ESD protection for a low current device - TVS leakage is too high - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: September 27, 2020, 01:49:20 pm
content: I'm making a small handheld project with a tiny84a and 5 LED's/6 buttons. It is going to run on a CR2032 battery and I've got the firmware so it is pretty miserly. There is no power switch, but the firmware will auto soft power off reducing current to 0.2 uA. At this rate, it would never use a CR2032 capacity and it will die of old age first.Now to the issue - I had planned on putting a unidirectional TVS on it like I do many of my projects. I tried a couple of parts and one was 7+ uA and the other was 35 uA.What other ESD protection can/should I consider that won't raise the power off usage much?

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: September 27, 2020, 02:13:30 pm
content: A low leakage TVS! Example :https://www.semtech.com/products/circuit-protection/low-voltage-esd-protection/uclamp2501t

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2020, 02:57:29 pm
content: A couple things:1. TVS diodes are just zener/avalanche diodes. Under 6V, the Zener effect dominates, and the knee is rather soft. A "3.3V" diode might regulate at 3.3V at say Ir = 10mA, but has a generous slope, maybe it's 1mA at 3.1V and so on. The leakage at lower voltages is rather poor, and the clamping voltage at high currents isn't great either.TVSs aren't very practical below 5V. A typical 3.3V TVS has about the same surge clamping voltage as a 5V part.Example:https://assets.nexperia.com/documents/data-sheet/PESDXS1UB_SERIES.pdfThese are pretty good, for what they are. Or they might not be TVSs exactly.Note the uClamp example above is a punch-through diode -- different physics from a zener diode, they exhibit some negative resistance (hence the snap-back spec). Ideally the negative resistance is tuned to equal the device's internal resistance, so the V(I) curve is very flat, even for quite high currents; you can see the advantage at the 1A clamping voltage.2. Small leakage currents are hard to measure. They may simply not be taking the time or effort to measure them -- the real leakage (at room temperature) may be much lower. Or, the given value is worst case, at max temperature, and the typical value at room temperature can be expected to be much lower. (Transistors are often rated in low uA leakage, but typically perform in the nA, or less. You have to shop around and pay extra for parts that are tested at, and guarantee, nA leakage. If you're okay with a potentially poor production yield, or with sorting parts yourself, it's okay to use average parts at low currents like this.)...Unless, of course, you measured them at 7 and 35uA, in which case that proves that.3. If you just need ESD, it may be adequate to put big enough capacitance on the supply (a few uF), to absorb the charge delivered by clamp diodes. The battery will then bleed off the excess charge.It sounds like this will be a small device, without many (any?) external connections to carry conducted ESD or other surge into the board. If so, the hazard is a lot smaller, and device internal ESD protection is probably sufficient.Tim

author: alank2
author_rank: Super Contributor
timestamp: September 27, 2020, 03:06:40 pm
content: Tim - it is basically a small handheld device - I'm trying to be minimal with the parts, but I'd like to have it have a reasonable amount of ESD protection. Essentially starting with the battery (CR2032), the AVR, and a single bypass cap - what can/should I add to this for protection. It will be possible to put the CR2032 in upside down, so reverse polarity protection is needed as well. I was planning on using a 10mA PTC + a TVS, but maybe I need to rethink it all. What do you recommend?

author: profdc9
author_rank: Frequent Contributor
timestamp: September 27, 2020, 03:47:42 pm
content: Another approach to ESD resilience can be to engineer structures into the PCB (if you are using one) to prevent a voltage across internal components. For example, a guard ring on the PCB at the edge shunts ESD through it at the edge of the board.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2020, 03:55:32 pm
content: A series schottky will be fine for polarity protection. A MOSFET if you really need the supply voltage to stay up. (Which eh, if you're blinking LEDs, it's probably a good idea. Avoid blue or white LEDs, BTW. Or, I might suggest a boost or SEPIC converter, if you need them..?)Heh, the short circuit current from those cells is so low (100s mA?), it'd probably survive anyway, without any additional protection.The PTC would probably be fine, too.A solid ground plane is probably adequate mitigation. Yeah, you can expose the edge to help deal with direct strikes. I guess it's insulated inside an enclosure, so that shouldn't be much of a problem anyway.Tim

