ESD protection IC only clamps in one direction? - Page 1
Forum section: electronics / projects

author: eecook
author_rank: Regular Contributor
timestamp: November 10, 2014, 11:26:31 pm
content: Ok, I am new to EMC. I've been working on a mobile design that is battery powered and I need an ESD protection, so I used an EClamp2455K chip from semtech. Now I am trying to asses how it works. So I took a look at Ott's (Noise Reduction....) chapter on ESD to try to understand the problem first :p. Maybe there is a problem with my understanding of ESD and the IC is fine, maybe I just don't get the IC but I do get the ESD phenomena....maybe both. Either way, here's how I understand things to work:If a human touches my battery powered device:I can model it with an RC circuit for the person and capacitive coupling of the device with earth to close the loop:When I go and see the IC's inner circuit (http://www.semtech.com/images/datasheet/eclamp2455k.pdf) in page 2 of the datasheet, I find that it can only clamp in one direction. Suppose I touch the "CLK In" line:Assumption: The highest coupling capacitance to earth will occur between the circuit's return path (which I'll call ground) and earth given that the return path has the highest copper area in the PCB. Is this assumption correct? How do I even tackle the problem if this is not the case?Now let's go back to me touching the "CLK In" line, I can think of 2 possibilities:1. I am at a higher potential than earth.2. I am at a lower potential than earth.If 1 is the case and if my Assumption is correct, then a diode will clamp the voltage between that line and ground.If 2 is the case and if my Assumption is correct, then a diode will be forward biased and clamp at a lower voltage than the previous situation, hence there will be more current flow.1. Is my understanding of the ESD correct?2. Is my understanding of the IC correct?Thanx if u got this far!Cheers!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2014, 03:41:42 am
content: Nice drawingsWhat you're missing on the voltage drop is, what's 1V (forward) or ~7V (reverse) out of 8,000V of ESD? The current isn't any different, and so the dynamics (requiring control of stray inductance, resistance to ground, and filtering if possible) are unchanged.Tim

author: eecook
author_rank: Regular Contributor
timestamp: November 11, 2014, 12:36:35 pm
content: Thank you Tim. Ha! good point. So, the diodes are operating as a protection when they are forward biased, it's a bit odd right?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2014, 04:04:37 am
content: No, it's quite normal to do that; you'll regularly see regular diodes (e.g., BAT54S, BAV99) used to clamp a signal to ground if it drops below, or supply if it rises above.A unidirectional zener/avalanche diode is often used when a supply voltage is not handy, or inconsistent with the signal range. Other examples: suppose you have an analog circuit with +12V and 0V supplies, and signal inputs with 0-5V range. It would be handy, not only to provide ESD protection to those lines, but also to clamp them to the intended range: using a 5V zener diode can achieve both at once. (Don't forget the current limiting resistor, in case someone accidentally shorts +12V to the input...)Likewise, a bidirectional TVS is often used when the range must be limited, but the direction isn't very critical. RS-232 is a good example: the signaling standard itself allows for +/-15V signals, and compliant devices (receivers and transmitters) must tolerate this as well. You probably won't have +/-15V supplies handy in such a circuit, so you can't use clamp diodes to provide additional ESD or surge protection; a bidirectional TVS is then used to clamp it to a safe range (perhaps +/-20V), minimizing damage to the ICs. (RS-232 is probably a bad practical example, because the receivers and transmitters are generally already rated for 8kV ESD, and don't strictly need additional protection. If you want a really robust device, you might go the extra step though.)Tim

author: EE-digger
author_rank: Frequent Contributor
timestamp: November 12, 2014, 04:38:48 am
content: Uni-directional devices are pretty common now for protection of single ended signals. You can get fairly low voltage devices and thanks to all of our USB2, USB3 and HDMI speeds, there are many devices available with super low capacitance that won't mess with your higher speed signal, if you have any.One of the best preventions against ESD is to prevent a discharge from taking place. If you have a plastic case and no I/O, you will be very tolerant of nearby ESD strikes (i.e. person's hand to a nearby metal object, desktop, equipment, etc.). To prevent upset of your circuit in this case, be sure to have good power supply decoupling.Personally, I haven't clamped to a supply in a long time, but most ICs have input protection which does just that. I prefer to clamp to ground ... and preferably that ground is either a chassis, inner shell, or at least a separate band of copper near the board edge that you would use as "ESD ground". This ESD ground can be tied to your circuit ground, but only near I/O points. Multiple ties to circuit ground can be made, at each I/O area if distributed around the board edge.By forcing the ESD discharge current to go elsewhere, your circuit can be very tolerant of ESD applied to your design.Ott is excellent and another good guy on ESD is Doug Smith, who has many tech tips on his website.Den

author: nctnico
author_rank: Super Contributor
timestamp: November 12, 2014, 09:51:02 am
content: Quote from: T3sl4co1l on November 12, 2014, 04:04:37 amI don't recommend clamping to supply voltages. Many regulators can only source current, not sink it. A large pulse on an input could be forwarded to the supply rails which is what you don't want. It is better to use zeners, varistors, MOVs or TVS diodes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2014, 11:53:36 am
content: Quote from: nctnico on November 12, 2014, 09:51:02 amESD doesn't carry nearly enough charge to cause problems (if there's a 0.1uF capacitor near by, you'll never tell the difference), but this is true of surges. In that case, place a surge rated TVS across the supply rail to keep it in check.Tim

author: DanielS
author_rank: Frequent Contributor
timestamp: November 12, 2014, 03:23:26 pm
content: Quote from: nctnico on November 12, 2014, 09:51:02 amFor bidirectional or output pins on CMOS chips, you do not get much of a choice in here since the pins are usually clamped to ground/VCCIO by the output FETs' body diodes anyway. The bigger worry with those would be preventing excessive clamp current which could destroy them.Most of the ESD's energy is wasted in the spark. Whatever ESD energy might get clamped and dumped into the PCB's decoupling caps will not be enough to produce much voltage as Tim said. This is why semiconductors tend to become far more ESD-tolerant once they are on an assembled PCB.