Re: Looking for two-input comparator with hysteresis

On Thu, 25 May 2006 07:57:02 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Wed, 24 May 2006 22:05:02 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

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.

John

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.

...Jim Thompson


The only "bad engineering practice" is doing something that doesn't
work. But usually BEP means "something I don't like."

Suppose I set up an HC14 and provide for measuring Icc. Now I force a
range of + and - currents into one input. Things that might happen
are...

1. Some parasitic transistor is turned on and Icc increases.

2. Some parasitic SCR turns on and the whole chip latches.

3. Some other phenom that I can't predict.

4. One of the above, plus the output level gets wrong.

5. None of the above, the diodes act like diodes.


So which will happen? If you'll make predictions, I'll go into the lab
and try it.

John

.

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