I have two datasheets for the same TL780–05C regulator one from Texas Instruments and another from MOTOROLA. I'm looking at the data for the TO-220 packages.
I need to estimate the total junction temperature without a heat-sink, and for that I will use the following formula:
Since I calculated the power loss P and know the max ambient temperature TA; what I need now is the parameter called θJA which is the thermal resistance between the junction temperature TJ and the ambient temperature TA.
Almost all basic parameters like the max output current, max junction temperature ect. are exactly the same. The inner circuit representative schematic diagrams look a bit different.
Now for the one from MOTOROLA the temperature parameters are given as:
And here is the temperature section from Texas Instruments:
My question is: These two regulators have exactly the same names where normally I would think all their critical parameters are very close. But the θJA differs a lot; one is given as 62.5°C/W and the other is as 19°C/W. This is a ratio of more than three. Is this something expected and usual or am I interpenetrating something wrong here?
(It is only by accident I came across this otherwise I would take granted that the θJA would be the same value no matter who the manufacturer is)
Best Answer
Notice the TI table has a "Board" column, specifying the part is tested when mounted on the JESD 51-5 high-K (high thermal conductivity) test board.
This board provides conductive cooling through the lead frame, reducing the thermal resistance \$\theta_{JA}\$ compared to a test done with the part resting in still air or mounted on a board with minimal thermal conductivity.
Most likely the Motorola values are based on tests done without the test board, probably before JESD even standardized thermal test boards.
TI has an app note here and an application report here giving some further details about the test board, but nothing specific to how they test the TO-220 package.