steering diodes for microcontroller input pin protection.  schottky or regular - Page 1
Forum section: electronics / general

author: snarkysparky
author_rank: Frequent Contributor
timestamp: April 01, 2021, 06:30:51 pm
content: Having finish laying out a board for the 64 pin version of SAMD21 with protection diodes on every IO I begin to wonder about the effectiveness of using this part.https://www.diodes.com/assets/Datasheets/DT6250-06MR.pdfThese diodes appear to have a higher Vf than the diodes intrinsic to the SAMD itself.Vf DT6250 = 0.8VSAMD absolute max ratingsGND-0.6V - VDD+0.6VIs there any protection to pins from this device or do I need to tear it up and put in Schottkys BAT54 ?

author: Gyro
author_rank: Super Contributor
timestamp: April 01, 2021, 07:07:30 pm
content: Ideally you would put a series resistor between the steering diode and the microcontroller input pin. When the steering diode clamps, this resistor limits the current through the input pin protection diodes to within their spec limits. That way you can be more tolerant of steering diode characteristics.If not including a series resistor, you would need to go for Schottky to get a forward voltage lower than the ones inside the microcontroller.

author: nctnico
author_rank: Super Contributor
timestamp: April 08, 2021, 04:41:32 pm
content: No. Use TVS diodes (and a series resistor between the TVS diode and the microcontroller input). Clamping inputs towards the power supplies is not a good idea; with enough energy you'll push up the power supply rail and wreck the entire circuit.

author: exe
author_rank: Unknown
timestamp: April 08, 2021, 04:55:52 pm
content: Why do you think you need extra protection?

author: snarkysparky
author_rank: Frequent Contributor
timestamp: April 08, 2021, 06:52:49 pm
content: Im making a breakout board for the SAMD21. It has 3.3V regulator and SWD debug header. Brings all the IO out to pins.My first versions had some ports go bad. Probably due to handling without any regard for ESD.So I want to put steering diodes on all the IO lines. In the application they can be inputs or outputs.I am not so worried about having a transient powerful enough to drive the supply rails. It is the very high speed glitches that mightaffect program operation and damage the device.My change of plans is to use a BAT30S on every IO pin. Yes... that will be a mess. But I couldn't find any reasonably priced high count schottky arrays with low Vf.Thanks

author: ledtester
author_rank: Super Contributor
timestamp: April 08, 2021, 07:11:00 pm
content: You might want to look at the Ruggeduino -- a ruggedized Arduino -- for some ideas:https://www.rugged-circuits.com/ruggeduino-1

author: snarkysparky
author_rank: Frequent Contributor
timestamp: April 08, 2021, 08:10:11 pm
content: Rugged Duino uses a 5.1V zener on each IO line.My research on the inter nets tells me this is a bad idea. The internal protection diodes inside CPU will begin conducting before the Zener climbs up its knee to shunt off significant current.And the resistor should be on the micro side of the zener.But... I dunno..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 08, 2021, 10:59:22 pm
content: Quote from: nctnico on April 08, 2021, 04:41:32 pmGood lord how much energy are you testing ESD with?A 1uF bypass is enough for single hits of IEC 61000-4-2 at 8/15kV.If more frequent pulses are expected, or sufficient capacitance is unavailable, a single TVS can clamp the main rail while saving cost with all the clamp diodes. Or use clamp-TVS arrays, which are more compact and don't need local bypass caps.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 08, 2021, 11:17:08 pm
content: Quote from: snarkysparky on April 08, 2021, 08:10:11 pmRight, for starters the zener is only functional if the pins are 5V-tolerant, which it seems is not what the OP is concerned about. Clamp diodes are fine, and preferable at lower supply voltages (you can't get <5V TVS that are any better than 5V types), or you need a different technology like snapback diodes, which perform much better at low voltages.Putting the clamp out in front of the MCU, keeps surge/fault currents away from it. A series resistor between them serves to limit peak current from the clamping voltage, which will be elevated (20-30V is a typical figure), so a modest resistance brings it into safe operating limits.Quote from: snarkysparky on April 08, 2021, 06:52:49 pmEh, dual duals are about the best you'll find. Not bad.Schottky aren't great for surge, anyway; they have higher internal resistance than PN diodes. Typically, under surge conditions, the PN guard junction (a PN diode constructed in parallel with the schottky junction, typically around it to improve robustness) carries the majority of the surge current anyway, so the schottky part is just dead weight (and a lot of weight it is, BAT30 is ~14pF vs. BAV99's ~1pF).Quad clamp arrays (and even more), typically with a TVS included, are cheap and plentiful.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: April 08, 2021, 11:20:44 pm
content: Quote from: T3sl4co1l on April 08, 2021, 10:59:22 pmSurge protection? In any case if you connect the wrong voltage on a IO it could blow up the entire circuit.I used to place steering diodes for ADCs and opamps or microcontrolers, when they were placed between different voltage rails. For example 1 opamp on +-12V rail driving the next on the 5V rail. And it could fry s circuit during turn on transients. Unless they had a beefy TVS diode on the voltage rail.

author: thm_w
author_rank: Super Contributor
timestamp: April 08, 2021, 11:43:43 pm
content: Quote from: nctnico on April 08, 2021, 04:41:32 pmYou TVS the supply rail, USB ESD diodes are a good example of this.A TVS on each input is fine but more expensive.

author: woofy
author_rank: Frequent Contributor
timestamp: April 09, 2021, 07:37:45 am
content: Quote from: snarkysparky on April 08, 2021, 06:52:49 pmI don't know of any breakout boards that crank up the BOM like this. Arduino's, Pi Pico, they all just bring the pins out. I think you'd be better looking to improve your ESD handling. Protection on specific pins is fine but as a general rule, its a waste of money.

author: woofy
author_rank: Frequent Contributor
timestamp: April 09, 2021, 07:45:18 am
content: Quote from: snarkysparky on April 01, 2021, 06:30:51 pmThe 0.8v (1.2 max.) is the conducting forward voltage at 15mA.The chips 0.6v the the max voltage beyond the rails you can go without pulling significant current.

author: nctnico
author_rank: Super Contributor
timestamp: April 09, 2021, 01:55:18 pm
content: Quote from: T3sl4co1l on April 08, 2021, 10:59:22 pmNot just ESD but also protection against overvoltage. Nowadays most microcontrollers run at voltages lower than the supply voltage used to supply the circuit.Quote from: thm_w on April 08, 2021, 11:43:43 pmUSB is a special case because there you also want to clamp the incoming supply voltage. For general purpose pins it is a good custom to bias the TVS diodes of such a multi-channel protection device from a higher voltage through a resistor otherwise the supply voltage can still be lifted up considerably. A TVS diode has a reasonably high clamping voltage (hence the series resistor in series with the I/O pin). Other parts of the circuit may not like the power supply being way off (think of microcontrollers which use the supply as the ADC reference; all your measurements will be off).The general setup is: series resistor, protection (clamping & filtering), series resistor and then the input pin. Actually the maximum voltage rating is not so interesting. What is more interesting is the amount of current that may flow into a pin. That tells you the limit of the internal protection diodes inside the chip and to avoid latch-up and so on. The second series resistor should limit the current into the pin with the clamping device at the worst case maximum voltage.