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TPS40400: FRA measurement

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Replies: 4

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Part Number: TPS40400

Hello,

We would like to do FRA measurement. We think feedback loop is removed then apply signal via resister 

Would you teach us how to apply the signal source for TPS40400 when FRA test?

Best regards,

Toshihiro Watanabe

  • Hi Watanabe-san,

         The system will be in closed loop when you are doing a bode plot measurement. You will typically have a 50 ohm resistor in series with the feedback resistor and signal will be injected (Vinj) across this 50 ohm resistor. The output will be measured at Vout. The ratio of Vout/Vinj will give you the loop gain which you would use to analyze your stability.

    Refer to figure 10 in this document.

    https://www.ti.com/seclit/ml/slup386/slup386.pdf

    Regards,

    Gerold

  • In reply to Gerold Dhanabalan - Online Design Tools:

    Hello Gerold-san,

    Thank you for the material. There is a schematic in datasheet figure 21. In TPS40400 case, Is it ok to add 50ohm series with R11?

    Best regards,

    Toshihiro Watanabe

  • In reply to Toshihiro Watanabe:

    Helo Gerold-san,

    Would you help for the question? TPS40400 seems different from typical DCDC converter. It would be helpful to let us know where we should apply the signal for FRA in TPS40400 case.

    Best regards,

    Toshihiro Watanabe

  • In reply to Toshihiro Watanabe:

    Hi Watanabe-san,

         Referring to figure 21 in the datasheet, you can inject the signal across R11 and don't need to add additional 50 ohm in series. Please note that the VSNS+ and VSNS- are serving for remote sensing purposes. So technically the output of the VSNS+ and VSNS- amplifier is the Vout (after the difference amplifier and is called DIFFO). So the R11 resistor is the 50 ohm resistor where the signal is injected. You can monitor the output at the DIFFO pin to find the loop gain.

    Regards,

    Gerold