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TPS65400: Feedback divider capacitor design

Bob Dvorak76
Prodigy 60 points
Part Number: TPS65400
Other Parts Discussed in Thread: PMP11414

In the TPS65400 EVK circuit and the PMP11414 reference design, capacitors are placed across the upper feedback resistor to shape the closed-loop response.  In the EVK, these are C11, C19, C27 and C37.

I do not see any mention of these capacitors in the TPS65400 data sheet or the EVK user guide.  When are these capacitors required and what is the best way to compute the proper values?

  • 0
    TI__Mastermind 19080 points

    Hi Bob,

    Capacitors in parallel with upper feedback resistor are called feed-forward capacitor. They provide a phase boost and bring higher phase margin.

    For the calculation, the feed forward capacitor formed a pair of zero and pole. 

    Fzero: RFBT & Feed forward cap

    Fpole: (RFBT//RFBB) & Feed forward cap

    Select the value to put the zero and pole at proper position.

    Regards,

    Sam

  • 0 in reply to Sam(MPAA) Zhang
    Prodigy 60 points

    Hello Sam,

    Thank you for the quick response!

    I am aware that the feed forward capacitor will add a pole-zero pair to the response.  What I am looking for is some guidance on when this would be required--the feed forward capacitor is shown in figure 6 of the data sheet, but is not mentioned in section 8.3.11 where compensation is discussed.  Would it normally be necessary to add an additional pole-zero pair to compensate the converter?

    Regards,

    Bob

  • 0 in reply to Bob Dvorak76
    TI__Mastermind 19080 points
    Hi Bob,

    Whether you need the Cff depends on how much bandwidth and phase margin do you have before adding it. If you have limited bandwidth(not good enough load transient) and phase margin, then an effective way to solve it is adding a Cff.
    In most cases, when you have a higher Vout like 5V, you may need a Cff because the DC loop gain is lower due to the lower value of (Vref/Vout) .

    Regards,
    Sam
  • 0 in reply to Sam(MPAA) Zhang
    Prodigy 60 points

    Thank you Sam.  We are including a feedforward capacitor in the PCB layout to be populated if we require it.

    Bob