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LM25085A: About Reduced ripple configuration (R3, C1, C2)

Tsukui Yusuke
Guru 21930 points
Part Number: LM25085A

Hi,

Could you tell me about reduced ripple configuration.
Is R3,C1,C2 a fixed value in Excel sheet?
www.ti.com/.../lm25085aquick-calc

It is different from the calculation formula of datasheet.
R3,R2,C2 looks like a fixed value.
Should I consider the datasheet formula?

LM25085A.pdf

Best Regards,
Yusuke/Japan Disty

  • 0
    TI__Mastermind 31900 points
    Hello Yusuke san, please use the approach detailed in the datasheet as the approach in the QS tool is a crude approach. Thanks.
  • 0 in reply to David Baba
    Guru 21930 points

    Daivid-san,

    Thank you for your information.
    I will use datasheet formula as reference.

    Let me ask you one more thing about datasheet formula.
    Customer made calculations based on the datasheet.
    However, Vout was unstable using the calculated constant.
    (By changing the constant, Vout stabilized.)

    R3:6.6k→100k
    C1:0.0033uF→0.01uF

    Customer is suspicious of the validity of the formula of the datasheet.
    Could you please give us an opinion on this reason?
    I attach an excerpt of the calculation formula below.

    VA = VOUT - (VSW x (1 – (VOUT/VIN(min))))
    tON=(1.45x10-7x (RT+1.4))/(VIN-1.56V+RT/3167)+50ns
    R3 x C1 =((VIN(min) - VA) x tON)/⊿V

    VIN

    19

    V

    RT

    103.3

    kΩ

    tON

    0.918873819

    μs

    VOUT

    12

    V

    VSW

    1

    VIN(min)

    12.4

    V

    C1

    3.3E-09

    F

    VA

    11.968

    ⊿V

    0.03

    V

    R3*C1

    1.32397E-05

    R3

    4012

    Ω

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 31900 points
    Hello,

    The datasheet speaks of choosing R3 to at least 10k. Customer started value at 6.6K? Also the formula given in the datasheet is an approaximation and final values may need to be modified on the bench depending on the specifications?
  • 0 in reply to David Baba
    Guru 21930 points

    David-san,

    Thank you for your response and support.
    Yes. Customer first tried 6.6k. Next Customer tried 10k.
    Finally calm value was 100k.

    From this,Customer is suspicious of the validity of the formula of the datasheet.
    (Calculation result is not very reliable.)
    Please advice the important point in selecting R3.


    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 31900 points

    Hello Yusuke san,

    I am glad that the circuit is now working. Please see link below that describes in detail the importance of selecting the right value for R3. I believe this should be calculated at Vin nom, not Vin min.


     www.ti.com/.../snva166a.pdf