ESD protection on USB data lines - Page 1
Forum section: electronics / beginners

author: derGoldstein
author_rank: Regular Contributor
timestamp: August 02, 2016, 12:41:37 pm
content: I've been looking at eval board circuit diagrams for USB hubs and chargers, and I sometimes see that there's a dedicated IC for ESD protection on the data lines, like the TPD2E001.My question is, when would this be necessary?Does it only apply if there's active high-speed data transfer, or does it also apply if I'm implementing a USB charging circuit (where the data lines are just set to a certain voltage and there's no data being transferred)?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: August 02, 2016, 02:54:09 pm
content: The ESD circuitry is used to protect any items on your board that are connected to the D+/- lines that might be susceptible to ESD damage. Think somebody plugging in a USB cable to your board with the other end not attached to anything, and then touching the unconnected end of the cable with their finger so as to induce an ESD spike on one of the data lines.If the data lines are just attached to resistors on your board, you can probably forego any ESD protection, but if you have an IC down the line that's used for handshaking (eg: TPS2540) you might consider it.

author: derGoldstein
author_rank: Regular Contributor
timestamp: August 02, 2016, 07:14:43 pm
content: Oh ok, so it doesn't have anything to do with the high frequency on the data lines.But then wouldn't the power lines (+5V an 0V in the case of the USB connection) also be susceptible to the same thing? I guess I'm kind of turning this into an extremely broad question, but couldn't any exposed conductor carry ESD to an IC on a board and fry it?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: August 02, 2016, 07:45:49 pm
content: Quote from: derGoldstein on August 02, 2016, 07:14:43 pmThe "high frequency" of the data line isn't what dictates whether or not it needs ESD protection, but if you decide you do need ESD protection, the required pass-band frequency needs to be taken into account so you don't inadvertently attenuate the signals you're interested in.Quote from: derGoldstein on August 02, 2016, 07:14:43 pmPower lines generally have a lot of filtering built in (chokes, coils, caps) that makes ESD less of an issue, but it can still be a problem. Sticking a TVS on the power line is a good way to shunt out any ESD spikes.Exposed connectors should almost always have some form of ESD protection on the data lines, as well as short circuit protection if the IC it attaches to has to output anything and could be damaged by a short.

author: derGoldstein
author_rank: Regular Contributor
timestamp: August 02, 2016, 07:49:42 pm
content: Thanks, that clarifies it!

author: savril
author_rank: Regular Contributor
timestamp: August 03, 2016, 08:52:18 am
content: Take a look at ST USB6B1, all the TVS and diode you need in one package.Do a search on USB6B1 in this forum, there's some threads about USB protection worth reading.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2016, 10:05:17 am
content: During regulatory testing, ESD is applied to any accessible metal parts/surfaces.If the connector sticks out (at the very least, its metal shell will), it will get zapped.USB connectors are designed to prevent direct contact (by contact discharge or sparking) from ESD events, but poor layout can expose the USB port IC to dangerous voltages, and cables can still retain a charge (that dumps directly into the pins when plugged in).ESD, by the way, is a tremendously destructive force. It doesn't seem like it, does it? Just a tiny spark that maybe makes your hand jump? Alas, bags-of-mostly-water are amazingly resilient to electromagnetic energy! An ESD spark delivers a peak current around 20A, not at all pleasant to a couple transistors made to deliver under 100mA!As an added bonus, you can pick a USB protectionand filteringdevice, which limits bandwidth to only that required.Only use the bandwidth you need.For USB Full Speed (12Mbit), typical filters roll off at 30MHz (the theoretical minimum is 6MHz, but practical filters start with a cutoff frequency at 12MHz or higher). For Full Speed, it's more like 500MHz.What good is filtering? If your device must be able to operate in the presence of RFI (which might be as ordinary as a Wifi router, or as powerful as a CB radio, or malfunctioning microwave, or a nearby commercial radio tower, to name a few possible sources), then filtering is a Good Idea. Even if not, it's practically a "categorical imperative" (so to speak) that you should only use the bandwidth youneedfor your signal.As for shielding, almost all USB appnotes show itpatently and unquestionably wrong. The shield must always be grounded. And by "ground", I don't mean galvanic ground -- that doesn't matter. What matters is RF ground. Because, the connector is carrying RF signals (at least several MHz, plus harmonics). The shield needs to be RF-grounded (that is, grounded directly, or bypassed with multiple capacitors) to the same ground as the USB controller IC is grounded to.About half the time, you'll see an appnote suggest a ferrite bead on the shield -- this is the complete opposite direction you need, and intentionally (I can only imagine?!) makes things worse, by orders of magnitude!Another "categorical imperative" is that appnotes are not to be trusted...Tim

author: derGoldstein
author_rank: Regular Contributor
timestamp: August 04, 2016, 06:11:51 pm
content: Quote from: T3sl4co1l on August 03, 2016, 10:05:17 amIs this the same ESD characteristics that can accidentally happen when you're handling the chips as you place them on the board for soldering, or is it worse because a long cable can already be charged?Quote from: T3sl4co1l on August 03, 2016, 10:05:17 amI haven't looked at appnotes specifically talking about what to connect the shield to, but in most schematics I've seen the shield being connected to ground.But what if you left the shield unconnected? It would become a very low-value, high-voltage capacitor, right? So wouldn't it be safer to connect the shield to ground using a high-value resistor, so that any charge that builds up will gradually bleed into ground rather than immediately discharge to it?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2016, 06:55:12 pm
content: Quote from: derGoldstein on August 04, 2016, 06:11:51 pmIntensity varies with voltage, of course. Part level ESD will be low energy (small capacitance). HBM has a modest impedance (~150 ohm) and can go up to quite high voltages (~10kV). Cable ESD isn't very high voltage (~kV?), but has a low impedance (50-100 ohms), and delivers more energy.Typical ESD waveforms are a ~1ns risetime, some ringing, and decay over tens of ns. Cable discharge waveforms are flat topped (because it's a transmission line discharge), for the length of the cable (~ns to 10s ns?).QuoteIf unconnected, then ESD to the shield (the most likely place to hit) is capacitively coupled (or, really, transmission-line-ally coupled) to the signal wires, or worse yet, the spark jumps wherever it can (maybe even to unrelated circuitry near the connector!).If you don't need to worry about galvanic grounding, you can do that: Ethernet uses a 1nF 1.5kV capacitor to "ground" the isolated side. This reduces a 8kV, 150pF (HBM) ESD surge to a nominal voltage (~1kV) which can be safely handled by the signal isolation transformer, while drawing zero DC, or insignificant mains-frequency AC, current. A high value resistor could be added to drain the charge buildup, though I don't think this is usually done.Tim