The maximum power a package may dissipate, written Pd. It is set by the maximum junction temperature and the thermal resistance, and above a certain ambient temperature (often 25 °C) it must be reduced in proportion to temperature.
Japanese name: 許容損失
Allowable power dissipation appears in 2 practice questions across 1 topic, including How to read datasheets.
Field: How to read datasheets
Answer: A. 0.8 W
Junction temperature is given by Tj = Ta + P × θJA. Solving for P gives P = (Tj(max) − Ta)/θJA = (125 − 85)/50 = 0.8 W. The higher the ambient temperature, the smaller the allowable temperature rise, and so the smaller the allowable power. 2.5 W is the mistake of forgetting to subtract the ambient temperature (125/50), and 1.7 W is the mistake of computing 85/50. In practice, considering that θJA varies with the board and allowing derating margin, design to keep power below the calculated value.
Answer: A. 0.6 W
It decreases by 1.0 W over the 125 °C span from 25 °C to 150 °C, so the slope is 1.0 W ÷ 125 °C = 8 mW/°C. 75 °C is 50 °C above 25 °C, so the reduction is 50 × 8 mW = 0.4 W, and the allowable dissipation is 1.0 − 0.4 = 0.6 W. 0.4 W is the reduction itself, and 0.5 W is the mistake of a proportional calculation starting from 0 °C. This curve is determined by the maximum junction temperature of 150 °C and θJA (here 125 °C/W); note also that, like θJA, it is a value for a test board.
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