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LM26480 Junction-to-case temperature coefficient

Holger Eliasson
Prodigy 115 points
Other Parts Discussed in Thread: LM26480, AM3505

I can't seem to find this in the datasheet for LM26480. I find it quite hard to get a good measurement of the ambient temperature, but I can easily measure the case temperature, so the JC parameter would help me very much.

 

Kind regards

Holger

  • 0
    TI__Intellectual 1955 points

    Holger,

    I think you need package and board model figures.  What kind of calculations, estimation or concerns you or user is contemplating?   JA, JB, JC, or JT info?  if TA-mx=Tj-mx-op  - (theta_JA * PD-mx)  is extraneous.  It is not readily available.  Let me know some more see if I may dig up anything.  NSC db is not available any more and TI is harder to navigate/search.  

    Kern

  • 0 in reply to Kern Wong
    Prodigy 115 points

    I have made case temperature measurements of several components of a design and I want  to calculate the junction temperatures for these. As the product will be specified for quite high temperatures and it will be enclosed in an IP67 housing, we want to make sure the components are operating within specified junction temperatures.

    I have successfully managed to calculate the case temperature for all components except for LM26480 and AM3505 (for which I have a seperate thread) by Tj = Tc + (theta_JC*PD). But for these last two compoenenents the theta_JC value is not available.

    It is quite hard to measure the ambient temperature in the housing as it is varying much depending on where you measure in the housing. Therefore I am interested in the theta_JC value.

    Regards
    Holger

     

     

  • 0 in reply to Holger Eliasson
    TI__Intellectual 1955 points

    Hi Holger,

    I got some figures from an old NSC archive, but does not have it for the 4mmx4mm (which has 2.6x2.6 DAP and different die size).  I am making a request for folks in Malecka to check.  My guestimate is the theta-Jc might be much lower that 25degC/W   perhaps down to 12 or 10. base on the larger die size and the compounds used, etc. In the mean time these are the range you may take for ref.

     (Measured data nsc:  die size 2x1.6, DAP 3.4x2.4, Body 5x4, 24pin LLP:   Theta_Jc (degC/W)= 38.9

    And by the way how do you measure, estimate the junction temp for you exparession of Tj = Tc + (theta_JC*PD). Thanks,

    Kern

     

     

  • 0 in reply to Kern Wong
    Prodigy 115 points

    Hello Kern,

     

    Thank you for you answer. I hope you come up with a value of theta_JC.

    I'm not sure what to do with the values:
    Measured data nsc:  die size 2x1.6, DAP 3.4x2.4, Body 5x4, 24pin LLP:   Theta_Jc (degC/W)= 38.9

    I was hoping I could calculate the junction temperature with the formula Tj = Tc + (theta_JC * Pd), where Tc is the value I have on the actual case, measured with PT100, theta_JC is still unknown, and Pd is max power dissipated, 1.17W according to datasheet.

    Maybe this is not the best way to go?

    Kind Regards
    Holger

  • 0 in reply to Holger Eliasson
    TI__Intellectual 1955 points

    Hi Holger,

    Got your figure from factory pd.

    ΘJC of LM26480 is 4.4° C/W

    Hope this helps, by the way what is the application and outfit using it,  automotive or security camera?

    Regards, Kern

  • 0 in reply to Kern Wong
    Prodigy 115 points

    Hi Kern,

    Great, thank you so much. This will help me alot!

    The application is an industrial network gateway fitted in a pretty well ventilated IP21 housing, but there is plans to do a completely sealed solution, so there is concers about heating.

    I have just now learned that the formula I intend to use; Tj = Tc + (ΘJC * Pd) might acually not be the best way to calculate the junction temperatures, as this formula, if I understand it right, is intended to be used when a heat sink is attached to the package.

    An article over at Electronic Engineering Journal suggest to use Tj = Tc + (ΨJT * Pd) instead, as ΨJT also takes into account for heat spreading to the PCB, and not only to the top. "Using theta-JC  would give misleading results as it tends to be a significantly larger value than Psi-JT".

    http://www.eejournal.com/archives/articles/20110721-tipt2/

    As ΨJT seems to be rarely used in datasheets I can't see how to calculate the junction temperature with great accuracy without using ΘJC instead. Using ΘJC would mean I end up with a higher value for the junction temperature, so I'm on the safe side at least.

    What do you think is the best way to measure the junction temperature?

    And also, is there any chance to get a value also for AM3505 ZCN in this thread? 

    Kind regards
    Holger

  • 0 in reply to Holger Eliasson
    TI__Intellectual 1955 points

    No best way, many ways depends on you resources, patience, etc.

    How accurate a few deg or tenth deg C?  usually using the pn junctions avail in the chip, but you must characterize making a curve a reference then try to ascertain in when chip is in operation.  Some notes:

    http://www.google.com/search?q=measuring+junction+temperature&rlz=1I7EGLA_en&ie=UTF-8&oe=UTF-8&sourceid=ie7

    http://electronicdesign.com/lighting/use-forward-voltage-drop-measure-junction-temperature

    Kern