What's the standard way of making ESD protected I/O? I see a lot of boards that just run wires straight from microprocessors to external connectors -- I know that's wrong, but what's right?
Any handy white papers, or suggested places to look? Things called "drivers" are obvious candidates.
I've got some plain old digital (0V, 5V) I/O, and some phototransistor receiver logic, that I need to implement.
Danke.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
My standard is this:
Resistor from connector to center of BAV99 (upper pin). Several kohms or tens of kohms, depends on speed your speed needs. Then another of a few hundred ohms from there to the uC pin. The BAV99 runs anode to GND and cathode to VCC. If VCC load is light there needs to be some sort of limiter such as TLV431 so the rail won't inch upwards from too many hits.
This way even with a big hit the uC won't see much substrate diode current. Of course, if you expect the ESD pulse from hell the resistor needs to be a "stretch type", very long. Vishay sells those.
Bitteschoen :-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Hi Tim
Have a look at:
Transient voltage suppression diode:
Comparison of transient suppressors:
Good teknical magazine:
ESD Myths and the Latency Controversy
A Basic Review of the Concept of No Free Charges in Insulators
Protecting PCBs from ESD
Mr Static: How Is Static Electricity Generated?
Induction: What It Means to ESD
Useful Static Electricity Most people are unaware of the important applications that are based on electrostatic principles.
Static Electricity and People
Ions
PCB Layout: The Impact of Lightning and Power-Cross Transients Milton Hilliard Helpful data can ease the process of designing for immunity to lightning and power-cross events to meet the requirements of GR-1089- CORE.
Avoiding EMI Filter Problems with Proper Selection and Installation:
Overvoltage Protection for Modems Using Gas Discharge Tubes:'
Filtering Out Interference Signals with Cable Ferrites Attaching a ferrite to a cable or looping a cable through a ferrite can help reduce unwanted high-frequency interference.
It needs some ESD protection -- it's going to be used over Astroturf, which I gather can create quite a discharge. But I'm not going to try to proof it against anything from the nether regions.
This is roughly what I was thinking of as the simplest solution -- I'll run with it, and see how it works.
Back when I was young enough to think that a language that wasn't my birth language was a super-secret code and that I had a Very Large Brain, I got into the habit of using tid-bits of German. I mostly stopped when folks would _respond_ to me in German, which just points up the fact that I can transmit in that language, but not receive*. Mostly I got out of the habit.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
If the zaps are super brutal put TVS up front, else it'll arc over that first resistor. But remember that TVS have a large capacitance.
Or Germans on the island of Mallorca after 6-7 beers each ;-)
Should have used a more secret code then, like "quatlho" which is Klingon for "thank you" :-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
The other important component is a ground plane, otherwise the TVS has nowhere to discharge the current to.
I use a TVS, then series resistor or ferrite (depending on required DC resistance), This can then go straight to the microcontroller pin etc. With modern parts with some integrated ESD protection this is all I have found to be necessary. The TVS limits the spike amplitude, the series impedance then limits the current.
-- John Devereux
Tim Wescott wrote: ...
Why not, "Hello, I'm an American tourist?" >:->
Cheers! Rich
I used to use the LittelFuse SP720AB. Unless the device was hit by lightning, or a 12 V supply with plenty of caps in it, it would survive. Now, I'm using TI voltage level translator chips, and they are vastly tougher than FPGA pins.
Jon
I'd try to buffer it with a device that's a bit more substantial. Even so, I've gotten into the habit of using a current limiting resistor a cap and a TVS diode, where I think ESD is going to be an issue. If EMI is an issue, add a ferrite to the mix. TVS diode arrays can be had pretty cheap.
These are around $.10 for five pins:
I wouldn't use them for power entry (use an SMB or some such, after a polyfuse) but they're fine for signal lines.
Oh -- I forgot to mention that. They'd like me for being American, but wouldn't be able to understand much. They knew Croatian*, but not much English. At least the folks that had to deal with tourists spoke German well enough, though. As for the rest, it's amazing what you can do with pointing and smiling (learning the local versions of 'please' and 'thank you' goes a long way, too).
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
I've never been to Europe, more's the pity, but I can say, "One Beer Please" and "Where's the toilet?" in Japanese, Korean, and Thai. When I went to Taipei, Taiwan on R&R, I didn't need to learn any Chinese because everybody spoke English almost better than I can. ;-)
A Chinese typewriter in action is truly a wonder to behold:
Cheers! Rich
What i have done for inputs is: input pin, 1K to dual diode (a pair in a SOT-23 type package) where one diode clamps to ground and the other clamps to +5V; said GND and +5 are what the outer world sees and are approximations. They are common planes supporting about 8 signal lines and tie to true GND and +5V via 1K and the planes as well as the true +5 all are bypassed. Maybe i will be able to find schematic and layout for ABSE Friday.
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