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TPS61093: About Cff calculation formula

Tsukui Yusuke
Guru 21930 points
Part Number: TPS61093

Hi,

Let me talk about the Cff calculation formula.
The data sheet has the following description.

''When the output capacitor is increased to 100 μF, then the fP is reduced to 10 Hz.

Therefore, a feed-forward capacitor of 10 nF compensates for the low frequency pole.

A feed forward capacitor that sets fz-f near 10 kHz improves the load transient response in most applications,

as shown in Figure 11.''

However,
When calculating based on the following circuit example and BB,It became 54 kHz.
Is it correct with understanding that it should be set to 10 kHz or more?
Which is 10 kHz or 54 kHz correct?
Could you give me your advice?



Best Regards,
Yusuke/Japan Disty

  • 0
    TI__Mastermind 46905 points
    For Figure 11, the feedforward capacitor is 100pF, then the fz-f = 1/(6.28*100p*249k) = 6KHz, No sure how do you get 54KHz
  • 0 in reply to Jasper Li
    Guru 21930 points

    Li-san,

    Thank you for your support.
    I understood your explanation about figure11.

    54 kHz is the calculation result using the circuit of data sheet Page9 Figure 6.
    fz-f=1/(6.28*10^3*10*10^-9)≒54kHz
    (Cff=10nF,Cout=100uF)

    However, the following description in the data sheet recommends 10kHz.
    「When the output capacitor is increased to 100 μF, then the fP is reduced to 10 Hz. Therefore, a feed-forward capacitor of 10 nF compensates for the low frequency pole.
    A feed forward capacitor that sets fz-f near 10 kHz improves the load transient response in most applications, as shown in Figure 11.」

    Customers are confused because of contradiction between recommended circuits and sentences.
    Which of 100pF or 10nF should you recommend?
    Which do you recommend 100pF or 10nF?

    Best Regards,
    Yusuke/Japan Disty



  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 46905 points
    R1 should be used in the calculation.
    if using 100uF output capacitor, the Cff can be 10nF. but if the COUT is only 1uF or several uF, please use 100pF.