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TPS7B70-Q1: thermal test results

Dongbao Zhou
Expert 4170 points
Part Number: TPS7B70-Q1

Hello,

Could you help share the thermal test results of tps7b70-q1? A customer would like to evaluate the thermal performance under 13.5V input, 5V/300mA output in room temp, thanks.

Dongbao

  • 0
    TI__Genius 10490 points

    Hi Dongbao,

    The junction temperature of the LDO can be estimated with the following equation:

    Tj = Ta + Rja * Pd

    Tj = Junction Temperature

    Ta = Ambient Temperature

    Rja = Junction-to-Ambient Thermal Resistance

    Pd = Power dissipation

    Using the Rja specified in the datasheet and your customer's parameters:

    Tj = 25 + 39.7 * (13.5 - 5) * 0.3 = 126.2 C

    This value of Rja is derived from the JEDEC High-K Thermal Test Board described in JESD51-7. It is a four layer PCB that contains only traces on the top layer. As the anticipated junction temperature is greater than the maximum recommended junction temperature of 125 C, some additional copper will be needed on the top layer of your customer's PCB to act as a heat sink. The datasheet includes some layout guidelines that can reduce Rja. The TPS7B70EVM can also be used as a reference for a 2 layer board:

    http://www.ti.com/lit/ug/sbvu047/sbvu047.pdf

    If your customer would like to know the Rja performance for a specific area of copper on the top layer, please let me know the exact area and I will submit a request with the thermal team. Keep in mind that these results will typically take up to two weeks.

    Thanks,

    Gerard

  • 0 in reply to Gerard Copeland
    TI__Expert 4170 points
    Hi Gerard,

    Thanks for your feedback.
    According to Semiconductor and IC Package Thermal Metrics www.ti.com/.../spra953c.pdf, Rja is not recommended to be used to calculated the thermal, it can only be used to compare the package thermal performance under same standard.
    Besides, tps7b70 have thermal pad, i think it should behave better in most application (more than 4 layer board, big enough space) than the calculation use Rja. So, i am asking for the thermal test results to help customer evaluate the thermal performance. Otherwise, the application will be restricted by the thermal calculation use Rja.

    Dongbao
  • 0 in reply to Dongbao Zhou
    TI__Genius 10490 points

    Hi Dongbao,

    This application report points out that Rja varies significantly based on the PCB and other parameters.The datasheet spec can still be used as a baseline to determine how the PCB should be designed for a specific application.

    I apologize for making a mistake in my previous post. The maximum recommended junction temperature for the TPS7B70 is 150 C. In your customer's use case, designing a PCB that is similar to or more thermally efficient than the JEDEC High-K Thermal Test Board is sufficient to ensure that the device operates as expected. Adding more copper to the PCB on the top layer will further ensure that the junction temperature is much less than the 150 C maximum.

    The thermal test results for this device are the metrics given in the Thermal Information table of the datasheet. They are derived from the industry standard JEDEC High-K Thermal Test Board described in JESD51-7. If you register an account with JEDEC here, you can access JESD51-7 for free here for all the details about the test board. Can you clarify what you mean by thermal test results?

    Thanks,

    Gerard