Excessive impedance on low current zeners?

Question

Hello Folks,It's the old zener issue but I started a new thread because it's not about sources anymore. Rather, I found that quite a few data sheets specify a really high impedance at the rated zener current. For example, these low current zeners are spec'd at 250uA yet the data sheet says that their impedance at that current is over 1000ohms:That would make the voltage slide all over the place. The only situation where I have encoutered such high impedances is when operating zeners in the knee. My question is: Could this mean that they aren't really low current zeners but just regular ones that have been labeled with a lower voltage and really operate in the knee when run at their rated Izt?-- Regards, Joerg

Jim Thompson · Accepted Answer

Yep, Joerg.  Looks like you're being taken for a ride ;-)                                        ...Jim Thompson-- |  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  |             ||  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  ||                  |    1962     |             I love to cook with wine.      Sometimes I even put it in the food.

Joerg · Answer

Hello Jim,Hmm, that wouldn't be so good. Even larger companies' low current zeners appear mushy on the data sheet. Question to the chip guru: Why don't they make "real" low current zeners? After all, a smaller silicon area would allow a lot more per wafer and thus increase the profit margin. Or maybe there is not enough market anymore :-(-- Regards, Joerg

bill.sloman · Answer

These are all low current, low voltage zener diodes - which s to say real zener diodes, breaking down by the Zener mechanism rather than avalanching.

Their performance is always pretty horrible, particularly when compared with the nice sharp knees you get with avalanche breakdown.

Bandgap voltage sources are a lot nicer in the 1.2V to 5V range.

Joerg · Answer

Hello Bill,That's where I have some doubts, that these are really low current zeners. I couldn't otherwise explain their high impedance.Not so. Why should a properly scaled down zener be worse than a classic zener? Example: My last own design with a 5.6V zener used a 5mA verion. It had 15ohms impedance at that current level. So:5.6V/5mA -> 11001100/15 -> a factor of 73 (meaning a pretty good zener)Low voltage versions:5.6V/250uA -> 2240022400/1400 -> a factor of 16The latter is not such a good zener. Unless it would be a high current zener marketed as low current, IOW in the knee. Clarence Zener would not have like that.5.6V is where the zener and avalanche effects meet.It's a legacy design, can't change it now :-(-- Regards, Joerg

Excessive impedance on low current zeners?

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