Unidirectional TVS on I2C prevents it from working - Bi-directional one is ok - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: March 01, 2022, 09:38:36 pm
content: (Updated with the part numbers and details about the voltage/length. See underlined addition)Hi,we have a board manufactured for years and always worked. On the I2C channel there are two separate connectors so we had two "dual TVS", one on each connector protecting both the SCL and the SDA lines.The original TVS were thePESD3V3L2BTfor some reason bi-directional (not required because I2C only needs unidirectional). We recently changed it toPESD3V3X2UTa uni-directional andsome boardsstopped working but when we remove it they work perfectly.The Ir" is running at 400KHz. The system and the I2C run at 3V3. Both connected like in the attached screenshot. The I2C and the 3V3 lines are clean and stable.The I2C is on the PCBs only connected by a ribbon cable 3 inches long. The external I2C connector does not have anything connected to it, not even a cable.Waiting for a PCB so I can have a look at the signals but any ideas/suggestions what could cause the intermittent faulty behaviour on the I2C channel with the new (uni-directional) TVS?The capacitance in the new one is extremely low (less than a pF) compared to 100pF of the original, so that cannot be the reason.)This is thePESD3V3L2BT, the original (bi-directional) which worked well for years:https://assets.nexperia.com/documents/data-sheet/PESDXL2BT_SER.pdfThis is thePESD3V3X2UTwhich, when used in place of the original one causes some PCBs to not be able to communicate with the I2C ICs:https://assets.nexperia.com/documents/data-sheet/PESD3V3X2UT.pdfThank you

author: ricko_uk
author_rank: Super Contributor
timestamp: March 01, 2022, 11:35:23 pm
content: Screenshots attached of the original and new TVS types (both connected as usual as shown)

author: m98
author_rank: Frequent Contributor
timestamp: March 01, 2022, 11:50:17 pm
content: Just shooting in the dark, could the ground potential at the diode float higher than the signal voltage for some reason? Are there some high current paths on your PCB?

author: uer166
author_rank: Super Contributor
timestamp: March 01, 2022, 11:56:24 pm
content: Check the footprint pinout, potentially you've flipped some pins on the TVS. If it's bidirectional, the breakdown from any pin to any other pin is >3.3V, so even if you screw up it'll keep working. No longer the case with 0.7V drop.

author: AndersJ
author_rank: Frequent Contributor
timestamp: March 02, 2022, 12:28:12 am
content: OP is not saying which type is used when it works.Most likely the clamping voltage is lower for the non working alternative.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 02, 2022, 03:37:11 am
content: Is it exposed to more than 3.3V? Is there any ringing on the line?Most importantly -- why is I2C being exposed on a connector!I would stick with the traditional zener type TVS. If nothing else, the capacitance gets you a bit more filtering (may want to add ferrite beads or other additional filtering to this end), and a mild resistance (say 10-100 ohms) toward the driver(s) increases ESD immunity at the expense of drive strength (which should be irrelevant as, it's I2C, it's not supposed to run any kind of length).Tim

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: March 02, 2022, 05:34:28 am
content: OP give us the exact part numbers please ;^)

author: Berni
author_rank: Super Contributor
timestamp: March 02, 2022, 06:20:08 am
content: Leakage can also be a problem sometimes.The more zenner kind of protection diodes have a pretty soft knee at these low voltages, so they can leak a significant amount of current in the area before the actual breakdown voltage. On a I2C bus that is only pulled up by resistors this could pull the bus down slightly.On the other extreme there are TVS diodes that operate more like a Diac where they have excellent low leakage up to a point where they trip and they start pulling current until the voltage drops sufficiently low to let it relax again. This can be a issue on I2C that has constant pullup resistors.And as others said running I2C on a cable outside of the device is usually a bad idea. I seen I2C be finicky even between chips on large PCBs.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 02, 2022, 12:52:39 pm
content: Quote from: Berni on March 02, 2022, 06:20:08 amThat's done on every HDMI link and it works just fine, although the cable is shielded.

