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TPS22990: Tj Calculation

Jim CHEN
Expert 4795 points
Part Number: TPS22990

Hi Team,

My customer would like to calculate Tj temperature. They measure Tc temperature which is 96.7℃.

Pd=I^2*R=8.7168^2*0.006=0.46W

RθJC(top) =65℃/W

Tj=Tc+(θJC*PD)=96.7+(65*0.46)=126.6℃ > SPEC=125℃

Is there any error in above calculation? It seems like my customer cannot use this chip.

Thank you.

Regards,

Jim

  • 0
    TI__Genius 13760 points

    Hi Jim,

    Thank you for reaching out to the E2E forum!

    To calculate Tj(Junction temperature) you need to use the equation Tj = Ta + ( RθJA * PD ).

    Ta = Ambient temperature

    RθJA = Junction to ambient thermal resistance (51.4 °C/W)

    PD = Power disssipation (Vout * Iout)

    I have a couple questions to find out what your Tj is?

    1. Is your ambient temperature the 96.7℃?

    2. I see from the question that your output current is 8.7168 A. What is your operating voltage?

    Once we the values mentioned above we can calculate to correct junction temperature.

    Regards,

    Andy Robles

  • 0 in reply to Andy Robles
    TI__Expert 4795 points

    Hi Andy,

    My reply as below

    1. Is your ambient temperature the 96.7℃?
      => That is Tc temperature not Ta temperature. The customer expects that the Ta should be around 55℃. And Ta would be 85℃ without any fan. 
    2. I see from the question that your output current is 8.7168 A. What is your operating voltage?
      =>3.3V

     If we calculate with Tj = Ta + ( RθJA * PD ) = 85 + ( 51.4 * 0.46) = 108.43℃, Ta will be the problem in this formula.

    Ta must lower than 76.5℃ then result of Tj will keep lower than 100℃ (SPEC 125℃ * derating 80%).

    Could you please help us to define how to measure “Ta” temperature? How far it is between the chip and measure point?

    Could we use below formula and parameter to do the calculation? 

    Regards,

    Jim

  • 0 in reply to Jim CHEN
    TI__Genius 13760 points

    Hi Jim,

    You can use the equation you mentioned to more accurately calculate the junction temperature of your system.

    With your power dissipation = 0.46 W, the value for the junction to top characterization parameter = 2.1 °C/W, and your measured Tc = 96.7 °C  the total Tj = 97.6°C. This is under your desired temperature.

    Usually Tc is a couple degrees above Ta which validates the value they obtained, but how did the customer calculate Tc?

    Regards,

    Andy Robles

  • 0 in reply to Andy Robles
    TI__Expert 4795 points

    Hi Andy,

    They measure the temperature at the case directly to get Tc = 96.7 °C.

    But I feel confused that what is the difference between below two parameters in yellow?

    With 65°C/W, I got Tj= 126.6°C => Failed

    With 2.1°C/W, I got Tj= 97.6°C => Pass

    When should I choose 65°C/W?

    Regards,

    Jim

  • 0 in reply to Jim CHEN
    TI__Genius 13760 points

    Hi Jim,

    I will be closing this thread. We can keep discussing this over email.

    Regards,

    Andy Robles