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LM2664: Question about flying capacitor and output capacitor.

Kazuya Nakai59
Guru 21645 points
Part Number: LM2664

Tool/software:

Hello guys,

One of my customers is considering using LM2664 for their new products.

At this moment, they have the following question.
Could you please give me your reply?

Q.
Their Vin/Vout/Iout conditions are the follows.
Vin=+5V, Vout=-5V, Iout=1mA each.

In this case, what is recommended capacitor value
for flying capacitor placed between CAP+ and CAP- and output capacitor?

Your reply would be much appreciated.

Best regards,
Kazuya.

  • 0
    TI__Genius 17100 points

    Hi Kazuya,

    Sorry for the late reply because of weekend. Let me check this and give you feedback later today. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    TI__Genius 17100 points

    Hi Kazuya,

    Pls use 3.3uF for these two capacitors. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    Guru 21645 points

    Hi Aurora,

    Thank you very much for your reply.

    They have the following questions as additional.
    Could you please give me your reply?

    Q1.
    Their LM2664 output current is about 1mA.
    Is there any formula to calculate the capacitor(Flying capacitor) value placed between CAP+ pin and - CAP pin corresponding to the output load current?
    Also is there any formula to calculate the output capacitor value corresponding to the output load current?

    Q2.
    They wanted to reduce the output ripple voltage.
    So they changed the flying capacitor from 3.3uF to 0.1uF.
    Is it problem to use 0.1uF for flying capacitor? At this moment, they use 10uF
    as the output capacitor.

    Q3.
    Is it problem that large difference of capacitor value between flaying capacitor and the output capacitor as the above?
    If it is problem, could you please tell me the reason?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Genius 17100 points

    Hi Kazuya,

    For detail formula, pls see our datasheet. It is not recommended to use 0.1uF, which is too small and not conducive to stability. Customers can try to use 3.3uF first. Because the absolute values ​​of the input and output voltages are the same, we generally recommend using the same capacitance for both input and output. Of course, customers can make adjustments based on actual needs, but please pay attention to whether there are abnormal changes in indicators such as output ripple. Hope this can help. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    Guru 21645 points

    Hi Aurora,

    Thank you very much for your strong supports.

    Could I ask you an additional question?

    Q.
    We understood that lower ESR of output capacitor and larger output capacitance help to reduce output ripple
    from equation 2 on page 10 of the device datasheet.
    For example, is it no problem to change the output capacitor from 10uF to 100uF (Flying capacitor is 3.3uF) to reduce output ripple voltage?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Genius 17100 points

    Hi Kazuya,

    Yes, generally speaking, the larger the output capacitance, the better the ripple will be if the loop is stable. Also, do not use electrolytic capacitors or other capacitors with large ESR, use ceramic capacitors instead. Because the output capacitor participates in the system loop, you can gradually increase its value. If the system is unstable, you will see an abnormal increase in output voltage ripple. Hope this can help. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    Guru 21645 points

    Hi Aurora,

    Thank you very much for your strong supports.

    I understood that 3.3uF capacitor is recommended for flying capacitor
    and low ESR ceramic capacitor should be used for lower output ripple voltage.

    Could I ask you the following questions?

    Q1.
    Is it no problem to use larger flying capacitor than 3.3uF? 

    Q2.
    Currently, the customer uses 10uF for flying capacitor and 10uF for output capacitor.
    Is this no problem?

    Q3.
    They want to use 100uF for the output capacitor instead of 10uF to reduce output ripple voltage.
    Is this no problem?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Genius 17100 points

    Hi Kazuya,

    It's best to use 3.3uF capacitor. 10uF is also ok. But not suggest use 100uF, it's too large, need to be evaluated by customer. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    Guru 21645 points

    Hi Aurora,

    Thank you very much for your strong supports.

    Could I ask you the following questions?

    Q1.
    If 100uF is used for the output capacitor with 3.3uF or 10uF flying capacitor, what is the concerning point?
    In the evaluation, what point should we confirm? Is it the output voltage stability?

    Q2.
    I think that when the flying capacitor is 10uF and the output capacitor is 100uF,
    the output voltage rising time becomes slower than 10uF output capacitor case.
    But I think the output stability is not changed between 10uF and 100uF.
    Is my understanding not correct?

    Thank you again and best regards,
    Kazuya.   

  • 0 in reply to Kazuya Nakai59
    TI__Genius 17100 points

    Hi Kazuya,

    Yes, you need to see if the vout is stable, and also check the thermal of the device with different capacitance. Pls check it on customer's board at first. Thanks.

    Aurora