ESD Diodes within PIC Microcontrollers ratings and specs - Page 1
Forum section: electronics / projects

author: PICmonster
author_rank: Regular Contributor
timestamp: October 07, 2014, 02:20:11 pm
content: Dear AllA while back I posted a question but I guess I did not word it very well as I did not get a proper answer to it. So I will give it another bash.I am looking for documentation about ESD diodes used within popular PIC microchip microcontrollers, this documentation should detail what PIC has what ESD protection and the current ratings of the ESD diodes would be nice.I am looking for documentation to backup a design with documentation rather than just assuming all will be well, as that would not be good enough for the bossThe standard datasheets do not specify or show any in-depth technical details such as current ratings of ESD diodes (analogue ports do show ESD diodes but that about it). I am wanting to use these as simple voltage clamps on digital inputs via high value resistor to keep the current low, there are some old application notes from 2002 that use this principle below.http://ww1.microchip.com/downloads/en/AppNotes/00521c.pdfDoes this application note app;lay to ALL PICs today, my guess is not so I am seeking the details for 24FJ series PICsIf I had a document that say ALL PICs have these ESD diodes on all ports pins and they have current rating of x mA, that would be fabulous.Phew.Jim

author: lapm
author_rank: Frequent Contributor
timestamp: October 07, 2014, 02:47:13 pm
content: Basically you should design your circuit in way you don't need ever to use those inbuilt diodes.. They are last resort sort of thing when everything else fails. Since they are on die of microcontroller i dough you can drive any significant current throw them.I dough any manufactures makes guaranties how much those protection diodes can handle, since some people would then go and design circuit that abuses those diodes to max....

author: Rufus
author_rank: Super Contributor
timestamp: October 07, 2014, 03:00:39 pm
content: Quote from: PICmonster on October 07, 2014, 02:20:11 pmMost PIC datasheets have input and output clamp abs max current ratings. 3v parts with 5v tolerant pins generally don't and the +ve clamping mechanism on 5v tolerant pins isn't documented.Almost no PICs are specified to operate correctly with voltage on pins which would allow any clamp current to flow.A few PICs do have a maximum injection current rating (PIC18F66K80 for example) and tell you any injection current may screw up ADC operation.For the majority of PICs you are not going to get any assurance from Microchip that they will operate correctly with any clamp current flowing - you are on your own. The first thing to get messed up will be analog functions including oscillators and voltage brown out detectors.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 07, 2014, 06:36:49 pm
content: In many CMOS chips, the IO "protection diodes" are nothing more than the NMOS body-diode in the output drivers and can clamp current up to about the same magnitude as the drivers can output. In those cases, you would be looking at clamping current in the neighborhood of 20mA.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 07, 2014, 06:48:54 pm
content: Had a strange issue on one of my senior design boards where the offset on one of the internal ADCs (of a dsPIC33) seemed to vary with the USB supply voltage, despite it running from a LDO. Turns out I had a 5V I/O from the host Arduino going to one of the 3.3V I/Os on the dsPIC through a resistor, causing injection current that was interfering with the ADC. Solution was to add another resistor to make a voltage divider.

author: Niklas
author_rank: Frequent Contributor
timestamp: October 07, 2014, 06:54:42 pm
content: Clamping diodes would be a more proper description than ESD protection diodes. We know nothing about the speed on the built in diodes and it is probably not guaranteed from batch to batch. If the diodes are too slow, then the actual voltage on the "protected" pin can exceed the expected clamping voltage. TVS diodes can have response times measured in ps.For slow signals the built in diodes might do the job together with a series resistor but not as the primary protection for ESD.

author: LukeW
author_rank: Frequent Contributor
timestamp: October 07, 2014, 08:34:43 pm
content: Atmel's AVR182 application note shows some interesting "abuse" of the protection diodes for a practical application... although it's not a PIC, obviously.www.atmel.com/images/doc2508.pdf