• State Resolved
  • Locked Locked
  • Replies 10 replies
  • Subscribers 62 subscribers
  • Views 698 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.

LM2675: boot-strap capacitor

K.Yaita
Guru 29690 points
Part Number: LM2675

Hi Team,

I would like to ask the following.

1)
The datasheet shows 470nF for CB to VSW however typical application shows 10nF.
Which is correct value? I believe 10nF is correct however would like to confirm just in case.


2)
Absolute Maximum Ratings shows the following however my customer said CB pin voltage exceed VSW+8V in normal operation.
Is it problem?
Could you also tell me CB to VSW voltage that should be observed when Vin=13.8V, Vout=5V application?



Best Regards,
Yaita

  • 0
    TI__Genius 11040 points
    Hello Yaita,

    I would recommend using a 0.010uF capacitor which is used on the EVM:

    www.ti.com/.../snva011c.pdf

    Please let me know if you have any additional questions or if anything remains unclear.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 29690 points

    Hi Katelyn-san,

    Thank you for your kind support.
    I understand 0.010uF is correct and my customer actually uses 0.010uF.

    How about the question 2)?
    ---------------------------------------------------------------
    2)
    Absolute Maximum Ratings shows the following however my customer said CB pin voltage exceed VSW+8V in normal operation.
    Is it problem?
    Could you also tell me CB to VSW voltage that should be observed when Vin=13.8V, Vout=5V application?

    ---------------------------------------------------------------
    Boost Capacitor(CB) is required to use 50V rating capacitor so I believe more than VSW+8V can be observed at CB pin in normal operation.

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    Guru 29690 points

    May I have your comments?
    My customer wants to know if it is problem or not as soon as possible.

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Genius 11040 points
    Hello Yaita,

    The BT-SW voltage will be about 8V, and this is okay. You want to prevent damage as the input voltage increases and switch voltage increases as well as the boot voltage.

    Please let me know if this is still unclear or if you have any additional questions.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 29690 points

    Hi Katelyn-san,

    Thank you for your kind support.
    I understand BT-SW voltage will be about 8V when when Vin=13.8V, Vout=5V application.

    I would like to clarify the following Absolute Maximum Ratings.
    I believe it indicates CB pin shouldn't be externally applied more than VSW+8V, generated internally is okay.
    Is my understanding correct?


    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Genius 11040 points
    Hello Yaita-san,

    Yes, you do not want to apply an external voltage of greater than VSW+8V. However, if the device did apply a voltage much above the abs maximum, then the boot circuitry could be over-voltaged and damaged. However, the part should not generate this voltage on its own since it would damage itself.

    As you mentioned, the way the CB pin can exceed abs maximum in operation is if an external voltage is supplied above VSW+8V.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 29690 points

    Hi Katelyn-san,

    My customer see the following waveform that CB-VSW exceeds 8V (approximately 10.4V max).
    Vin=13.8V, Vout=5V in this case.
    I believe it is no problem because external voltage isn't supplied to CB pin in this case.
    -------------------------------------------------
    Green (CH2): CB
    Blue (CH3): VSW
    Yellow (CH2-CH3): CB-VSW



    -------------------------------------------
    May I have your comments if it could be a problem or not?

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Genius 11040 points
    Hi Yaita-san,

    This should not be a problem. Are they measuring the waveforms with tip and barrell method? This will make sure that there is not any additional voltage due to measurement techniques.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 29690 points

    Hi Katelyn-san,

    Thank you for continuous support!

    >Are they measuring the waveforms with tip and barrell method?
    I believe you referred about probe to measure, if so they used the probe with short GND connection as the following.
    I think it is no problem in terms of measuring method.


    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Genius 11040 points
    Hi Yaita-san,

    Yes, I agree there is no problem with this method.

    Best Regards,
    Katelyn