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LP38853: RθJC(bot) of TO220

Yuval Minster
Intellectual 800 points
Part Number: LP38853

Hi,

I use the  LP38853, TO220 package in one of my designs. This is an old design.

When I used it, the RθJC(bot) in the datasheet of the TO220 package was 3 C/W.

Now I saw that there is a new datasheet but in the new datasheet, the RθJC(bot) is NA.

Any reason why it was changed to NA ?

Do you know what should be the RθJC(bot) value?

I am attaching the 2 datasheets (old and new)TI LP38853.pdfTI LP38853 new.pdf

Yuval

  • 0
    TI__Genius 13205 points

    Hi Yuval,

    I'm unsure why the RθJC(bot) value was removed, we do update our thermal models occasionally so it's not abnormal for them to change slightly with the better models. However for RθJC(bot) 3 C/W seems higher than I'd expect, when I look at other LDOs in the TO-220 package it is it is usually below 1 C/W. 

    May I ask what you are using the RθJC(bot) value for? This thermal metric can be hard to use since the bottom is usually next to the PCB making measurements of the bottom case temperature pretty difficult/impossible.

  • 0 in reply to Kyle Van Renterghem
    Intellectual 800 points

    Hi,

    This is  the way I am cooling the device, from the bottom..

    I need to make calculate\ions to make sure that the I can cool the device with the power it consumes.

    Yuval 

  • 0 in reply to Yuval Minster
    TI__Genius 13205 points

    Hi Yuval,

    If the bottom of the device will be attached to the PCB, then the RθJA or RθJB/ΨJB thermal metrics can be used since the PCB is acting as your heat sink. It is less common to use RθJC(bot) for this since you would still need to be able to determine/estimate how much thermal resistance there is through your solder and then how much thermal resistance there is through the PCB to either the ambient air or a known point with a specific temperature on your board (it could probably be done with an in-depth thermal modeling tool but difficult to do by hand). 

    So my recommendation is if you know the PCB temperature right next to the device you can use RθJB/ΨJB, if you don't have that information and plan to base your calculations off the ambient air temperature then I'd recommend using RθJA but it's important to note that RθJA  is very dependent on the PCB layout and how much metal is connected to and around the hot IC. What is published in the datasheet assumes the JEDEC standard High-K board (2S2P), but your PCB layout may be better or worse than the JEDEC standard. We have an App Note comparing the actual RθJA of various layouts which you could use to get a better estimate for your PCB (the results for the TO-252 would be the most applicable to your use case). 

  • 0 in reply to Kyle Van Renterghem
    Intellectual 800 points

    HI,

    I am not cooling the device with the PCB but with a water cooling system.

    Do you know what is the Qjc for this device. It will help me to calculate the Pd of the device. 

    Thanks,

    Yuval

  • 0 in reply to Yuval Minster
    TI__Genius 13205 points

    Hey Yuval,

    Ah I see, yes that would be the right thermal metric to use for your use case. The package hasn't actually changed so I would continue to use what was originally published in the older version of the datasheet which was 3C/W.