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Original question:

TPS7A8300: Thermal Performance

TPS7A8300: Which thermal metric to use?

Izik
Expert 1545 points
Part Number: TPS7A8300

Hi 

We are running thermal analysis on a board with multiple LDO's and other components. 

We are not sure which metric to use when trying to calculate the TPS7A8300 junction temperature. The application note SPRA953 is not clear. 

According to the thermal simulation, we know the PCB temperature on the ground exposed pad footprint just below the LDO. Which metric should we use  in order to calculate the junction temp, junction to case or junction to board?

Thanks

  • 0
    TI__Genius 13085 points

    Hi Izik,

    If you know the PCB temp at the exposed thermal pad then the best thermal metric to use would be ψjb (Junction-to-board characterization parameter). It is better to use the characterization parameter since the θjb (Junction-to-board thermal resistance) thermal metric assumes all of the heat transfer is occurring through the exposed pad. On a real PCB most of the heat will flow through the exposed pad but technically there will be some heat transfer through the top of the package as well. 

    For this device the difference between the characterization parameter and the thermal resistance is minor, but I thought it would be good to clarify the concept since they are not always the same.  

  • 0 in reply to Kyle Van Renterghem
    Expert 1545 points

    The junction to board parameter is ~12 times more than the junction to case parameter... What would be the case to use junction to case parameter?

    Thanks

  • 0 in reply to Izik
    TI__Genius 13085 points

    Hey Izik,

    I don't find the junction to case thermal resistance very useful for calculating the actual temperatures of things since it is impossible to measure the bottom of the case on a physical PCB and thermal sims will usually tell you the PCB temperature but there will be a difference between the PCB temp and the case temperature based on the thermal resistance between those two points (solder and PCB resistance to the measurement point).

    The thing that the junction to case thermal resistance can be useful for is when you are comparing two devices' thermal performance because it tells you that if the PCB layout is the same the device with the lower junction to case thermal resistance will have a lower junction temperatur. You might ask why not use the junction to ambient thermal resistance, but this value is VERY dependent on the PCB layout and some companies report values which do not adhere to the JEDEC standard or two companies may intereperet the JEDEC standard slightly differently so it may not accurately represent the real world difference between the two devices on your PCB. 

    Below is the thermal information table for easy reference, for others who might come across this thread in the future.  

  • 0 in reply to Kyle Van Renterghem
    Expert 1545 points

    Hi 

    Thanks for the explanation. 

    Do you have thermal measurement of this device or something similar? 

    It would help to see the temperature difference on a part soldered to a pc board, using FLIR or other thermal sensor. 

    Thanks

  • 0 in reply to Izik
    TI__Genius 13085 points

    Hey Izik,

    We don't really use thermal imagers much since we don't use this type of data for development. But we do have two EVMs which you could order and use if you'd like to do some comparisons. If you are using the RGW package I'd suggest this EVM and if you are using the RGR package then this EVM is the one you want.