12Derating and Uprating
Derating is the practice of limiting thermal, electrical, and mechanical “stresses” to levels below the manufacturer's specified ratings, to improve reliability. Derating allows added protection from anomalies unforeseen by the designer (e.g., transient loads and electrical surge).
12.1 Part Ratings
Ratings set by manufacturers of parts and subsystems on their environmental and operational limits affect the decision-making by the part users and equipment manufacturers. This section explains the ratings with the examples of electronic parts.
Part datasheets provide two types of ratings: absolute maximum ratings and recommended operating conditions. In general:
- Absolute maximum ratings (AMR) are provided as a limit for the “reliable” use of parts.
- Recommended operating conditions (ROC) are the conditions within which electrical functionality and specifications given with the part datasheet are guaranteed.
Intel (1995) considers the difference between absolute and maximum ratings as guidance to users on to how much variation from the recommended ratings can be tolerated without damage to the part. Motorola (Boyle v. United Technologies Corp. 1988) states that, when a part is operated between the ROC and AMR, it is not guaranteed to meet any electrical specifications on the datasheet, but the physical failure or adverse effects on reliability are not expected. Motorola notes margins of safety are added to the absolute maximum ratings to ensure the recommended ...
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