• State Resolved
  • Locked Locked
  • Replies 11 replies
  • Answers 5 answers
  • Subscribers 62 subscribers
  • Views 616 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS650861: For BUCK3 to BUCK5, May we adjust the slew rate and soft-start time?

Edward Lee
Intellectual 1655 points
Part Number: TPS650861

Dears,

       May we adjust the power on slew rate of BUCK3 ~ BUCK5 ?

       For the estimation of rising time during powering on, can we just use the expected voltage divided by the slew rate 4mV/us ?

       

      As to another question is about the soft-start. May we set the buck 3 ~ buck 5 to enable the soft-start during power on? 

      It's some like what datasheet depict the DVS as below

Thanks

BR

Edward

  • 0
    TI__Intellectual 950 points

    Hi Edward,

    It seems that B3-B5 slew rate is fixed, but let the responsible applications engineer answer to your question. your question has been forwarded to correct person,

    Thanks

    Tuomo

  • 0 in reply to Tuomo Alatalo
    TI__Genius 12735 points

    Hi Edward, 

    This Slew Rate you highlighted in the data sheet is the slew rate when using Dynamic Voltage Scaling, not on startup. The start up slew rate is not fixed, but is dependent on the inductor and output capacitors selected for BUCK3/4/5. This statement is incorrect, refer to my next answer for the correction.

    I believe BUCK3/4/5 start up in soft start mode by default.

    Best regards,

    Layne J

  • 0 in reply to Layne J
    TI__Intellectual 1655 points

    Dear Layne,

          Thanks for your feedback.

          As to the soft start mode set by default during power-on, does that means the ramping up time will be at least more than 200us?

          My customer gave the ramping up time measured on their side, the period in each rail is less than 200us.

          Is there possible our customer disable the soft-start function?

    Voltage Rail Voltage (Volt) Ramp Time (us) Which buck 
    AVCC 0.9 103 4
    AVTT 1.2 140 3
    AUX 1.8 200 5

    Thanks for your help

    BR

    Edward

  • 0 in reply to Tuomo Alatalo
    TI__Intellectual 1655 points

    Dear Tuomo,

         Thanks forward for me.

    BR

    Edward

  • 0 in reply to Edward Lee
    TI__Intellectual 1655 points

    Dear Layne,

        My customer enlarged the output capacitance for buck 3~5 and found nothing changed at the slew rate!

        Would you kindly advise how to adjust or change the ramp-up time for these three buck converts?

    Thanks a lot

    BR

    Edward

  • 0 in reply to Edward Lee
    TI__Genius 12735 points

    Hi Edward, 

    Soft start on TPS650861 is always enabled and cannot be disabled. I misspoke before when mentioning output capacitance and inductance, the ramp up time is tied to VREF. As VREF ramps up from 0 to 0.4V, output voltage also ramps to the target output voltage. This timing is set and not changeable. Unfortunately I do not have an exact number for how long this ramp up is.

    As a reminder, we recommend using a maximum of 400uF output capacitance and 470nH inductance.

    Best regards,

    Layne J

  • 0 in reply to Layne J
    TI__Intellectual 1655 points

    Hi Layne, 

        Thanks for your explanation and kindly reminder.

        As you mentioned the soft start mechanism is related to the VREF, what if we enlarge the capacitance at the VREF pin?

        Is that possible to change the overall power on ramp time? Or will there be any stability issue involved?

    Thanks so much

    BR

    Edward

  • 0 in reply to Edward Lee
    TI__Genius 12735 points

    Hi Edward,

    Changing the capacitance on VREF will not change the start up ramp time. Changing this capacitance will either cause instability in the system or have no effect on timing. This timing is set internally and cannot be changed.

    Best regards,

    Layne J

  • 0 in reply to Layne J
    TI__Intellectual 1655 points

    Hi Layne,

          Got it!

         Thanks for your feedback and reminder. 

    BR

    Edward

  • 0 in reply to Edward Lee
    TI__Intellectual 1655 points

    Hi Layne, 

     

        Sorry for bothering again. Would you kindly advise to add external circuitry to increase the ramp up time of these three buck converters?

        Like using mosfet with RC time constant or other simple way to implement it.

    Thanks so much

    BR

    Edward

       

  • 0 in reply to Edward Lee
    TI__Genius 12735 points

    Hi Edward,

    It is not possible to add any external circuitry to change ramp up time, the device is internally referenced and the ramp up time is not user adjustable.

    Best regards,

    Layne J