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Original question:

TPS650864: TPS6508641 SW6 output high

TPS650864: TPS6508641 Buck6 output 6-8V abnormally

An Hui
Prodigy 10 points
Part Number: TPS650864

HI, TI experts:

we design a new board using TPS6508641RSKT with Xilinx XCZU2EG.

schematics is the same with the reference design. but when we power up the board, the buck6 of TPS6508641 output 6-8V abnormally , instead of 1.8V.

i searched TI forums, and found that the following thread is almost the same problem and SW pin's wave with us.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1085907/tps650864-tps6508641-sw6-output-high

but the thread above do not have any conclusion anyone can use.

could you please help us to solve it?

thanks.

  • 0
    TI__Mastermind 21765 points

    Hi An Hui,

    We should go through some of the same steps in that thread as a start.

    Could you please provide a PDF version of the schematic where we can clearly see all the connections for the PMIC and the external components? Could you also provide a scope capture of the power-up sequence showing Buck5, Buck2, Buck1 and Buck6?

    Where the equations on the datasheet (section 9.2.1 Design Requirements) used to calculate the inductor as well as the output capacitance and ILIM resistor for each of the Buck controllers (Buck1/2/6)?  

    Note that the thread you linked has a number of suggestions to try and debug the issue, namely

    • remove the load from all the PMIC rails (if possible) and take the power-up captures. This will help to determine whether the issue is coming from some external connection or not
    • make sure the VSYS input is stable before pulling CTLx pins high.

    I would look into some of these suggestions since it might be the case that the issue was actually solved in that thread, but the previous poster simply did not confirm the solution.

    Regards,

    James

  • 0 in reply to James Steenbock
    Prodigy 10 points

    Hi，james

     thanks your reply. we have found the root cause of issues in my thread.

    we think, for TPS650864 device, the resistor(2.37Kohm 1%) we used on pin45 of ILIM6 for buck6 is too small to work normally. we change this resistor from 2.37Kohm to 20Kohm, and then Buck 6 output expected 1.8V voltage.

    we review the datasheet <TPS650864 @SWCS138E – JUNE 2017 – REVISED DECEMBER 2022> , found the depict below:

    "If RILIM is too low for the chosen inductor and voltage conditions, then the ripple current at no load will trigger the negative current limit, forcing the low side FET to turn off. This will eventually result in the output voltage increasing above target regulation point due to irregular duty cycle created by current limit being triggered."

    so, we think that "too small ohm value of ILIM6 resistor " is the root cause. but the value of 2.37Kohm is selected according to Alinx ACU3EG/ACU2CG coreboard reference design. maybe some mistakes exist in it. this issue make us burned three XCZU2EG devices by outputing over-voltage.

    So, whatever it is; we will re-calculate the suitable resistor value of ILIM6.

    above is the updated situation from us.

    thanks .