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LMR33640: CFF calculation

Jeferson Pehls
Expert 1125 points
Part Number: LMR33640

Hello.

About LMR33640, at page 21/44 of the datasheet is shown this equation.


My question is about the value of Cout. For example, if I am going to use 3x 22uF (22uF nominal) ceramic capacitors at output. Then at the equation should I input 66uF? or should I input the effective capacitance of them (summed) at the DC bias (at the calculated output voltage)?

Regards.

  • 0
    TI__Genius 10941 points

    Hi Sir,

    I think you can input 66uF under the premise of enough DC rating cap, thanks.

    Colin

  • 0 in reply to Colin Chen
    Expert 1125 points

    Hello, thanks!

    Could you help me with this? Are the calculations correct?

    ------------------------------------
    FEEDBACK RESISTORS FOR 8V OUTPUT
    ------------------------------------
    Vout = 8V
    Vfb = 1V
    Ratio = (8-1)/1 = 7
    Resistors = 820K and 115K (100K + 15K)
    Vout = 1+1*(820/115) = 8.13V
    8.13V / (935K) = 8.7uA

    FEEDFORWARD CAPACITOR
    CFF < (Vout * Cout) / (120 * Rfbt * sqrt(Vref/Vout))
    CFF < (8.13 * Cout) / (120 * 230000 * sqrt(1/8.13))

    Output capacitors = 3x the capacitor with this description:
    Capacitor ceramic SMD, 22uF, +-10%, 25V, 1206, X5R, -55°C ~ 85°C, CL31A226KAHNNNE (Samsung)
    LINK: product.samsungsem.com/.../CL31A226KAHNNN.do
    Effective capacitance @ 8.1V ~= 8.8uF (~ -60%)

    Calculations of CFF with 22uF and 8.8uF:
    22uF:
    CFF < ( (8.13 * (22*10^-6)) / (120 * 935000 * sqrt(1/8.13)) ) *10^12
    CFF < 4.54pF
    8.8uF:
    CFF < ( (8.13 * (8.8*10^-6)) / (120 * 935000 * sqrt(1/8.13)) ) *10^12
    CFF < 1.81pF (consider this)


    ------------------------------------
    FEEDBACK RESISTORS FOR 3.3V OUTPUT
    ------------------------------------
    Vout = 3.3V
    Vfb = 1V
    Ratio = (3.3-1)/1 = 2.3
    Resistors = 230K (220K + 10K) and 100K
    Vout = 1+1*(230/100) = 3.3V
    3.3V / (330K) = 10uA

    FEEDFORWARD CAPACITOR
    CFF < (Vout * Cout) / (120 * Rfbt * sqrt(Vref/Vout))
    CFF < (3.3 * Cout) / (120 * 230000 * sqrt(1/3.3))

    Output capacitors = 3x the capacitor with this description:
    Capacitor ceramic SMD, 22uF, +-10%, 25V, 1206, X5R, -55°C ~ 85°C, CL31A226KAHNNNE (Samsung)
    LINK: product.samsungsem.com/.../CL31A226KAHNNN.do
    Effective capacitance @ 3.3V ~= 17.6uF (~ -20%)

    Calculations of CFF with 22uF and 17.6uF:
    22uF:
    CFF < ( (3.3 * (22*10^-6)) / (120 * 230000 * sqrt(1/3.3)) ) *10^12
    CFF < 4.77pF
    17.6uF:
    CFF < ( (3.3 * (17.6*10^-6)) / (120 * 230000 * sqrt(1/3.3)) ) *10^12
    CFF < 3.82pF (consider this)

  • 0 in reply to Jeferson Pehls
    TI__Genius 10941 points

    Hi Sir,

    As you said: "Output capacitors = 3x the capacitor with this description", the Cout should input 3x22uF=66uF.

    Thanks

    Colin

  • 0 in reply to Colin Chen
    Expert 1125 points

    Hello Colin.

    Actually, what I wanted to ask is, does those results of the calculations for the CFFs of my previous post, with few pF values, does these values make sense for you?

    Thanks.

  • 0 in reply to Jeferson Pehls
    TI__Genius 10941 points

    Hi Sir,

    Generally, CFF are often range from tens of pF to hundreds of pF. From what Cout you choose, the CFF should be:

    CFF < ( (3.3 * (22*10^-6*3)) / (120 * 230000 * sqrt(1/3.3)) ) *10^12
    CFF < 14.3pF
    17.6uF:
    CFF < ( (3.3 * (17.6*10^-683)) / (120 * 230000 * sqrt(1/3.3)) ) *10^12
    CFF < 11.46pF (consider this)

    B R

    Colin

  • 0 in reply to Colin Chen
    Expert 1125 points

    I understood my mistake now. (multiplying by 3 capacitors).

    Thank you so much.

  • 0 in reply to Jeferson Pehls
    TI__Genius 10941 points

    You are very welcomed.