Matrix user forums

I need to calculate the value of the current limiting resistor for an I/O pin on a PIC16LF1936 but, I can't seem to find the rating for the internal ESD diodes in any document. My working voltage is 18vdc (measured) and i need to connect that directly to a pin configured as digital input thru a current limiting resistor and I don't want to blow one of those internal diodes. Anyone know what the rating is or where I can find it?

Hi Kenrix2,
If it was me, I would limit the current to about 100uA.
Or use a potential divider to reduce voltage down to say 4.5V
I have found this link on interfacing which may or may not help:
http://cq.cx/interface.pl

Martin

Thank you for your response medelec35. I finally did locate the rating after changing my search to "clamping diodes" instead of "ESD diodes". The last sentence on page 2 of AN521. It is +-500ua. So your value of 100ua is well under the max rating. Good call.

Just came across this.

Martin is quite right of-course, as there's maximum 'withstand' and the more unpredictable 'latch-up current', which have bearings in different ways. It's therefore wisest to keep OVP/ESD diode current as low as comfortably possible.

For future information, the absolute maximum rated clamp current however (+/- 20mA) can be found in the Electrical Specifications in the following PIC datasheet on page 357.

http://ww1.microchip.com/downloads/en/D ... 41364C.pdf


All the best,

Brendan

Hi Brendan,
Thanks for the useful information.
The reason I limit current to uA instead of mA is because if you limit to uA when connected to mains, then transient spikes should not be an issue.
If you limit to mA and connect to the mains then even though normally current will but OK, any transient spike could take the current over the maximum permitted for ESD protection.

IMPORTANT: Do not connect to mains unless your very competent, since mains can and does kill!

If only connecting to a much lower voltage that does not suffer form spikes then should not be too much of an issue.
Just make sure that fridge/freezer, fluorescent & street lighting etc does not cause spikes that induce a voltage which will cause clamp current to exceed maximum specifications.

Martin

Indeed Sir!... We concur

http://www.matrixmultimedia.com/mmforum ... 46&t=12235


All the best,

Brendan

Morning,

Just seen this post whilst browsing forums ,

Have you seen these ?

http://www.maximintegrated.com/app-note ... mvp/id/287

Not sure if they are any use to you or not

Steve

Thanks for the links Brendan and Steve,
any addition information is always useful.

Martin

Thanks for that very informative link Steve001. I never considered the situation where the over voltage might not have a path to go to. I'll be adding the recommended zener diode (in my case a 3.6v Zener) across the micro's supply rail.