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TPS566238: Inquiry about TPS566238RQFR Specifications

Takumi Kimura
Prodigy 40 points
Part Number: TPS566238

Tool/software:

Hi,

We are currently planning to use TPS566238RQFR with internal soft start in our new product.

CSC has responded that SSpin is fine for open processing when using internal soft start.

However, we would like to connect a capacitor of several pF to several nF for noise suppression.

If I connect this capacitor, will it affect the internal soft-start time?

Also, if we use the internal soft start time, is there any recommended pin treatment for SSpin?

Regards, Takumi

  • 0
    TI__Mastermind 29440 points

    Hello Takuma, 

    The default will be 1.9ms. There is no need for cap on this pin. The device has a internal current source, if there is no cap, it I will default directly to 1.9ms default. If a cap is connected outside, eventually, it will charge up and the time can be calculated with equation  2 according to DS.   

    https://www.ti.com/lit/ds/symlink/tps566238.pdf?ts=1718312870942&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPS566238

    Thanks 

    Tahar

  • 0 in reply to Tahar Allag
    Prodigy 40 points

    Hello Tahar

    If a cap is attached to an external cap with a soft-start time calculated from Equation 2 that is smaller than the internal soft-start time of 1.9 ms,
    Is it correct to assume that the internal soft-start time is valid for the soft-start time of the cap?

    For example, if an external cap of 100pF is attached, Tss ≈ 0.13ms from Equation 2.
    In this case, since the internal soft-start time of 1.9ms is larger, would 1.9ms be valid?

    Thanks

    Takumi

  • 0 in reply to Takumi Kimura
    TI__Expert 4295 points

    Hi Takumi,

    It is possible to have a soft-start time that is smaller than the default soft-start time of 1.9ms. For example, in the datasheet Application Implementation section, the reference schematic has a 10nF capacitor on SS, which equates to 1.2ms soft-start time, per Equation 2.

    So it is possible to have a soft-start time smaller than the default 1.9ms if you connect the correct capacitor.

    Hope this answers your question. If this answers your question, please don't forget to click "This resolved my issue". If not, please reply and the thread will open to clarify any further questions.

    Thanks!

    Calan

  • 0 in reply to Calan Elaya
    Prodigy 40 points

    Hi Calan,

    I understand that it is possible to set a soft start time smaller than the default soft start time of 1.9 ms.

    The datasheet states the following:" If the application needs a longer soft-start time than 0.5 ms, the time can be set by connecting a capacitor on the SS pin."

    Is the soft-start time by the external cap still valid when connecting an external cap that has a soft-start time less than 0.5 ms from equation 2, such as 100pF?

    Thanks

    Takumi

  • 0 in reply to Takumi Kimura
    TI__Expert 4295 points

    Hi Takumi,

    Typically I think about it as the external SS cap and internal SS cap are in parallel. So even if a very small external SS cap is connected, it will more than likely be dominated by the internal cap. There is certainly a minimum SS time that the device can handle, so once a very small capacitor is connected to external SS, then at some point, it will not be valid and the internal time/capacitor will dominate.

    Thanks,

    Calan

  • 0 in reply to Takumi Kimura
    TI__Expert 4295 points

    Takumi,

    I am also available to test this out on the bench. I can install a 100pF cap and measure the start-up to see what is the SS time.

    I will do the test and reply to you with my results.

    Thanks!

    Calan

  • +1 in reply to Takumi Kimura
    TI__Expert 4295 points

    Hi Takumi,

    I was able to test this experiment on the bench with an EVM. I initially had a 10nF cap on SS pin which resulted in a SS time of about 1.35ms. I replaced this cap with a 100pF cap on SS and the soft-start result time decreased to about 1ms. This means there is a minimum SS time, and installing a small enough capacitor will invalidate the soft-start time calculated from equation 2. I attached a scope shot of Vout ramping up.

    Hope this helps answer your question.

    Thanks,

    Calan

  • +1 in reply to Calan Elaya
    Prodigy 40 points

    Hi Calan,

    Thanks for the experiment.

    I now understand that even with a tiny external cap connected, it works at the lower limit of the internal soft start time.

    My question has been resolved.
    Very helpful. Thank you very much.

    Thanks,

    Takumi