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How to read datasheets

Which is a correct caution when using the θJA (junction-to-ambient thermal resistance) given in a datasheet for the thermal design of real equipment?

  1. AIt is determined solely by the package structure, so the value does not change regardless of the board it is mounted on
  2. BThe larger the copper area on the board, the more heat is trapped, so θJA increases
  3. CIt is measured on a test board, so it varies greatly with the actual board's copper area, number of layers and airflow
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Answer: C. It is measured on a test board, so it varies greatly with the actual board's copper area, number of layers and airflow

θJA is measured with the IC mounted on a test board defined by standards such as JEDEC, inside an enclosure with still air. Much of the heat flows from the package through the pins and the exposed pad on the bottom into the board, so θJA depends strongly on the board. Even for the same package, it can differ by nearly a factor of two between a small single-layer board and a multilayer board with many solid planes. Enlarging the copper area or adding airflow makes θJA smaller (easier heat dissipation). Treat the datasheet value as a guide for comparison, and verify by measuring temperature on the actual equipment.

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More How to read datasheets questions

  1. An IC with maximum junction temperature Tj(max) = 125 °C and θJA = 50 °C/W is used at an ambient temperature Ta = 85 °C. What is the upper limit on allowable power dissipation?
  2. When estimating the junction temperature of a power device used with a heatsink, which combination should be used as the junction-to-ambient thermal resistance?
  3. Which is the correct use of the ψJT (junction-to-top-of-package thermal characterization parameter) given in a datasheet?
  4. 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?
  5. A silicon power MOSFET's datasheet gives RDS(on) = 10 mΩ (Tj = 25 °C, VGS = 10 V). Which is the correct approach when using it at around Tj = 125 °C?