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TPSM843620: Is feedforward resistor, RFF, is MUST for this device?

John11513
Expert 5520 points
Part Number: TPSM843620

Tool/software:

Hi experts,

TPSM843620 datasheet and EVM schematic both recommended to use a feedforward resistor (RFF) in serial with feedforward cap. Is this RFF MUST? Can it be omitted? What should do if it is omitted?

Thank you!

John

  • 0
    TI__Mastermind 20870 points

    I would say it is a must, if you want the best load regulation. 

    The Rff helps improve load regulation in case of non-ideal AGND to PGND connection in the PCB layout. 

    If you don't want to use the Rff, replace 0 ohm or short with a pcb trace in pcb layout. 

  • 0 in reply to David_Daniels
    TI__Expert 5520 points

    Thank you for reply. My understanding is feedforward cap CFF is used to improve the load regulation through introducing a zero and extend loop bandwidth. Do not what is the purpose of RFF.

    Regards,

    John

  • 0 in reply to John11513
    TI__Mastermind 20870 points

    Yes, the Cff adds a zero in the loop and improves the load transient response by increasing the phase margin allowing higher bandwidth.   

    The Rff improves dc load regulation.   

    The Cff will ac couple noise onto the FB pin and create an offset when the pcb layout is not ideal.   As the load current increases, the offset becomes greater and output voltage droops.   Adding the Rff in series with Cff eliminates the offset.   If the pcb layout is good, the Rff is not needed on the converter.  The module has been sensitive than the converter, so it is recommended to use the Rff.