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LM137QML-SP: How the PDmax @25oC is calculated as 2.5W?

Kazuhiko Hirano
Genius 13428 points
Part Number: LM137QML-SP

Tool/software:

Hello,

The datasheet 6.1 states as follows.
PDmax = (TJmax - TA)/RθJA, and TJmax = 150oC.
PDmax at TA=25oC for T0-39 = 2.5W.
Here, RθJA is calculated as follows.
RθJA = (TJmax - TA)/PDmax = (150 – 25)/2.5 = 50 (oC/W)

However, the datasheet 6.4 states RθJA is 174(oC/W) (still air @0.5W), and 64 (oC/W) (500 LFM @0.5W), so both are different from 50(oC/W) calculated above.
How the PDmax @25oC is calculated as 2.5W?

Best regards,

K.Hirano

  • +1
    TI__Genius 9430 points

    Hi Hirano-san,

    The thermal metrics are not precise values since they are influenced by the layout and physical properties of the board that the device is mounted onto - they are better thought of as estimates. Thermal metrics are mainly useful as a comparison between different packages since the tests are standardized.

    The calculated RθJA is relatively close to the 500 LFM moving air RθJA (considering that the thermal metrics are not precise). It seems likely to me that this is the mismatch between the thermal model value and a physical test board. Keep in mind that the equation is likely a simplification of the device's heat dissipation since heat is also being dissipated by other means besides just the atmosphere (such as sinking heat into the board).

    Thanks,

    Sarah