How to read datasheets
A package's allowable power dissipation is 1.0 W up to an ambient temperature of 25 °C, and above that it decreases linearly to 0 W at 150 °C. What is the allowable power dissipation at an ambient temperature of 75 °C?
- A0.6 W
- B0.4 W
- C0.5 W
Show answer and explanation
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.