How to read datasheets
Which is the correct use of the ψJT (junction-to-top-of-package thermal characterization parameter) given in a datasheet?
- AAdd power dissipation × ψJT to the ambient temperature to find junction temperature in place of θJA
- BUse it as the thermal resistance when a heatsink is attached to the top of the package, for selecting a heatsink
- CMeasure the package top temperature on the actual equipment and add power dissipation × ψJT to estimate junction temperature
Show answer and explanation
Answer: C. Measure the package top temperature on the actual equipment and add power dissipation × ψJT to estimate junction temperature
ψJT is the difference between junction temperature and top-surface temperature divided by power dissipation, under normal mounting conditions where almost all heat flows somewhere other than the package top (mainly into the board). Because it is not a thermal resistance representing a heat-flow path, it is denoted ψ rather than θ. If the top temperature Tt is measured on the actual equipment with a thermocouple or similar, junction temperature can be estimated as Tj ≈ Tt + P × ψJT. For example, with Tt = 70 °C, P = 1 W and ψJT = 2 °C/W, Tj is about 72 °C. ψJT is a small value, so calculating from the ambient temperature greatly underestimates junction temperature.