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BQ24171: LC resonant constant

Hirotaka Matsumoto
Guru 27585 points
Part Number: BQ24171

Hi all

Would you mind if we ask BQ24171?

<Question1>
Our customer would like to use constant with C=4.7uF, 10μF or 22uF and L=10uH.
Current condition is maximum 800mA.
We assume that it doesn't matter.
If you have some comments, could you let us know?


<Question2>
There is the description which LC resonant constant must be within 15kHz upto 25kHz on the datasheet P23.
So, should we consider followings?
Cout : DC bias, temperature characteristics
L : DC superimposition characteristics
As the background of our question, we would like to know whether the resonant constant(15kHz upto 25kHz) means ideal value or nominal value.

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Guru**** 166696 points
    Matsumoto-san,

    Regarding Q1, with L=10uH, I recommend either C=10uF. Even though C=4.7uH is within the small signal resonant constant range, I think it will too small for large signal events like transients.

    Regarding Q2, I would consider capacitor DC bias and temperature effects. So, it might be best to place two 4.7uF capacitors with much higher voltage rating in parallel to get 10uF instead of one large 10uF capacitor.
  • 0 in reply to Jeff F
    Guru 27585 points

    Jeff san

    Thank you so much your support always!
    OK, we got it.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Jeff F
    Guru 27585 points

    Jeff san

    Our customer has an additional question.

    Does LC resonant 15kHz～25kHz on the datasheet have some tolerance?
    Because, when our customer configures target LC resonant = typ20kHz, according to tolerance of parts(DC bias, temp condtions), it might go over 15kHz～25kHz.
    Therefore, if you have tolerance for it, could you share us?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru**** 166696 points

    Matsumoto-san,

    I do not have tolerance data.  In general, for buck converters it is okay to increase output capacitance, i.e. go lower in frequency.  I would estimate but cannot warranty that the converter small signal stability will be unaffected down to 10kHz.