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TPS65400: Purpose and value calculation for C1 across the top feedback resistor

Jeff Richelsoph
Prodigy 90 points
Part Number: TPS65400

Hello.  A capacitor C1 is shown on the TPS65400 datasheet, section 8.3.11 figure 17.  No other reference to this capacitor is present.  The EVM schematic shows this capacitor varies for each rail (C11=120pF, C19=160pF, C27=68pF, C37=68pF).  There is no explanation for the purpose of this capacitor or how the value is calculated.

Please provide information concerning the purpose and value of this part.

Thank you.

  • 0
    TI__Guru**** 191445 points

    Jeff,

    That is a feed forward capacitor (Cff).  It will provide some phase boost to the loop compensation.  It places a pole zero pair with the zero at 1/(2*pi*Rtop*Cff) and the pole at 1/(2*pi*Rtop||Rbottom*Cff).  Since Rtop and Bottom are fixed by the output voltage, the distance between the zero and pole is fixed and proportional to Vout/Vref.  For maximum phase boost, the zero and pole should be symmetrical about the crossover frequency.  For ease of design, I generally place the zero slightly above the crossover frequency as placing the zero below the original crossover frequency will change the frequency to a higher value, necessitating an iterative design.  I hope that answers your question.

  • 0 in reply to JohnTucker
    Prodigy 90 points

    Hello John,

    So, on the datasheet C1 is actually Cff and there is an equation to find that.  Please help me with the math.

    On the EVM schematic, the RCLOCK_SYNC resistor is 374k.  Therefore FSW(kHz) = 138644 * 374 ^ -0.948 = 504.5kHz.  This doesn't match the graph in figure 12 which shows something closer to 400kHz, but lets go with the 504.5.  Fc = FSW / 10 = 50.45kHz. Looking at Vout1 where Rfb1 = 12.7k:

    Cff (hoping I got my units right), comes out to 247pF.  However, the value on the schematic is 120pF.  What am I doing incorrectly?

    Thank you.

    Jeff

  • 0 in reply to Jeff Richelsoph
    TI__Guru**** 191445 points

    Jeff,

    If you are referring to figure 17, then yes C1 is the feed forward cap (Cff)  The equation for it is given in Table 3.  They just use the first equation in my previous post and solve for Cff rather than Fzero.

    As far as the EVM is concerned, 12.7k and 120 pF give a zero frequency of 104.4 kHz.  You are assuming the crossover frequency is 50.45 kHz.  While placing the loop crossover at Fsw/10 is one possible option (and a reasonably good rule of thumb), the designer may have chosen a higher crossover frequency.   Nowhere in the Users Guide is there any information about the actual loop crossover.  So your math is correct as long as you use your assumption.