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Absolute Max

Discussion in 'Electronic Design' started by Jon Slaughter, Jan 21, 2009.

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  1. What are the suggested derating from the absolute max? How are those values
    calculated? Is that essetially the threshold for failure for reduced
    lifespan by some factor or what?

    If 30mA is the absolute max for an LED, will running it at 25mA max(lets
    assume that we know for a fact that it will not ever get > 25mA) be ok? (the
    devices in question don't give a typical operation on their datasheet)

    I've heard people say that you should stay at most 50% of the absolute
    values. Not sure where this value comes from and how the absolute max is
    computed. The absolute max you might want to drive my card on the highway is
    75mph but you wouldn't then drive around at 37.5mph.
     
  2. There is no simple rule. It will usually depend on the aging/failure
    mechinism.

    With leds higher current means more heat and shorter life. For most
    applications leds are more than reliable enough so you can push near
    the limit.

    On the other hand I like a nice margin on a fets safe operating area.
    Otherwise some unexpected glitch blows them up.

    Standard electrolytics are normally specified with 2000 hours life at
    85C and this doubles for every 10C drop in temperature. Reducing V
    and ripple also improves life.

    TTL is specified at something like 5.5V max. It's designed to run at
    5V, you can't have a 50% margin.
     
  3. That sounds right, long time since I've used TTL. But I wouldn't run
    it at 50% of 7V.
     

  4. I wonder if anyone has written a book on such topics? Would be nice to have
    some reference.
     
  5. Jon Kirwan

    Jon Kirwan Guest

    Theory and experimental result of failure mechanisms are discussed in
    a book by HP, "Optoelectronics Fiber-Optics Applications Manual," (2nd
    ed.) These include flexing of the bond/wires, if memory serves. But
    there is an entire chapter on the subject there. It's a little old,
    but I suspect that although there may be some new additions to the
    list the ones listed there still remain in some fashion.

    Jon
     
  6. Thanks, I'll look that up.
     
  7. Guest

    Generally the abs max compress from the process/reliability people,
    not design. In other words, you are given a blah blah blah voltage
    process in which to design. Now you could do something stupid and
    achieve less than absmax.

    It's a little worse than I describe since the absmax isn't an over
    temperature spec. Worse yet, it varies from semi to semi. I've worked
    at companies where they would check functionality at abs max, which I
    always thought was dumb. That is, why stress the part. [The wafer
    itself had it's test pattern flogged to insure devices are working at
    absmax, no diodes are breaking down at lower than spec voltages, etc.)
    Some do a QA test at absmax, rather than stress every part. Some just
    do a lab test to look for obvious design error that make the part not
    function at absmax.
     
  8. Jon Kirwan

    Jon Kirwan Guest

    The book should be fairly easy to find. I think HP put out a bunch of
    them at one point (gave them away, even.) 1981, I think, is the pub
    date. I'd be willing to try and copy out parts of that section for
    you as a personal matter but I suspect you can get your own copy
    without much fuss or money. It will definitely give you some things
    to follow up on.

    Jon
     
  9. rex

    rex Guest

    OK, with proper safety precautions, no doubt.

    One criterion is how much the part costs or how much you can afford to
    spend.

    Murphy says that if the part is really expensive, you will probably
    perform the "to destruction" experiment even if you didn't want to.
     
  10. I've seen them online for 40$+ bucks but I thought I saw it used for 4$
    somewhere ;/
     
  11. Jon Kirwan

    Jon Kirwan Guest

    Try:
    http://www.alibris.com/

    I think I saw some cheap ones there. Conditions were "good." They
    also seem to list a couple of pristine copies, but _expensively_.

    Their standard shipping rate is $4 (a penny less) for all sources of
    books so a $4 book will set you back a real $8.

    Jon
     
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