EC-24.2 Datasheets, Tolerance and Derating

Work through these

• Absolute maximum against recommended operating: two very different promises

  The first is the level beyond which damage may occur and the second is the range in which the specifications hold. Designing anywhere near the first is a decision to have field failures.

• Typical, minimum and maximum, and which of the three is guaranteed

  Typical figures are frequently unguaranteed and sometimes not even tested, while the limits are what the manufacturer will stand behind. A design that only works at typical values works on some units.

• Tolerance stack-up: worst case, root sum of squares, and which to use

  Adding every tolerance in the same direction is safe and often absurdly pessimistic, while combining them statistically is realistic and occasionally wrong. Which is appropriate depends on how many units you are making and what failure costs.

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• Temperature coefficients, ageing and the drift a specification hides

  A one per cent resistor is one per cent at twenty-five degrees on the day it was made. Over a temperature range and several years it is something else, and the data sheet says how much else.

• Derating voltage, current, power and temperature

  Running a part at a fraction of its rating extends its life by a large factor, and organisations that care publish a policy saying what fraction. Capacitors and semiconductors are the two families where this matters most.

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• Reading the characteristic curves rather than the headline table

  The table gives one operating point and the curves show how the part behaves across its range, which is usually where the design actually sits. The most useful information in a data sheet is often in a graph.