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TPS54202: NC Parameter in the Calculation for Icout (RMS)

Mark Williams
Intellectual 510 points
Part Number: TPS54202

I am currently working through the design calculations for the TPS54202 IC.

On page 16 of the datasheet the calculation for ICout(RMS) has a parameter "Nc" which is not described anywhere. Could you confirm what this parameter is please?

http://www.ti.com/lit/ds/symlink/tps54202.pdf

Thanks, Mark

  • 0
    TI__Mastermind 19080 points
    Hi Mark,

    It's the number of output capacitor. As the more capacitors you have in parallel, the less current you have on each capacitor.

    Regards,
    Sam
  • 0
    Intellectual 510 points
    Also could you help clarify the feedforward capacitor requirements. Equation 14 shows f0=3.95/(Vout*Cout). The paragraph around this seems to suggest that below 40kHz the feedforward cap isn't required.

    But then later on Table 2 shows with Vout 1.8V, Cout 66uF that a 47pF cap should be used. But with these values f0 is 33.25kHz (below 40kHz).

    My calcs for 22uF Cout, 100k R2 and 5V Vout are coming out with:

    f0 = 35.9kHz (3.95/(5*22E-6))
    Feedforward cap requirement: 44.3pF (based on eq 16)

    I just want to make sure I am understanding this right as f0 is below 40kHz but calc 16 would suggest I need a capacitor? Is this to give extra phase margin? I will add the footprint to the design anyway but good to get clarity.
  • 0 in reply to Mark Williams
    TI__Mastermind 19080 points

    Hi Mark,

    The equations you mentioned are just an estimation. Whether to place a Cff here is a thing of good or better, not good or bad. The final criteria should be based on the bode plot. The estimation is just a quick reference suggestion.

    From the perspective of time domain , Cff provide a fast path for small high frequency error signal to go to the FB pin.

    From the perspective of frequency domain, Cff forms a zero with RFBtop, a pole with RFBtop//RFBbot. The zero is at lower frequency than the pole. So it enlarges the bandwidth, and introduces a phase boost, which is good for stability and fast loop response.

    For your application of 5V Vout, I suggest you can place a Cff here. 

    Regards,

    Sam