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TPS2556: Thermal question about maximum expected pin and package temperatures

Ronnie Hughes
Expert 3090 points
Part Number: TPS2556

Customer is using TPS2556 with a 49.9K current limit programming resistor, thus Ios maximum should be about 2.5A.  Customer would like to know expected package and/or lead temperature when the die is near its 135C thermal shutdown threshold.  They notice the high temperature of the device during a fault condition. 

  • 0
    TI__Mastermind 36751 points
    Hi Ronnie,

    Thank you for your question. I have assigned the expert for this device and he should get back to you on Monday.

    Thank you,
    Eric
  • 0
    TI__Expert 8980 points

    Hi, Ronnie,

    To calculate junction temperature, you can use R-thetaJa as below.

    Tj= Ta+P(dissipation)*R-thetaJa= 135deg+2.5A*2.5A*35mohm*41.5deg/W=135deg+9.08deg=144.08deg.

    To calculate case temperature, you can use Psi Jt.

    Tt=Tj-P(dissipation)*Psi Jt= 144.08deg-0.15deg=143.93deg.

    Regards,
    Bob

  • 0 in reply to Bob Ma
    TI__Expert 3090 points

    Bob,

    I understand how to calculate junction and case temperatures based on thermal resistance and/or thermal characteristics and power dissipation.  Before posting my question, I had tried to calculate what the pin and case temperatures might be assuming the device was in current limit and had reached its internal 135C thermal cycling threshold.  However, you need to know what the average power dissipation is to accomplish this calculation.  The peak power dissipation will be high and the junction will hit 135C and the device will shut off until the junction cools by 20C.  What I'm not sure of is how the duty cycle and thermal inertia will impact the overall case and pin temperatures. 

  • 0 in reply to Ronnie Hughes
    TI__Expert 8980 points
    Hi, Ronnie,

    Sorry for misunderstanding your question. First of all, I want to confirm the case temperature calculation at current limit condition.
    Tc=Tj-(Vout-Vin)*Ilimit*PsiJT.

    Because part will thermal shutdown when Tj over 135C, so the case temperature's worst case is 135deg. Usually, top(case) dissipation is 5%, pin(lead) dissipation is 40%, bottom dissipation is 55%. PsiJt reflect the thermal relationship between top(case) temperature and junction temperature.

    Regards,
    Bob