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TPS7A4501KTT - Calculation for Case Temperature

Rizwan Hirani
Intellectual 450 points

Okie.. So I am using 4/5 TPS7A4501_KTT in my design. The board I am designing is kind of board to be used in testing.  So, the person working on that board is going to probe different locations on PCB so some or other testing.

Now - What I need is to calculate "How Hot these LDO will become" so that by mistake if somebody touch them during testing it doesn't hurt or burn the fingers. So , to do that I calculated the power dissipation in each LDO based on input output current/voltage.

To find case temp - First we calculated junction temperature - Using

Based on junction temperature we calculated case temperature (just to make sure IC doesn’t get too hot to touch during testing on board). Now data sheet has two different parameter for θJC – θJCtop and θJCbot. So using following equation for both top and BOT we calculated the Temperature of case for TOP Side and bot side.

 

So , now issue is – using above equation for top and bot side , the numbers we are getting for case temperature doesn’t seem to practical. As power dissipation increases bottom case temperature increases and TOP temperature decreases.  Are we missing something here ? Is there any error in calculation or mis interpreted thermal parameters of IC ? If it is wrong – what would be the correct method to calculate case temperature of IC for given power dissipation and Ambient temperature ?

Following is the table of calculation of temp rise:

I know, there is something wrong. Can somebody guide me here, how to calculate the rise of temperature on chip case ? Please suggest..

  • 0
    TI__Mastermind 41800 points

    Hi Rizwan,

    Please see the post regarding this below. Theta JC top assumes all heat is transfered to the top of the IC which in reality is not the case. Most of the transfer will be at the bottom. Therefore, case top temp calcualtion will be different.

    http://e2e.ti.com/support/power_management/linear_regulators/f/321/p/251240/879344.aspx#879344

    In addition to the above link, Theta parameters are highly dependent on the board layout. The board for the parameters in the datasheet uses JEDEC standards and are described in the notes below the thermal parameters table.

    I recommend you simply measure the temperature of the IC at different operating conditions of the LDO so the tester knows how hot the device will be at those conditions.

    Regards,

    Darwin