EC-24.2 Datasheets, Tolerance and Derating

What this is and why it exists

A data sheet is a legal document as much as a technical one.

Reading it the way a student reads a textbook produces designs that work on the bench and fail in the field. The numbers are promises with conditions attached, and the conditions are the part beginners skip.

Three habits come out of this topic. Never design at an absolute maximum. Always ask over what temperature and voltage range a figure holds. Always propagate tolerances rather than assuming nominal values.

The vocabulary

• Absolute maximum rating — the level beyond which damage may occur.
• Recommended operating conditions — the range in which the specifications hold.
• Typical value — a representative figure, often untested and unguaranteed.
• Guaranteed limit — a figure the manufacturer will stand behind.
• Tolerance — the permitted deviation of a component from its nominal value.
• Stack-up — the combined effect of several tolerances in one circuit.
• Temperature coefficient — how much a parameter changes per degree.
• Derating — deliberately using a part below its rating to extend life.

The mental model

The two tables at the front of a data sheet mean very different things. Absolute maximum ratings say where damage begins. Recommended operating conditions say where the specifications apply. Designing near the first is a decision to have field failures, and the gap between the two is not spare margin.

Inside the specification table, not every number is a promise. Typical figures are often untested, and sometimes derived from a simulation. The minimum and maximum columns are what the manufacturer will honour. A design that only works at typical values works on some units, which is worse than a design that fails on all of them.

Tolerance stack-up is the arithmetic nobody enjoys. Adding every tolerance in the worst direction is safe and often absurdly pessimistic. Combining them statistically is realistic and occasionally wrong. Which is appropriate depends on how many units you are making and what a failure costs. Stating which you used is part of the design record.

Drift is the tolerance you forgot. 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. The data sheet says how much else, usually in a graph rather than the table.

Derating is the cheapest reliability improvement available. Running a part at a fraction of its rating extends its life by a large factor. Organisations that care publish a policy saying what fraction. Capacitors and semiconductors are where this matters most, and the aluminium capacitor is the classic example.

Finally, the most useful information in a data sheet is often in a graph. The table gives one operating point; the curves show behaviour across the range where your design actually sits.

What you should now be able to explain or do

• Distinguish absolute maximum ratings from recommended operating conditions.
• Say which figures in a specification table are guaranteed and which are not.
• Perform a tolerance stack-up and justify the method you chose.
• Account for temperature coefficient and ageing in a design margin.
• Apply a derating policy and say what it buys.
• Extract a design figure from a characteristic curve rather than the table.

Check yourself