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TPS543620: Should I use WEBENCH or Calculator Spreadsheet?

Thai Phan
Prodigy 55 points
Part Number: TPS543620

Hi team,

I have been trying both WEBENCH and the Calculator Spreadsheet to get values for passive components for our design with TPS543620. WEBENCH seems quite nice and easy to use. However there is a considerable difference between inductance and capacitance values obtained from WEBENCH and the ones from the spreadsheet. WEBENCH even puts a 4.7uF cap for pin BP5 while it should be 2.2uF as recommended in the datasheet. 

Which tool should I use for the design?

Thanks,

Thai

  • 0
    TI__Mastermind 18370 points

    Hi Thai,

    Sorry for the confusion caused.

    There is no single value of inductor and capacitors that is optimal for all needs.
    For example, these values will change if you set for higher or lower current ripple.
    This will provide a tradeoff between efficiency and transient response.

    But overall, I suggest that for calculating the values of the passive components you use the calculator.

    I use the calculator every day and know that it is up to date.
    WEBENCH can be used for estimating the efficiency and other features that the calculator does not provide.

    If this addresses your concern, please set the status of this post to resolved.

    Regards,
    Yitzhak Bolurian

  • 0 in reply to Yitzhak Bolurian
    Prodigy 55 points

    Thanks Yitzhak. Can you please clarify what the feedforward resistor Rff is in the calculator spreadsheet? Is it optional and how should it be connected?

    Thai

  • +1 in reply to Thai Phan
    TI__Mastermind 18370 points

    Hi Thai,

    Rff should be connected in series with Cff (the capacitor that is in parallel with the top feedback resistor).

    It is not a must to have Rff, the alternative to having Rff is to have the feedback resistor values low.
    Meaning the bottom feedback resistor value to be less than 4 KOhms.

    If this addresses your concern, please set the status of this post to resolved.

    Regards,
    Yitzhak Bolurian

  • 0 in reply to Yitzhak Bolurian
    Prodigy 55 points

    Hi Yitzhak,

    Thank you for the information. Could you please explain the purpose of Rff in more details? Just to understand a bit more..

    The evaluation schematics and calculator spreadsheet both recommend a 0ohm placeholder for Rff. What could go wrong if we just follow this instead of using a specific value?

    Thanks,

    Thai

  • 0 in reply to Thai Phan
    TI__Mastermind 18370 points

    Hi Thai,

    If Cff is very large, the DC voltage regulation set point may be a bit off.
    Rff helps to prevent this issue. 
    You may start with Rff=0 Ohms, and only if there is any regulation issues, then increase the value.
    If the values of the feedback divider resistors are low, there will usually be no need for Rff.

    If this addresses your concern, please set the status of this post to resolved.

    Regards,

    Yitzhak