What is the relevance of "typical values" in datasheets?

Question

When working on a design I always work with minimum/maximum values from the datasheet (whichever is worst case), never typical values. I was reminded when in another discussion the leakage current for a BAS416 diode came up: 3pA typical, 5nA maximum. That's a factor 1000! In this case I surely would dismiss the 3pA.
What's the relevance of "typical values" in general? Do you use them in a design?

score 6 · Answer 1 · answered Dec 03 '10 at 09:09

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When estimating battery life in a device, I tend to use the typical values. If you use maximum values, you'd end up with a much shorter battery lifetime than would "typically" be achieved. Usually it is the typical battery life that one would be interested in, and not some extremely short time that will never result anyway.

Sure, when you're designing power supplies..etc, I would take max current consumption values for all the devices that it's going to power as a safeguard.

answered Dec 03 '10 at 09:09

cksa361

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@cksa361: "...will never result anyway". I don't agree here. Since they're mentioned as maximum values it means they may occur, you just don't know how often. Maybe some statistic like standard deviation (typical value should be the mean) would help. – stevenvh Dec 03 '10 at 09:14
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@cksa361: re battery life. Suppose you design a handheld device which according to typical values would work for 10 hours on a single battery charge. If the current may be 100 times as large (not even the 1000 like in the BAS416) on some devices the battery will be drained in 6 minutes. Is the device defective? No, you say, it was designed according to the given specs. Yes, I say, your product looses credibility and you'll have to give the customer another one. Which may also be defective. How about 10% of your production?.... – stevenvh Dec 03 '10 at 10:00
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but then if you start working with maximum's it's not useful. Go design a product with a 10 hour battery life based on maximum current ratings. You'd end up compromising on increased price, increased battery size and weight, and a battery life that is typically far more in excess than what the user needs. – cksa361 Dec 03 '10 at 10:05
i think being a perfectionist and overengineering products is counter productive. Take into account typical values because it is what would typically happen (i.e. most of the time). – cksa361 Dec 03 '10 at 10:09
@cksa361: "most of the time". Exactly, like in 90% of the time. Which means 10% rejects, which is unacceptable in any production IMO. Let's see what others have to say about this. – stevenvh Dec 03 '10 at 10:28
The other thing is that the specifications define a device operating envelope that can be much larger than what your design will allow. For example, that diode that you're talking about has a 3pA reverse leakage under normal conditions - at 75V reverse bias. If you're using it in a circuit with a 3.3V regulator, you might not need to worry so much. – W5VO Dec 03 '10 at 19:24
@stevenvh: If a product specifies that leakage will be at most 100uA in a 0 to 70C temperature range, but typically 10uA at 25C, then it's very unlikely that a product which was kept below 40C would draw 90uA. Such a thing could happen in theory, except that most parts would likely leak much more at 70C than at 40C, so parts which would leak 90uA at 40C would leak more than allowed at 40C. That having been said, I do wish manufacturers would write data sheets which could be used without making what would, strictly speaking, be unjustifiable inferences. For example... – supercat Mar 30 '15 at 17:22
...many data sheets will provide specs for how the part will behave at certain precise voltages, but don't make any promises about any voltages beyond those. If a part is rated for 1.8 to 5.5 volts, odds are pretty good that a device's behavior at 4.94V will be closer to its behavior at 5V than to its behavior at 2V, but a lot of data sheets don't actually promise that. – supercat Mar 30 '15 at 17:24

score 4 · Accepted Answer · answered Dec 03 '10 at 11:45

One thing to bear in mind when reading technical datasheets, is they are finalised by marketeers!!! not engineers. If it was up to engineers to publish datasheets you would only have the min/max relevant values and more information about the statiscal variations that you can expect.

Many people are involved with the writing of the datasheets, and ultimately it is people in marketing that have the final say!

So when you read a typical value that is so far away from the min/max values that is simply marketers doing there thing. Generally when doing a parametric search typical values are what are listed and get you in to review the datasheet. Latter on you discover that 3pA is a maximum of 5 nA, sometimes latter in the design process!!

I will generally review the min/max values to really appreciate the range and perform worst case calculations/montecarlo analysis to really work out the expected performance!

Often, the only useful information on the first page is the part number - The rest is marketing mumbo-jumbo. — Kevin Vermeer, Dec 05 '10 at 19:51

score 2 · Answer 3 · answered Dec 09 '10 at 18:53

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You may optimize for the typical case. For example if I need 5 A max but 2 A typical, I will design/buy a 5 A supply but choose the one that is most efficient at 2 A.

answered Dec 09 '10 at 18:53

Brian Carlton

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score 1 · Answer 4 · answered Dec 03 '10 at 16:49

Often when companies spec those values they have not done that much work determining it. I have talked to many companies that their specs are purely off of simulation of what variances they will have in production.

If you are making a million phones, check the spec on what you are buying, determine what is feasible. if you have to trash 1 phone out of 1/1000 because you get a perfect storm of devices being out of spec, that is probably much better than manufacturing problems. If the diode is almost always worst case, get a different diode.

If you are making 3, fab them for typical, allowing some tolerance, and if it ends up having a major variance in one part, replace it with another. If it is mission critical part, check it first.

If over-spec is not an issue for your 3, just spec worst case. You will spend more on components then you need, but you are building three, 10 dollars only makes 30. No real cost lost. If you are making millions, you need to confirm conformance yourself.

Someone downvoted me. I could see many reasons someone would disagree with me, although I would like to know which one they chose. — Kortuk, Dec 04 '10 at 23:34

score 0 · Answer 5 · answered Oct 20 '12 at 00:28

The word typical is "content free". Real statistical estimates require at least a range, and a confidence. Better yet, a density function.

For instance, quantity X falls between 3.7 and 4.3, with 95% confidence means something, whereas quantity X is typically 4.0 is absurd. What does that mean? What is the probability that X lies between, say, 3.999 and 4.001?

If there is any earnestness in such a "typical" claim, the interpretation should be like this. Since the 4.0 is given as two significant figures, it means that there is some high confidence (like 95%: two standard deviations, or 99.7%: three standard deviations) that it will not fall bellow 3.95, which would cause it to be rounded down to 3.9, nor rise to or above 4.05, which will cause it to be rounded off to 4.1.

That is to say, if, say, 95% or more of the time a parameter measurement, when rounded to two significant figures, does not show 3.9 or less, nor 4.1 or greater, then we have justification in claiming that it is "typically 4.0" (but not necessarily that it is "typically 4.00").

I don't know of any source of assurance that datasheets apply this sort of standard to "typically".

What is the relevance of "typical values" in datasheets?

5 Answers5