Integrated Circuit Derating Factor
Here are four of the most common design aspects to consider when derating an IC:
Derate the Fan-Out of an IC to 80% of the possible maximum fanout [if applicable].
Derate the Open Collector output voltage to 80%; OC resistor calculation [if applicable].
Derate the Maximum clock frequency to 85% of maximum allowable frequency.
Maximum junction temperature is limited to 1000C [Recommendation]
When possible use the smallest available power supply the device will accept.
Remember these are recommendations and are based on the engineering organization your design reports to. In this case the guidelines come from NASA and may seem a bit strict. Of course there could be a trade off between added reliability and increased complexity.
For example why add another IC gate to increase fan-out to what may be required when the 80% maximum fanout is an arbitrary number. Violating the derating guidelines in favor of reduced complexity is a trade-off that would need to be discussed at the design review.
Design Example; Some Integrated circuits draw so little current that their Junction temperature is assumed to be the Ambient air temperature surrounding the device, when this is the case there would be no derating required up to the maximum operating temperature of the component. However; the guideline stated above would still apply recommending to keep the junction temperature at or below 100C.
As an example the TPS3808 Supervisory Circuit may operate from -40C to +125C, but the derating factor would limit the maximum operational temperature to 100C. This insures that the junction temperature of the device is never exceeded, allowing for 25C margin of safety. In fact in this case the absolute maximum junction temperature allowed is 150C, so there would be a total of 50C of head room before the device was damaged.
How to Derate other components; Guideline for Derating Electronic Components.
Keep in mind that although manufacturers guarantee that their devices meet the temperature ranges specified in the data sheet, they still may recommend some kind of forced air cooling. The recommendation may be in the data sheet or it may reside in an application note, but in either case the junction temperature needs to be controlled as far below the maximum listed in the data sheet as possible.
Recommendations for Linear ICs are listed under Op Amp Derating.