From: Jim Thompson
Subject: Re: isolated ckts for LM339 quad comp
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Date: Wed, 30 Oct 2002 18:34:24 GMT
NNTP-Posting-Date: Wed, 30 Oct 2002 13:34:24 EST
Organization: Cox Communications
On Wed, 30 Oct 2002 10:13:22 -0800,
John Larkin ,
In Newsgroup: sci.electronics.design,
Entitled: "Re: isolated ckts for LM339 quad comp",
Wrote the following:
|On Wed, 30 Oct 2002 16:15:52 GMT, "Dan"
|>I want to use LM339's in an automotive app. Can I have the inputs (+ and -)
|>on a separate 5vdc ckt which is isolated from the normal 12vdc which powers
|>the chip (VDD, VSS) and supplies the +12vdc pullups for the output? BTW, I
|>am using TVS's and decoupling caps to suppress transients and the +5 ckt is
|>on the output side of a 12v->5v dc-dc converter. See my previous post: "can
|>someone recommend a flyback zener diode value..." and thanks to all who
|******DO*****NOT***** (sorry for shouting) *******EVER******* let any
|of the LM339 inputs go below the chip ground pin, even by a few
|hundred millivolts. It will make the entire chip go haywire.
|Nasty gadget, overall.
That's a wee bit of an extreme recommendation.
Each input can safely go almost a vBE below ground before there *may*
be excessive current drawn... protect by using a finite source
As the negative input gets near a vBE below ground the output phase
will flip. That can be a problem in digital loops.
I used to use Ge diodes to ground to avoid such problems.
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
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