author: alank2
author_rank: Super Contributor
timestamp: September 27, 2020, 04:23:30 pm
content: I'm using some Kingbright Heli2 LED's - I'm running them at 0.5mA each, so maximum 2.5mA+maybe another 0.5mA from the AVR so 3mA total.I tested a Schottky I use and it was 67mV drop with 1 LED on, or 110mV with all 5 on. I am thinking (but I could be wrong) that that just pushes the battery curve down by that voltage amount, but since a CR2032 has a pretty flat curve anyway, it might not be too bad. Maybe - what do you think? I was down on the MOSFET concept because I'm a bit worried about that gate getting zapped, but I know there are some MOSFETS with protection for the gate. Is a Schottky going to be more robust?Can you say more about how the ground plane will help? (Is this if I have a TVS, or even if I don't?).I was thinking about trying those Oshpark after dark PCB's that have black FR4 and clear mask so I was debating whether to have ground planes or not (it would look better not). Essentially on the top, what is exposed is ground connections, button sense connections (40K pullup via AVR), LED anode (after resistor). On the bottom is the battery (+5V exposed completely via a metal shield you push the battery into) and the AVR connections.Thanks for your advice and help!!

author: profdc9
author_rank: Frequent Contributor
timestamp: September 27, 2020, 05:00:24 pm
content: The purpose of a guard ring or a ground plane is so that if an electrostatic discharge occurs to the PCB, rather than traveling through the circuit traces and into the pins of the devices, destroying them, a solid path of copper around the device or sensitive components shunts the charge away. For example, if one side is a solid copper plane with many vias going to the other side around the perimeter, if the device is charged and discharged, it will tend to be through the conductor. If you can't have a solid ground plane, a ring around the edge of the board can offer such a path with stitched vias between top and bottom layers. If you have buttons, you can place TVS diodes (or sometimes even high-value resistors) to bleed the ESD to the ground plane or ground ring from the button conductors.

author: alank2
author_rank: Super Contributor
timestamp: September 27, 2020, 05:02:42 pm
content: Also, no enclosure. Hopefully the user grounds themselves before handling it, but what can I reasonably to offer some protection.

author: Marco
author_rank: Super Contributor
timestamp: September 27, 2020, 05:53:13 pm
content: Quote from: T3sl4co1l on September 27, 2020, 03:55:32 pmBut then the cell can no longer absorb energy through the I/O protection diodes (and I/O resistors).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2020, 06:29:51 pm
content: Quote from: alank2 on September 27, 2020, 05:02:42 pmOh, well then something would be a good idea.Clamp diodes for each signal (pushbuttons and LEDs?) would be best. I'd probably excuse any that are short, or generally not reachable from the outside. Like if the LEDs are at the end of the board, probably they'd get a strike, but if they're up close and not routed with vias, it might be okay. Likewise pushbuttons can be had with a ground shield, that will take a strike away from the signal pins.I suppose this would be the better option,https://www.semtech.com/products/circuit-protection/esd-protection/uclamp3301djust the 3.3V version. Same low leakage. Excellent clamping voltage.Quote from: Marco on September 27, 2020, 05:53:13 pmYes, a TVS might be a good idea then.It probably doesn't matter much, really; ESD is a low duty cycle phenomenon, it would be very unusual to have enough hits in a row that supply voltage is made to rise dangerously. If the CPU depends on stable supply voltage, even while sleeping (Idunno, maybe an RTC would have a problem with that..?), that might not be great, and a more stable solution (MOS, TVS, shunt reg) might be desirable.Also, note the schottky has substantial leakage: just as it has a low forward drop at these currents, so too it will have a low drop at small reverse currents. Probably it takes just a fraction of a second for the supply to return to quiescent.And yeah, a TVS is a good combination with MOS polarity protection.Tim

author: alank2
author_rank: Super Contributor
timestamp: September 27, 2020, 09:06:19 pm
content: That UCLAMP3301D.TCT with the 0.5 uA maximum leakage would be fine - dropping the battery life to only 37 years.What do you think about that combined with a 10mA PTC :https://www.mouser.com/ProductDetail/bel/0zcm0001ff2g/?qs=uwxL4vQweFOSr3rpPGffpA%3D%3D&countrycode=US&currencycode=USDMy thought is that as I can't do much to protect the + side of the battery (metal bracket that solders to the PCB and you push in the cell (+ side exposed), that I will put the PTC on the negative side near the pad that the cell connects with. Then put the TVS across the vcc/gnd - but there isn't a lot of ground/vcc rail - pretty much just connecting to the AVR'c vcc/gnd and its bypass cap near the same.Still not sure about having a ground plane or not, but again, having that +3V exposes doesn't make me want to have a ground guard ring exposed too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2020, 09:43:58 pm
content: The bracket will serve as the guard ring, that's fine.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: September 28, 2020, 08:07:14 am
content: Check the ESD spec of the ICs and other parts. Overkill, No diodes needed.Jon