• State TI Thinks Resolved
  • Locked Locked
  • Replies 8 replies
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 1542 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM25085A: LM25085A Ripple Configurations Circuit Question.

Bob Lee62837
Mastermind 7230 points
Part Number: LM25085A
Other Parts Discussed in Thread: TPS53318, LM25085

Dear support member

My customer used LM25085A.

I have two question.

Q1.
Even in the configurations of Fig 28, Fig 33, and Fig 34 in the data sheet
Is the equation for calculating the output voltage to be Equation (2) on page 10?


Q2.
When R3 resistance of Fig28 is removed,
Can we guarantee the design?


When R3 resistor of Fig28 is inserted, it outputs higher than the set output voltage.
I want to remove R3.


Best regard.
Bob Lee.

  • 0
    TI__Mastermind 23825 points
    Hi Bob,

    Please see my answers below:
    1) For Q1
    Yes the equation (2) on page 10 can be used to calculate the output voltage for those figures.

    2) Q2
    When R3 is removed, you must have enough ripple at the output in order to make the controller stable. In other words you have to have enough ESR on the output capacitor to make a noticeable ripple at the output. You can use the Aluminum electrolytic capacitor for example with high enough ESR. Or you can follow configuration Figure 33 or 34 by adding a resistor in series with output capacitor.

    In general the R3 and C1 network should only slightly affect the DC operating point of FB because you have the AC coupling capacitor C2. If you do not have the capacitor C2 on this then i can see how the DC output voltage can be shifted but with the C2, your operating output voltage should be close to what you set in the feedback resistor divider.

    Thanks and i hope that helps
    -Arief
  • 0 in reply to Arief Hernadi
    Mastermind 7230 points

    Dear Arief Hernadi,

    Thank you very much for reply.

    May I ask add question?

    Just to be sure for sure, TPS53318 also has the same ripple injection circuit.
    Although ripple and Rr and Cr are added to the equation of VFB in equation (11)
    Can not say the same thing for LM25085 A?


    Best regard.
    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Mastermind 23825 points

    Hi Bob,

    Yes, the idea is similar. As you can see on the datasheet of TPS53318, they recommend the injection value is less than 50mV. and the injection circuit is on figure 45 instead of figure 34. If you follow the equation and go through the number, the FB will move slightly due to injection circuit. This will be the same for LM25085A. 

    What i would normally do in designing these type of converter is:

    1) Calculate what FB resistor normally to get the output voltage that you wanted

    2) Set the RC value for injection circuit and also the AC coupling capacitor (1nF - 10nF)

    3) Power up and check if the design is stable throughout load

    4) Adjust the top FB resistor to fine tune the output voltage that you wanted. 

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Mastermind 7230 points

    Dear Arief Hernadi

    Thank you very much for reply.


    May I ask a add question?

    Is it no problem to use the same formula to say that it is similar?


    When using the formula of LM25085 A in customer's specification, the calculation result and measured value do not match.
    When using the formula of TPS53318, the calculation result matches the measured value.


    I have attached file.

    Could you check attached file?


    Best regard.
    Bob Lee.

    LM25085A_Question.pdf

  • 0 in reply to Bob Lee62837
    TI__Mastermind 23825 points
    Hi,

    Could you measure what is the FB voltage on LM25085A with and without the injection circuit with a digital multi meter?
    Also, what happened if you changed C2 into 0.1 uF instead of 0.01uF

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Mastermind 7230 points

    Dear Arief Hernadi,


    Thank you very much for reply.


    I ask customers to confirm.

    I have  two question.


    Q1.
    Please tell me the reason for changing C2 from 0.1 to 0.01 μF.


    Q2.
    Can you answer the question of the other day?
    Is it no problem to use the same formula to say that it is similar?


    Best regard.
    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Mastermind 23825 points
    Hi Bob,

    For
    Q1:
    The reason changing C2 from 0.01 uF to 0.1uF is to move the corner frequency of the highpass filter formed by C2 and bottom feedback resistor. Therefore, less DC will be injected from the injection circuit

    Q2:
    Yes, i believe the formula is similar

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Mastermind 7230 points

    Dear Arief Hernadi,

    Thank you very much for reply.
    I will feedback from my customer.

    Best regard.
    Bob Lee.