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TPS543B20: Feedforward Cap

GARETH OU
Guru 11435 points
Part Number: TPS543B20

Hi

Please suggest Feedforward Cap needs much?

  • 0
    TI__Guru 56925 points

     

    A feed forward capacitor from VOUT to RSP is design to bypass the feedback  divider and change the attenuation factor from VREF/VOUT to 1/1  at high frequency in order to improve bandwidth and phase margin at cross-over.  In this case, with the feedback divider ratio so small (1 / 1.2) the boost in phase and gain will be quite small and may not be worthwhile.

    With your existing Cramp selection and output capacitor configuration, I would not recommend a feed-forward capacitor.  If you were going to lower the bandwidth by changing the Rramp from Open to 187kΩ then I would recommend adding a feedforward capacitor sized to provide a 55kHz cross-over with the 2kΩ Vout to RSP resistor.  that would be 1.5nF.

  • 0 in reply to Peter James Miller
    Guru 11435 points

    Hi Peter

    Thank you very much for your careful explanation

    1. How to figure out that feed-forward capacitor is not required?

    2. How much is recommended in Cross Frequency?

  • +1 in reply to GARETH OU
    TI__Guru 56925 points

     

    I entered the design into the attached design spreadsheet to evaluate the loop to find the approximate cross-over frequency.  Without the feed forward capacitor and selecting the smallest ramp (largest Cramp value) I estimated the crossover frequency well above 100kHz, with the added boost of the feed-forward divider, I am concerned the bandwidth could get too high and unmodeled parasitic delays in the power delivery path could degrade the loop phase margin and stability.

    Loop bandwidths over 100kHz or 1/5 of the switching frequency can get tricky as propagation delays and board parasitics start to additional poles into the loop and degrade phase margin, but a typically target cross-over is between 1/10 and 1/5 of the switching frequency.

    TPS543C20 Bode Plot Design Tool.xlsx

  • +1 in reply to Peter James Miller
    Guru 11435 points

    Hi Peter

    got it , Thanks