author: ricko_uk
author_rank: Super Contributor
timestamp: March 02, 2022, 02:33:29 pm
content: Thank you all!Replies below to various questions/comments:Quote from: T3sl4co1l on March 02, 2022, 03:37:11 am@Tim,it is not exposed to more than 3V3 and no ringing. Why would the I2C being exposed to a connector be a potential issue in general?By "traditional zener type TVS" do you mean to use an actual Zener instead of a TVS?@ANTALIFE,parts updated also in OP. The one that worked (bi-directional) isPESD3V3L2BTand the one thatonly for some boardsdid not work was the uni-directionalPESD3V3X2UT. So both parts are rated for Vrwm = 3.3 V and the entire system runs at 3V3 only.@Berni,why is an I2C cable outside an enclosure a potential issue?Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 02, 2022, 06:34:26 pm
content: Quote from: ricko_uk on March 02, 2022, 02:33:29 pmI2C on connector is bad for EMI and ESD. Fine if well shielded (as the HDMI example (Display Data Channel (DDC)), also used in VGA and DVI -- all use shielded connectors and cables), but the low bandwidth may give the false impression that it doesn't matter. But there's hardly anything special about I2C transmitters or receivers; there's a minor amount of filtering (maybe 50ns worth) and that's about it. Extra transitions corrupt the state easily, pins don't have extra robustness towards ESD, and EMI banging into ESD clamp diodes shifts the baseline which can also corrupt logic levels, even if the RF isn't being read directly as signal.Besides, it's no good for long cable runs, not that that's relevant here I guess, but cable length is an easy way to soak up extra loading capacitance compromising clock rate.Finally, it's not easy to filter, because the impedance isn't constant. You want to minimize loading capacitance, but you can't get the cutoff frequency very low just by increasing inductance: when a driver pulls down, it'll ring (because R(driver) < filter Zo), and then when driver(s) turn off, it'll rise slowly again -- the edges are asymmetrical, which I mean, you already know this, but trying to put a filter on it just doesn't work very well for the same reason all over again.It truly is well suited to exactly what it is -- onboard communication!There are extenders and translators out there, if you absolutely need to use it over longer distances (e.g. differential I2C, which physically works something like CAN, but it's the same old I2C protocol)... or there are other standards to choose from.As for TVS, the regular kind is just a zener, a beefy one that's rated for surges. You can indeed use regular zeners, if appropriately rated; they might not be, so it's simply safer to get the TVS version. They might well put the same chips into both products, give or take appropriate testing; who knows.There's also the snapback diode (the unidirectional one). This has a negative resistance characteristic, and, it's not clear if it'll turn off with the pullups in there; they don't specify the valley (turnoff) current or voltage.You'll also see low-capacitance TVSs, which are of the usual (zener) type, but have a series diode so that the TVS capacitance charges up to signal peaks, only loading the signal initially. Often the TVS comes out on a pin that can be tied to VDD, so it stays out of the way normally, and also protects VDD. See dual and quad arrays for USB and such.The other kind of negative resistance TVS is the SIDAC type, which has low leakage until breakdown, then stays latched on until holding current is released. Advantage of course being much higher surge energy handling for a given voltage rating. They aren't available at such low voltages though (I think?).Tim

author: Berni
author_rank: Super Contributor
timestamp: March 02, 2022, 08:13:53 pm
content: Quote from: ricko_uk on March 02, 2022, 02:33:29 pmSome I2C chips and MCU peripherals can get confused into weird states if they see a voltage spike on the lines. At the same time the bus is only driven high by a pullup so it is more sensitive to extra capacitance on the lines (long cables can have quite a bit of it)Not saying it can't be done, just that some care has to be taken when doing so. Like short shielded cables with consideration to there capacitance.

author: ricko_uk
author_rank: Super Contributor
timestamp: March 02, 2022, 09:21:13 pm
content: Thank you Tim and Berni

author: Scutarius
author_rank: Regular Contributor
timestamp: April 17, 2024, 02:53:17 pm
content: Hi ricko_ukJust wondering if you ever found out the root cause?

