Re: Looking for two-input comparator with hysteresis

On Thu, 25 May 2006 07:57:02 -0700, Jim Thompson wrote:
On Wed, 24 May 2006 22:05:02 -0700, John Larkin
On Wed, 24 May 2006 23:21:16 -0400, krw <krw@xxxxxxxxxx> wrote:
Some day I'll explain ESD diodes to you, if you ask real nice.
...and why you *really* don't want to use them as part of your
circuit, I presume.

OK, why not? I's been a couple of decades since people figured out
CMOS latchup and took suitable precautions. Modern HC parts are spec'd
to handle numbers like 50 mA without problems.

The "suitable precautions" only prevent frying. They don't prevent
copious amounts of substrate current flow... substantially more than
you inserted, due to PNP action.

The "diodes" (they're actually isolation tubs that, with the adjacent
diffusions, can act as bipolar transistors) are only rated for
transient ESD events, NOT for clamping slow signals.

But go ahead and use them as clamps... I certainly don't care if your
circuits up and fry for "magical" reasons.

I NEVER rely on ESD diodes for clamping... VERY BAD engineering
practice.


I've always used diodes, but not for any reason I learned in school - for
one thing, the data *** recommends them, and for the other, every piece
of "real" equipment I've ever worked on, from military avionics to video
games and pinball machines, used diodes.

That is on I/Os that go off-board, so I'm probably not even talking about
the same thing, but in this case, I'd use diodes, just because it feels
more comfortable.

That's my position, and I'm sticking to it! ;-)

Cheers!
Rich


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