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TPS65131: FBN-VNEG capacitor and resistor

Yuto Sakai
Mastermind 6100 points
Part Number: TPS65131

Hi,

Datasheet P.11 Figure 8 has no resistance, but EVM User's Guide P.8 Figure 6 has a 51.1 kΩ resistor connected.



I understand it as a resistor to adjust the phase margin, is it correct?
How do we determine this resistance value?

Best regrads,
Yuto Sakai

  • 0
    TI__Expert 6330 points

    Hi,

    I have notified our expert regarding this topic. Please expect a response by 6/19/19.

    Thanks,

    Aaron

  • 0
    TI__Mastermind 18550 points

    Hello Sakai-san,

    In some noisier designs, we have seen loop stability issues due to excessive noise coupling into the FBN pin through the feed forward capacitor (C9). This series resistor reduces the noise coupling in to FBN pin to make the circuit more stable but the drawback will be a bit slower loop response. Recommended value for this resistor is between 10K and 100K and 51K has worked well with TI EVM design. A particular customer design may need smaller or larger value depending on the amount of noise coupling. 

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    Mastermind 6100 points

    Hi Liaqat-san,

    Does this device require this resistance?
    If yes, could you add it to the datasheet?

    Best regards,
    Yuto Sakai

  • 0 in reply to Yuto Sakai
    TI__Mastermind 18550 points

    Hello Sakai-san,

    This device does not require it but we have seen that some customer designs are quite noisy and having this resistor in series reduces noise coupling into feedback pins. Datasheet does discuss this briefly in section 8.2.2.6.1 Please see excerpt below from datasheet.

    Kind Regards,

    Liaqat

    8.2.2.6.1 Feedforward Capacitor As a recommendation, to speed up the control loop, place feedforward capacitors in the feedback divider, parallel to R1 (boost converter) and R3 (inverting converter). Equation 11 shows how to calculate the appropriate value for the boost converter, and Equation 12 for the inverting converter. (11) (12) To avoid coupling noise into the control loop from the feedforward capacitors, the feedforward effect can be bandwith-limited by adding a series resistor. Any value from 10 kΩ to 100 kΩ is suitable. The greater the resistance, the lower the noise coupled into the control loop system.