author: ricko_uk
author_rank: Super Contributor
timestamp: May 20, 2024, 12:12:39 pm
content: Sorry for the late reply but took several months off.Kind of... that behavior apparently was shown in one of the graphs's curves. You can barely see it that after clamping it goes back to a voltage lower than the initial (rated) one.But it was not clear why it kicked in in the first place because even the manufacturer's tech support replied saying that it should not have kicked in in the first place (the voltage never overshot over the triggering voltage).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 22, 2024, 12:20:57 pm
content: I am wondering about your choice for unidirectional TVS'.As long as the signals stay on the PCB, then you don't need any extra TVS diodes, and when you connect it with a connector to the outside world, then the polarity of ESD events is unknown.Edit: Oops, silly me. As Langwadt mentioned below, unipolar TVS also works as a normal diode.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2024, 12:34:54 pm
content: Was this hot-plugged at all? Or firmly seated and only then power-cycled?Tim

author: langwadt
author_rank: Super Contributor
timestamp: May 22, 2024, 01:19:00 pm
content: Quote from: Doctorandus_P on May 22, 2024, 12:20:57 pma unidirectional TVS is a regular diode it the other direction

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 22, 2024, 08:17:58 pm
content: Quote from: ricko_uk on May 20, 2024, 12:12:39 pmIf it was a thyristor-type trigger TVS, maybe it has a dV/dT trigger, like triacs do ?Many are designed for protecting rails, not signals,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2024, 09:22:27 pm
content: Sort of. It's a snapback diode. It's sort of like a zener that has BJT avalanche-discharge (punch-through) behavior. Or perhaps other designs, I'm not sure. Not dV/dt AFAIK. The important part is, like the thyristor, there's a hysteresis loop with some breakdown voltage, peak roll-over current, and valley voltage and current. Unlike the thyristor, they don't specify these...Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: May 23, 2024, 12:05:26 am
content: Quote from: T3sl4co1l on May 22, 2024, 12:34:54 pmIt was firmly seated and only power-cycled. that is why it was weird that it kicked in. But since then I changed it to a "classic" type and it works as expected.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 23, 2024, 02:41:44 am
content: Quote from: T3sl4co1l on May 22, 2024, 09:22:27 pmYes, data is vague. I was musing if one could make an oscillator using theseThis is AOS data, for their new low C TVS diodes in teensy packages, they do include curves, but no mention of HOLD current.

author: Berni
author_rank: Super Contributor
timestamp: May 23, 2024, 05:20:46 am
content: You can still get brief spikes during power cycling.It is possible for power regulators to slightly overshoot on startup because the regulation loop doesn't handle the surge of filling capacitors at the start.It is also possible to get large voltage spikes above the supply voltage when hard switching the power via switch contacts and having a long power cord. The initial surge in current from filling caps stores some energy in the inductance of the cable that then kicks back and causes a spike upwards in voltage.If you are particularly unlucky it is possible for these spikes to fry something on rare occasion. This is a nightmare to debug because it just happens randomly after like 10 to 1000 power cycles, you can't capture what happened, you are just left staring confused at the blown chip when it does.

author: ricko_uk
author_rank: Super Contributor
timestamp: May 23, 2024, 04:54:16 pm
content: Quote from: Berni on May 23, 2024, 05:20:46 amI guess just putting an inrush current limiter, or just an inductor in series at the regulator output, might be a general simple and cheap solution to avoid that. Any other ones too?

author: Berni
author_rank: Super Contributor
timestamp: May 23, 2024, 06:05:43 pm
content: The solution for avoiding that tends to be the use of EMI ferrites to burn off the pulse energy into heat, and avoidance of using high Q capacitors on the input (like most MLCC capacitors)Similar can be used around regulators to limit how sharp of a current the regulator is exposed to. Otherwise softstarting a regulator is also a good idea

