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I am working on a custom board for the PCDuino3 Core Computer on Module. The design for the reference board for this has bi-directional ESD diodes on every port that is likely to be touched. I am unfamiliar with this type of device, but would like to follow the reference design if possible, but the one the manufacturer uses is difficult to come by (the Semitel ESDPSA0402V05).

What is the criteria for finding a suitable bi-directional ESD protection diode for a 5v device like the PCDuino3c?

From what I have found after an hour or so of searching, I should be aiming for one that has 5v Reverse Standoff Voltage and 0.05 pF capacitance or lower to support high speed data rates. Can anyone confirm this?

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    \$\begingroup\$ 50nF seems too high to allow high-speed data to pass unmollested. \$\endgroup\$
    – brhans
    Jan 10, 2015 at 8:25
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    \$\begingroup\$ The data sheet says 0.05pF... that's a pretty good value for a protection device. \$\endgroup\$ Jan 10, 2015 at 13:46
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    \$\begingroup\$ Yeah that was my fault. I fixed it now. \$\endgroup\$
    – Nate
    Jan 10, 2015 at 22:31
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    \$\begingroup\$ Yeah I am working my way around the schematic, replacing parts. I am mostly concerned with what are the important parameters for finding a replacement. I am not familiar with ESD diodes and need to learn what stats are important for them before I can hope to find a replacement. That is what this question is meant to discover. "Am I correct about the Reverse Standoff voltage being 5V-6V? How do I figure out how much capacitance I should have? How do I know how much dynamic resistance I need? \$\endgroup\$
    – Nate
    Jan 12, 2015 at 19:25
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    \$\begingroup\$ Found this tidbit in a document by wurth elektronik "For USB 2.0 line capacitances up to 5pF shouldn’t cause any trouble." While I have other signals to deal with, this at least gives me an idea of what I should look for. \$\endgroup\$
    – Nate
    Jan 13, 2015 at 0:27

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Ok, here are a few basics on ESD diode parameters:

  • \$\mathrm{V_{RWM}}\$, Reverse Max Working Voltage, sometime called Reverse Standoff Voltage: this is the highest voltage that the ESD diode is guaranteed not to open at. You select the diode to have this parameter higher or at worst equal to the highest "normal" voltage on your pin. You have some leeway in selecting a higher \$\mathrm{V_{RWM}}\$ than the circuit. E.g., 5.3V or 5.5V and even a 6V \$\mathrm{V_{RWM}}\$ diode would be fine on a 5V-rated input pin. The higher you go however, the less overvoltage protection you have. This isn't an ESD-type of event (which would be kV) overvoltage but mismatched components being plugged in etc. An ESD diode is a glorified TVS diode after all, so protects against those kinds of overvoltages too.
  • \$\mathrm{V_{BR}}\$, Breakdown Voltage, somewhat higher than \$\mathrm{V_{RWM}}\$. This is the "corner" voltage if you like, whereabout the diode is guaranteed to open. If you somehow choose a diode with \$\mathrm{V_{BR}}\$ below the max normal operating voltage of the pin, it will short it out (occasionally at least), something that you surely don't want. As example, the PESD5V0F1BRSF (mentioned in your comments) has 10V for \$\mathrm{V_{BR}}\$ but 5V for \$\mathrm{V_{RWM}}\$. Once \$\mathrm{V_{BR}}\$ is exceeded the ESD diode starts conducting pretty well, being basically equivalent with a resistor rated at...
  • \$\mathrm{R_{DYN}}\$, (possibly with variations in case e.g. \$\mathrm{r_{dyn}}\$), dynamic resistance, a measure of the diode's resistance for voltages above \$\mathrm{V_{BR}}\$. The lower it is, the more protection it offers, because it is in parallel with whatever it is protecting, ICs etc.
  • \$\mathrm{V_{ESD}}\$, the maximum ESD rating of the diode for contact ESD events; usually conforming to the test procedure in IEC61000-4-2. This is in the kV range, 10kV for PESD5V0F1BRSF. ESD voltages higher than this may destroy the diode itself and then it will provide no protection. For GPIO ports, NXP also offers 30kV-rated ESD diodes.
  • Diode capacitance, variously denoted as \$\mathrm{C_L}\$, \$\mathrm{C_D}\$ and probably others. Protection doesn't come free now does it? This is where you pay for it in engineering terms: the ESD diode will distort high-speed signals. The lower its capacitance, the lower the distortion will be. In general the ESD diode manufacturer will save you the trouble of much transient calculations by suggesting an application for the diode, e.g. HDMI, in which case you can look up the max bit rate (and thus frequency) of signals that usually are okay with that diode. "Usually" here refers to the fact that a particular application like HDMI has certain parameters for its eye diagram that are good enough for that application/standard. ESD diode manufacturers usually have a selection guide in which they suggest ESD diodes by application, e.g. here's NXP's, here's littlefuse's, here's Infineon's etc. It's possible that for a different, more sensitive application at the same speed, a certain diode may not be good enough. And there are tradeoffs of this diode capacitance vs dynamic resistance and vs \$\mathrm{V_{ESD}}\$ that a manufacturer has to deal with. So the less distortion a diode has... the less protection an ESD diode will provide.

As for "I just want it to work as designed." What I see is that their original diode provides 8kV ESD protection... and a dynamic resistance that I can't figure out (not published). So take a guess on that parameter "as designed". Besides that, I've already told you in the comments above and I'll repeat here is that its very low capacitance value appears silly for GPIO ports. That board uses much faster diodes on GPIO than on the HDMI port. The "USB-ICTRL/GPIO1" pin does appear shared with USB so perhaps select ESD diodes intended for whatever USB standard that chip supports.

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