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LMG3411R070: Junction Temp (Tj)

BJ Han
Expert 2035 points
Part Number: LMG3411R070

Dear TI,

For developers, only the case temperature of the device can be measured.
What is the formula for converting the case temperature to the junction temperature?

  • Tj = θja x Pd + Ta or Tj = Ψjt x Pd + Tc
  1. θja: Thermal resistance [°C / W]
  2. Ψjt: Thermal resistance between package top center[°C / W]
  3. Pd: Power consumption [W]
  4. Ta: Ambient temperature [°C]
  5. Tc: Package Top Center Temperature [°C]
Is this formula correct? If so, how do I apply # 1 and # 2?
  • 0
    TI__Expert 3225 points

    Hello BJ,

    Yes, the temperature difference between the top package and junction is the product of thermal resistance from junction to top and power dissipation. However, please keep in mind that the power that dissipates from the top package is very small, since most of the heat are transferred from the bottom with a heat sink and lower thermal resistance.

    The device dissipates heat from two path, just like current flows from two resistors in a circuit. Therefore, you have to know what the thermal resistance between junction to bottom ambient and junction to top ambient are, and then to know how much heat are dissipated from the top. By this way you can get to know the junction temp easily. According to our experience, the temperature difference between top and junction will not be high, usually within 5C when you have a heat sink. Very little heat is transferred from the top.

    Please let me know if you have any questions.

    Thanks and regards,

    Lixing

  • 0 in reply to Lixing Fu
    Expert 2035 points
    Hi Lixing,
    Thank you for your response.
    I will heat the PCB without heater sink.
    3, 4 and 5 can be measured in the above calculation formulas.
    How do I calculate 1 and 2 and assign them to the formula?
  • 0 in reply to BJ Han
    TI__Expert 3225 points

    Hello, 

    For the 1 and 2 that you mentioned, they are the thermal resistance from junction to top case and to ambient, which are shown in our datasheet. Please look into LMG3411R070 datasheet for this information. 

    Thanks and regards,

    Lixing