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2024, 06:33:03 pm
content: Without schematic, obviously we don't know, but you can compare against these ideas and check or test whether it's relevant to your case.https://electronics.stackexchange.com/questions/713381/correct-placement-of-series-ferrite-beads-to-avoid-dc-disconnect-during-power-cy/713473#713473Quote from: Berni on May 23, 2024, 06:05:43 pmFerrites tend not to do much in power supplies, because of the low DCR (if it were higher, it could be used more easily as series damping) and saturation current (not much flux is absorbed). Lossy caps are the most common go-to, especially as electrolytic or tantalum where the ESR comes for free.Tim

author: Njk
author_rank: Frequent Contributor
timestamp: May 23, 2024, 06:49:53 pm
content: I doubt it's necessary to select the part with documented standoff voltage equal to the max. working voltage (3.3 V or something). What's the point for that? Whatever the standoff voltage is, the breakdown voltage is always higher. And the clamping voltage is even more high. Moreover, the clamping voltage depends on current and that voltage typically far exceeds the absolute max. DC voltage for the IC you're going to protect anyway. The key assumption is that the overvoltage event is very brief. I doubt adding or subtracting several volts to the clamping voltage parameter of the part will make a difference as the current and duration are not exactly known. The IEC charts provide estimated values for modeled spikes but who knows what real spike will be encountered by the device. All that ESD protection stuff seems murky.

author: Berni
author_rank: Super Contributor
timestamp: May 23, 2024, 07:15:27 pm
content: Quote from: T3sl4co1l on May 23, 2024, 06:33:03 pmYes the ferrites having near 0 DC resistance is exactly why they are good for this. They won't burn any significant amount of power due to resistance, but up above a MHz they have at least a few Ohms. Enough to eat up the energy trying to ring itself around a high Q capacitor. It can also be just enough AC impedance to hide away large low ESR capacitance from regulation loops to prevent them from going unstable. Using regular inductors as power filters is no good as they return most of the AC energy back. While increasing the ESR of capacitors could result in slightly worse ripple.Tho admittedly it is not the primary reason i like to use lots of ferrites around power rails(but is a nice bonus). More of a reason is to keep the noise of switching regulators from escaping out, or to enhance the ripple rejection of linear regulators up into the MHz where they can't really do much. Other nice bonuses are also getting more control of the power rails return path in the ground plane(especially for switchmode converters), or as debug switches that can isolate a part of the circuit away from the rest to help find a short circuit fault or easily measure the current consumption.(They also light up nicely on a thermal camera if there is a short)Might not be a perfect catch all solution for inrush spikes, but it does help in mitigating it while providing other benefits in the same cheap small component.

author: thm_w
author_rank: Super Contributor
timestamp: May 23, 2024, 09:54:24 pm
content: Quote from: Njk on May 23, 2024, 06:49:53 pmNot sure what you are trying to say here. Data sheets are linked in the OP: both diodes have a clamping voltage well above 3.3V tolerance, they are designed to be used on 3.3V systems (hence the 3v3 in the part number).The original diode was 5.8V new was 4.2V minimum clamp.The issue is the new diode has a snapback voltage of 2.6V or even less, as teslacoil points out, its not well documented.This type of ESD diode is probably more appropriate for an ultra sensitive component where you want the voltage to rise as little as possible? They say in the datasheet for "USB2.0, HDMI, DisplayPort, eSATA and LVDS".I know people complained recently about EMI shutting off their monitor signal, maybe a similar device was the cause?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2024, 10:22:02 pm
content: Quote from: Berni on May 23, 2024, 07:15:27 pmMaybe... when, is the question. Saturation can be as little as 20mA, for small (0603-) high-impedance chips. It can be 200mA or more for larger (1210+) chips, and some low-impedance parts (<30 ohm?) are actually made for filtering and saturate at high currents (8A+).Since inrush can be 10s of A, it's very easy to saturate a bead, and then only a tiny delta of energy is removed per cycle; the ringdown envelope looks like a triangle rather than an exponential.Tim

