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TPS65023B: Soft start current will be above the max current for the device and the inductor

Paul Thomas
Prodigy 50 points
Part Number: TPS65023B
Other Parts Discussed in Thread: TIDA-01480

I am using the PMIC to power an FPGA. 

Due to the large number of decoupling capacitors required on the rail the soft-start current will be above the rating of both the PMIC and the saturation current of the inductor. Please advise whether this is safe and if not how I resolve as even if I replaced the inductor with one with a higher saturation current, the soft-start current would still be ~ 3.5A

I have ~900uF on a 3V3 rail driven by DCDC3. 

  • 0
    TI__Mastermind 19060 points

    Hi Paul,

    Typical DCDC3 output capacitance is 10uF or 22uF. As far as the TPS65023B is concerned, my advice would be to lower the capacitance value substantially. From my perspective, such a high current spike would not be desirable. There's not much that can be done from the PMIC side to mitigate this if the built in soft start isn't enough:

    Regards,

    James

  • 0 in reply to James Steenbock
    Prodigy 50 points

    Hi James,

    The output capacitance of the PSU itself is only 44uF. The problem is the decouplers on the FPGA that it is driving. These cannot be lowered as are required. I have copied your reference design TIDA-01480 which is specifically designed to power the Xilinx Ultrascale+ MPSoC. I am only re-creating your reference design - are you saying your reference design does not work?

    Thanks and Regards,

    Paul

  • 0 in reply to Paul Thomas
    TI__Mastermind 19060 points

    Hi Paul,

    As long as the local capacitance isn't 900uF, this should be fine. 44uF local capacitance should not be a problem.

    As for the soft start current, the reference design is made to be a working example so I wouldn't expect the basic setup to cause issues with either the PMIC or the FPGA. I would need to know more about how you are calculating the expected soft start current.

    Regards,

    James