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LM1084: What to use, RθJC(top) value or the ψJT value?

Art Mecina
Genius 16505 points
Part Number: LM1084

Hello Guys,

We  usually received this kind of inquiry where the customer is asking which of the thermal parameters to use θ or ψ.

This time around, a customer is asking what to use, RθJC(top) value or the ψJT value for his thermal analysis? His use case is below.

He is measuring the temperature on the top of the KTT package with a thermocouple. 

The regulator has an input of 12V and output of 5V.  The operating current is between 0.2A and 1A.  The board is FR-4, 4 layer and has a thermal interface pad connecting the backside of the board.  He is seeing top-of-IC temperatures of ~77C in a 49C ambient ,and measuring the temperature on the top of the KTT package. Do he make his temperature calculation using the RθJC(top) value or the ψJT value?

Thanks for the confirmation!

Art

  • 0
    TI__Genius 13085 points

    Hi Art,

    In this case ψJT should be used. This is because when using RθJC(top) it assumes all of the heat is being transferred through the top of the package which is not accurate, the majority of heat will be transferred through to the PCB through the bottom of the package. The characterization parameter  ψJT  accounts for all heat transfer while measuring the top of the package. 

    There is an app note put out by the packaging team describing all of the thermal metrics and explaining how they are calculated if you'd like to dig in deeper to any of these thermal metrics.