Source for low current zener in China?

You cannot depend on mfg data sheets...you need to do a reality check with anactual part.

If you do not mind using an oddball part in an un-documented and unspeced manner, try the SOT-363 package dual PNP by Rohm: the UMT1NTN; use the E-B junction as a zener (somewhere in the 7-8V region). Carried by DigiKey. If i remember correctly, it is has a well-behaved knee in the nanoamp region and has no negative resistance or oscillations to somewhere in the milliamp region.

Look at Fig. 10 though. It shows the dV/dI slope of a 5v6 device being about 2000 ohms at 50uA, dropping to 350 ohms at 250uA.

On Wed, 27 Sep 2006 17:13:02 GMT, Joerg wrote in Msg.

So how come this thing has worked in the past? (You said it was a legacy design so I assume it has been successfully in production for a while.) Were those real "low-current zeners" that all of a sudden have gone to the land of unobtainium, or did the finished product work only by sheer luck in the high-impedance knee region of whatever diodes were designed in back then?

robert

A good way to evaluate the relative die size of two zeners is to look at their capacitance. Figure 7 of ON Semi's '4690 datasheet shows about 110pF at 1V bias. This matches the 100pF value in figure 10 of the 1n5232 datasheet. The '5232 is the classic 500mW 20mA axial-lead zener, which has notoriously-poor regulation characteristics at low currents. They also have identical peak-power ratings at 1ms.

We can speculate that these two zener families have similar, if not identical, dies and are made the same way. However, they should be graded at their respective test currents.

Hello Robert,

You cannot rely on either alone. The guaranteed values in a data sheet is what's important for production. Testing supplements that but if you test, say, in September then the die for December might come from a wafer where the process had veered substantially. That's what seems to have happened in this case (and the vendor is surprisingly non-responsive about it...).

Hello Win,

Many data sheets omit capacitance :-(

But when you measure they seem to be all the same :-(((

That's exactly what seems to be the problem, a "one size fits all" approach. Doesn't work in real circuits as much as it doesn't work for socks and T-shirts. What they often (maybe always?) do is tune the process so they get the right Vz range at the current they want it to be, regardless of how far it's into the knee.

Maybe the market for low current zener isn't sufficient. Otherwise I wouldn't understand why they would use the same die size and get only

Hello Robert,

Well over a decade without problems.

Not by luck. The design is within data sheet limits. The low cutoff is what matters. Data sheet says it's between 5.32V and 5.88V at 50uA. We run at 200uA and had a lot of them below our cutoff limit of 5.1V. Oops! So I wrote, asking about whether it could be a lot glitch. No call back so far...

Hello Robert,

Have test results on 50 samples now. However the behavior of samples from these two lots is not enough for a release. We need hard data the mfg stands by. Which I got yesterday from one and may be getting next week from another that would then become the 2nd source.

Ok, I guess they deserve credit here now: Centralsemi did exactly that. They were very open with me about process details and I appreciate that.

Thnaks. However, 7V to 8V wouldn't work here and a new package means a re-layout. In that case we'd do it right and move to the TLV431.

Face it, and do it. Otherwise the next lot will be out of the "spec" you need, and the client will be unhappy. Can't you make a little "teensy-weeny" daughter board in there with a TLV431 ?:-)

...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 | |

Hello Jim,

It's already on the suggestion list for that client. Depends on how long this design is going to remain in production. Daughterboards aren't really feasible because it's small and there is more than one zener. Plus it is automated mass production.

Out of spec on this one only happened once. I am still wondering how it could happen, considering that our spec is way more loose than what the mfg states in the min-max table. However, with smaller suppliers you can negotiate a deal where they make sure that every shipped reel is within spec. The small ones really care. I guess they know the old saying in sales "If you don't take take of the little guy, someone else will". And today's little guys have a habit of becoming tomorrow's big guys :-)