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Using snva419c for Thermal Design

BRANDON SMITH1
Prodigy 240 points
Other Parts Discussed in Thread: LMZ35003

Hello,

I am using the application note http://www.ti.com/lit/an/snva419c/snva419c.pdf to design a thermal area for the LMZ35003 chip. Reading through the document it appears that θJA is very important and PCB specific. When using equation (10) and calculating board area θJA is subtracted by θJC and 77.5in^2 is divided by that result. Because the 77.5 and θJC are constants (θJC is package dependent) it seems like a higher θJA will reduce board size. But I would think you would want to reduce θJA so that your thermal resistance is low and therefore dissipated more heat easily? 

It seems like using θCA would be a better calculation to use because it would give me the thermal resistance of the actual board i design. However θCA is not used in any of the calculations, can i substitute it for θJA? It would be great to talk to an FAE on the phone if possible to help clear this up and go through the example i currently have.

Thanks,

Brandon

  • 0
    TI__Guru**** 191445 points

    ThetaCA is thetaJA - ThetaJC as shown in the denominator.  ThetaJA is calculate, so yes higher thetaJA will reeduce the board size, but the result is higher junction temperature for a given ambient temp.  Is that clear?

  • 0 in reply to JohnTucker
    Prodigy 240 points

    "ThetaCA is thetaJA - ThetaCA as shown in the denominator.  ThetaJA is calculate, so yes higher thetaJA will reeduce the board size, but the result is higher junction temperature for a given ambient temp.  Is that clear?"

    I think you mean, ThetaCA is thetaJA - ThetaJC as shown in the denominator?

    ThetaJA is based on a 2 layer board though so it does not take into account the use of via's or varying board thickness. Do you know of an app note that shows how to calculate ThetaCA using Figure 3 and Table 1 in the app note i specified? 

  • 0 in reply to BRANDON SMITH1
    TI__Guru**** 191445 points

    Yes, I fixed my typo in the previous post.  For that example, the thetaJA is a calculated value to meet the  desired device junction temp given teh maximum ambient temperature.  It is not based on a particular layer count or board size (that is what you want to calculate.

    I would have to research whether there are other app notes.  They would come out of a different group than mine.

  • 0 in reply to BRANDON SMITH1
    TI__Genius 10095 points

    Hi Brandon,

    John is correct. 

    Theta CA is just the value calculated for Theta JA minus the device dependent value for Theta JC.  As he said this is the denominator of the right hand side of equation 10.

    I have a spreadsheet where I add the thickness of the board into the equations for theta JA as well as vias. If you want to calculate Theta CA from the spreadsheet set the Theta JC parameter to zero.

    This is still an approximation because I don't adjust for via spacing or more copper layers, it is just an averaged model.  For more accuracy you would need to use finite element analysis software.

    In general as you decrease the thickness of the board and increase the number of copper layers you will improve the thermal performance because you make it easier for the heat to transfer to the surface of the board on both sides and radiate into the ambient air  As I said in the application note thicker copper or more copper layers can improve the performance of your thermal design by as much as 20%

    7181.Thermal Resistance Estimate_ver1.0.xlsm

    Regards,

    Marc

  • 0 in reply to Marc Davis-Marsh
    Prodigy 240 points

    Marc,

    Thanks for the excel sheet this seems like it will be very helpful when trying to calculate the variables i need.

    Thanks,

    Brandon