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LM5642X: Question about Thermal measurement (theta JA vs ΨJT)?

AnBer
Mastermind 29430 points
Part Number: LM5642X
Other Parts Discussed in Thread: LM5642

Team,

In the LM5642x datasheet SNVS219K we do provide theta JA (for both package at page 4) to estimate junction temperature from the ambient temperature measured.
I have seen that Semiconductor and IC Package Thermal Metrics - SPRA953C suggest at section 3.1 that ΨJT seem to be the preferred method to calculate the junction temperature.

-Can we provide as well ΨJT (in case the case temperature is measured) to estimate the junction temperature?

-How to proceed in case that the only temperature that is measured is the top case temperature to approximate the junction temperature?

Thanks in advance,

Anthony

  • 0
    TI__Genius 11040 points
    Hello Anthony,

    We are looking into this, and we will provide feedback on your questions.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    TI__Expert 6215 points
    Hello Anthony,

    I cannot find data for this part so have looked up a number of devices that use the same package. They consistently have a ψJT between 0.4 °C/W and 0.8 °C/W. Since this value is small compared to RθJA, the top of this package is a good indicator of die temperature.

    Regards, Robert
  • 0 in reply to Robert Blattner
    TI__Mastermind 29430 points

    Hello Robert,

    Thanks for the answer, it helps!

    One more related question:

    LM5642X datasheet (SNVS219K) gives the max Junction temperature of 125C at page 4. Section (4) gives this additional information:
    "A thermal shutdown will occur if the temperature exceeds the maximum junction temperature of the device."

    At page 16 section "Thermal Shutdown" it says:
    "The LM5642 IC will enter thermal shutdown if the die temperature exceeds 160°C. ..... The IC will return to normal operating mode when the
    die temperature has fallen to below 146°C."

    -How do 146°C and 160°C relate to the max Junction temperature 125°C?
    -Does it means that when the device goes in shutdown it has already gone over the max unction temperature and it could potentially be damaged?
    -Will the life time of the device be reduced when it goes to Shutdown? Is there a way to quantify how much?
    -Do we have some document that explain more about max junction temperature vs temperature measured in the die?

    Thanks in advance,

    Anthony

  • 0 in reply to AnBer
    TI__Expert 6215 points
    Hello Anthony,

    TDS is normally set above the recommended operating range and often above the absolute maximum temperature. Unlike exceeding other absolute maximum ratings, exceeding junction temperature will dramatically shorten the life of the product but not instantaneously destroy the device. The TSD feature will allow the part to survive an over temperature event but will not allow the part to run for years while above the absolute maximum temperature.

    Regards, Robert