder: a 0.3 x 0.6 mm body with 0.4 mm pad pitch, expect to need hot air, and preferably magnification too. Some automated assembly lines may charge a premium for this as well.

You cannot find a TVS with clamping voltage as low as the supply voltage, so honoring the strict GPIO voltage limit is impossible that way, I'm afraid.

However, we can note that the GPIOs are rated for -- well, whatever they are, typically 1kV HBM or more -- and we can use a TVS, plus a series resistor between TVS and GPIO, to emulate the same or less stress to the pin, while handling a much larger event. Since the peak current at 1kV will be about 3A, and the peak clamping voltage is about 13V, the difference or 10V should deliver less than 3A or 3.3 ohms should do.

You can always go with higher resistance of course, which also has the value of filtering the signal against the pin capacitance (which can be increased with an external capacitor for better RFI rejection).

(Note that filtering is preferably done in front of the TVS, as it will clamp or rectify RF, distorting the signal before it has been filtered otherwise. Typically a series ferrite bead or resistor will be used here, then a capacitor in parallel with the TVS. A large enough TVS can also be used to give some capacitance itself, e.g. the ~20pF a BAT54S contributes, or the ~100pF of a zener-type TVS.)

Filtering, and reducing the peak current, have the value of extending the operating conditions: tolerating more ambient noise and peak transients. For ESD, many devices are rated for class C behavior (may require power cycle to restore normal operation); if you require class B (momentary disruption, recovers automatically) or A (no disruption to function), consider these mitigations.

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    \$\begingroup\$ I think the HBM spec is for unpowered devices. Stricter limits (namely those under abs max ratings) may apply when powered to prevent latch-up damage. \$\endgroup\$
    – tobalt
    Feb 4 at 5:40
  • \$\begingroup\$ Typically, devices are latch-up-free below 100mA (though this is rarely documented as such!). Again, different levels of behavior: you may need 0mA injection to satisfy analog inputs (meaning: the ADC/comparator/etc. meets all specs in and around the instant of that transient); perhaps a failed acquisition or two (out of tolerance, or totally bogus, whatever the case may be) is tolerable for your application, and more injection could be allowed (10mA? 100mA?). Agreed, when powered, probably over 100mA would be risky because of latchup (class C, requires power cycle -- if it survives!). \$\endgroup\$
    – Tim Williams
    Feb 4 at 7:55
  • \$\begingroup\$ Thanks so much for the answer!! As I answered tobalt to his/her comment, I'm afraid the diode break if it's not rated for the 24A, as that will be the current flowing througt it. Even if I put the resistor, that will protect the GPIO pin against the ESD event, if the TVS is working, but if TVS breaks after the first ESD event, then, the GPIO will be exposed (with the R) to the next ESD event, and that might damage the resistor, the gpio, or both, maybe... It's a quite good idea the resistor, but I guess I should find at least a 25A diode, am I right, or am I missing something? \$\endgroup\$
    – Alber
    Feb 6 at 7:36
  • \$\begingroup\$ As they commented, it's rated for the waveform. \$\endgroup\$
    – Tim Williams
    Feb 6 at 11:28

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