author: Berni
author_rank: Super Contributor
timestamp: May 24, 2024, 05:30:12 am
content: Quote from: T3sl4co1l on May 23, 2024, 10:22:02 pmYep that's why i said it is not a perfect solution for inrush spikes.Ferrites designed for power filtering are designed to operate a sizable amount of DC current, hence why they have amp ratings. Even after they do reach saturation they will be dissipating around the ringing zero crossings, then even in the saturation they still have some DC resistance that becomes significant at large currents due to I^2*R (the non low ESR caps are still well under 1 Ohm too). Tho if you are actually ringing with multiple up and down swings then you need more damping anyway. You want to suck up enough energy on the way up so that you don't even get a significant overshoot, let alone full on exponential decay ringing.Ferrites are also not a solution if you have like 10000uF of capacitance and expect it to solve your inrush problem(the ferrite for that would be enormous). Just that with a sensible amount of input capacitance (like you might have with MLCCs before a switching regulator) a ferrite will likely remedy the problem enough for it to not be a significant problem. You might want a ferrite there for filtering and EMI reasons, but even if the ferrite is not great at damping inrush, it might still be enough damping to fix the overshoot, so you can have 1 component do multiple jobs EMI and inrush.In a lot of cases just putting a 0.1 Ohm resistor in series with the input is already enough to fix it, but those burn more power at DC, so they tend to put the resistor in series with the capacitors.There is a similar problem with SD card sockets as some SD cards have a significant amount of power supply capacitance inside the card. So if you apply permanent 3.3V power you could have the card monetarily drag down your supply rail enough for your MCU to crash. I found that a ferrite in series with the power helps nicely with that too. There inrush limiting is admittedly a primary job of the ferrite, but it works and i likely already have one in the BOM somewhere as a cheap component. So i just reuse a existing part and avoid increasing the BOM line count.

author: Njk
author_rank: Frequent Contributor
timestamp: May 24, 2024, 06:42:40 pm
content: Quote from: thm_w on May 23, 2024, 09:54:24 pmI think the problem was created artificially, by choosing the part type too optimistically. It is not stated in the NXP data sheet for PESD3V3X2UT that the parts "are designed to be used on 3.3V systems", this is your interpretation. The vendor is not aware of the definition for your particular "3.3V system". So the data sheet merely states that the reverse stand-off voltage (VRWM) is of 3.3 V max, at 25 degrees C. As no further information about the test method for this parameter is provided, we can only assume that the protection will not be triggered and will not start leaking at 3.3 VDC. Seems fine but what will happen at a little bit higher voltage, say 3.4 or 3.5 V or at different temperatures? The document does not specify that. It's also reasonable to assume that every mfg. and measurement processes are associated with the tolerance so the actual parameter value may vary within the tolerance window. That's the reason for putting some engineering margins in the design.The other consideration. In the past, the margins were typically provided by every reputable vendor. For instance, if the part is rated for 10 A, it actually could withstand 11,2 A. So the integrators were provided with 12% safety margin for free. Not because the vendors were less greedy back then, but because it was the only method to guarantee that every part meets the rating. As the mfg. process matures, that goal can be achieved by technology and now the new part works fine at 10 A and blows up at 10.001 A. I think it's reasonable to expect something like that in the near future. You can't force the vendors to continue with the traditional approach without resorting to a nonmarket mechanism such as regulations. Perhaps this case is a good example. I'm not sure a parameter like (VRWM) is a subject for any regulation. Actually, the responsibility for margins provision is shifting from the vendors to the integrators. And worse, that creates a temptation for vendors to start manipulating a little with the numbers and the definitions, for the sake of marketing.For one reason or another, there is no perfection in the world. Just don't use the part at 100% of its rating, leave some margin to improve the design reliability.In that particular case, why do not choose a part with (VRWM) one step higher, e.g. 5 V? That will provide reasonable assurance that no shit will happen at 3.3V, and also will take care about all the usual suspects like the PSU output voltage tolerance, ringing on the bus, etc.

author: thm_w
author_rank: Super Contributor
timestamp: May 24, 2024, 08:27:32 pm
content: Quote from: Njk on May 24, 2024, 06:42:40 pmI don't disagree with your other points but NXP straight up recommend the part for USB2/HDMI which are 3.3V systems. As noted above.

author: Njk
author_rank: Frequent Contributor
timestamp: May 24, 2024, 09:45:00 pm
content: OK, not for an I2C system that was mentioned by OPEdit: Just looked into HDMI v 1.4 specification. Table 4-22 Required Operating Conditions for HDMI Interface:Termination Supply Voltage, AVcc: 3.3 Volts +/- 5 %No wonder all the recommendations are mentioned in the informative part of the data sheet, not in the normative one