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LM317A: Inquiry regarding thermal resistance difference between National Semi and TI part of LM317AMDT/NOPB

Iyappan Rajendran
Prodigy 30 points

Part Number: LM317A

Dear TI,

I can see the difference in thermal resitance (Juction to case and Juction to ambient) between National semi and Texas Instruments of LM317AMDT/NOPB.

As per National Semi datasheet the thermal resistance Juction to case is 12(°C/W) and Juction to ambient is 103 (°C/W), but as per TI datasheet the thermal resistance Juction to case is 51.3 (Top), 0.9 (Bottom) (°C/W) and Juction to ambient is 54 (°C/W).

could you please clarify how it is possible to get different thermal resistance value (Nationa Semi vs TI) for same package  LM317AMDT/NOPB? 

Screenshot TI vs National Semi 2025-11-03 211432.png

Regards,

Iyappan R

  • 0
    TI__Genius 9420 points

    Hi Iyappan,

    Thank you for asking us!

    While die size and lead frame changes can sometimes significantly alter thermal performance while remaining drop-in equivalent with existing devices, I think in this case it is due to a difference in specification between the National Semiconductor era part and the current TI one. 

    In our archived version of the datasheet that matches the E.C. there is this table/chart at the bottom:

    TI's thermal metrics are calculated in simulation for a JESD51-7 standard board. This has 4 layers, and the planes are 2oz 3inch x 3inch, or  9in^2. Looking at the actual thermal data in the datasheet, the 54'C is close to the asymptotic limit of the real θJA line, and the old 103'C thermal metric is the same as the minimal copper area at the top of the table.

    It this case it is reasonable to explain the difference as National Semiconductor measuring a value against a thermally poor board with little copper, while the current TI metric is calculated against a board with a very large copper area, and the large change in number reflects the very real thermal performance difference with wildly different copper areas.

    θJA is a system level metric and ultimately depends on board design. θJC exhibits a dependence on large changes in parallel thermal conduction paths.

    Best,

    Gregory Thompson