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TPS53355: Mode Pin

Julian Chen
Expert 2550 points
Part Number: TPS53355

Hi E2E,

We use TPS53355 and let mode pin connect to 3.3V by 200k.

Does the SS time will be affect? How to adjust it?

  • 0
    TI__Mastermind 18510 points

    Julian, About ss time and mode selection, details are shown in 7.4.1 section in datasheet, you can check the description to get it.

    Yuchang

  • 0 in reply to Yuchang Zhang
    Expert 2550 points

    Hi Yuchang,

    The key is  that datasheet  pull-high to VREG not 3.3V.

    Could you kindly let me know TPS53355 how to judge the SS time?

  • 0 in reply to Julian Chen
    TI__Mastermind 18510 points

    Julian, I know that, actually I am not sure how is the ss value calculated, but I think it should be no relative to Pull high level(3.3v or Vreg). You can see the ss are same for both tied to GND and PG. Because During the ss it is auto skip mode and PG will be pulled up only after ss.

    Yuchang

  • 0 in reply to Yuchang Zhang
    TI__Genius 14455 points

    Hi Julian,

       To add to Yuchang's comment, the SS time is determined by the value of the MODE pin resistor. In your case, for 200k, the soft start time would be 2.8ms.

    Depending on MODE pin being pulled to ground or to a specific voltage determines the Skip mode or FCCM mode of operation. Based on description in section 7.4.1, if the voltage on MODE pin voltage is greater than 1.3V, then the converter enters FCCM mode.

    The reason for tying the MODE pin to PGOOD is such that during startup, PGOOD will be low. So the converter would be in Auto-skip mode.

    After startup, PGOOD will be pulled high. Which means that MODE pin will also be pulled high. This will cause the converter to enter FCCM mode.

    So, PGOOD being pulled up to 3.3V or VREG does not change the MODE pin operation. However, you should check your downstream logic to ensure that with a PGOOD of 3.3V, everthing downstream will work.

    Regards,

    Gerold