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From: Winfield Hill
Subject: Re: 1.6V zener diodes?
Date: 17 Oct 2002 04:07:55 -0700
Organization: Rowland Institute
References: <firstname.lastname@example.org> <email@example.com> <10Pq9.508$Vv3.firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
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> Tom Bruhns wrote:
>> email@example.com (carltons) wrote
>>> All of the suggestions other than the use of a zener of any sort sound
>>> good to me. A Zener is okay for ESD protection as a last resort, but
>>> other than that, it has gone the way of the 6AL5 tube diode. ...
>> I'd say you're under-rating the zener's usefulness for things other
>> than ESD protection and noise generation (and perhaps even over-rating
>> it for those). There are some things they do very well for low cost
>> which would be quite difficult to do other ways. ;-) (Though we
>> haven't seen any confirmation from the OP, his application is likely
>> not one of the good ones, of course!)
> Coming from the world of low voltage portable equipment, I'm not a big fan
> of shunt regulation. There are also many ways to get better performance
> these days. Why use a zener? When I was doing ic design, suggesting a
> zener would have gotten you walked out the door immediately. Also, zeners
> have really lousy junction capacitance which made them perfect for esd on
> portable radios as they didn't rectify the rf past about 1 MHz. All our
> radios were 30 MHz and above.
You're right Steve, the use of zener (field-emission breakdown) technology
is not recommended for low-voltage design. But don't trash zener diodes
for all the "high-voltage" (over 5 volts) folks. High-voltage zeners
(avalanche technology) work very well in a wide variety of tasks, often
much better or at least more efficiently than the alternates. And well-
designed zener diodes in the 5.6 to 6.8-volt region (field-emission plus
avalanche) are very excellent performers indeed, featuring low noise,
low-resistance, and nearly zero temperature coefficient. Nice stuff.
You mention IC designers eschewing zeners? Low-voltage ICs often have an
internal zener across the supply to make sure they stay at low voltages.
:-) Certainly zeners are very common in ICs that operate from 8V or more.
They're often seen in current-source bias-startup circuitry, or to limit
the voltage at some point. Finally, don't forget that the top-performing
voltage-reference ICs employ buried-zener technology, because it's quieter
than the competing cheap band-gap stuff. Zener voltage-reference ICs even
outperform Analog Devices' amazing new proprietary XFET technology. E.g.,
compare NSC's LM399 or LTC's LT1000 and LT1021 with Analog's ADR421.
LTZ1000A LM399A LT1021C ADR421BR LT1019A
zener zener zener XFET bandgap
------ ------- ------- ------- -------
accuracy 4% 2% 0.05% 0.04% 0.05% max
tempco 0.05ppm 0.5ppm 5ppm 3ppm 5ppm/C max
noise 0.17ppm - 0.6ppm 0.7ppm 2.5ppm p-p typ, 0.1 to 10Hz
drift 0.3ppm 20ppm 15ppm 50ppm est 50ppm per 1000 hours
That old chestnut, "The right part for the right task," insures that zener
diodes will continue to have an important role in electronics. Some may
think that in the future few ICs will operate above 1.8 or 2.7 volts, etc.,
but I seriously doubt it. :-)
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