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TPS65400: What is the ranges of capacitances that will work for CA, CD and CG?

Per Magnusson
Intellectual 500 points
Part Number: TPS65400

The datasheet (section 7.7) gives typical values for the capacitors connected between VDDA, VDDD, VDDG and GND, but no max and min values. It is also not clear whether this refers to the nominal values of the components, or the actual capacitance at the applied bias voltage. The EVM has the values from the table as nominal values, but uses 0805 size caps which seems to be a size that is on its way out for these capacitance values.

I wonder what capacitors I could use here, as ceramic capacitors (especially high values in small packages) have a very large temperature coefficient, so a 4.7 µF, 10 V, 0402 capacitor might have less than 1 µF left when 6.1 V is applied. Is such a capacitor still a good choice for CA?

Would it be bad to have more capacitance than what is listed in 7.7, or is more perhaps better (or at least not worse)?

I would like to use the same capacitor for all three positions to limit the number of different components in the BOM. The following one is a candidate: Taiyo Yuden LMK107BBJ106MALT. 10 µF, 10 V, X5R, 0603. Would this capacitor be a good choice for all three positions? At 6.1 V it should have a capacitance of about 2 µF and at 3.2 V it should be about 4 µF.

Regards

Per

  • 0
    TI__Expert 7500 points

    Hi Per,

    1. 4.7uF 10V 0402 cap is not a good choice when DC bias is 6.1V.

    2. The cap listed in 7.7 is the nominal values, same as the EVM values. It's better to select the same cap (cap value, voltage rate...)according to EVM user guide.

    3. The cap you selected can be used for all three positions, VDDA, VDDD and VDDG to GND. I think for the actual cap higher than 2uF are all okay.

    Thanks.

    Lishuang

  • 0 in reply to Lishuang Zhao
    Intellectual 500 points

    Thanks for the reply, although I am not sure it clearly answered my questions.

    I do not want to use 0805 capacitors (due to space and availability reasons).

    I do not want to use three different values if that can be avoided.

    So going with the same components as on the EVM (as you recommended in point 2) would be highly undesirable for me.

    Could you perhaps clarify the following:

    4. Is there any downside (like worse performance in some regard) to doing like you say in point 3, i.e. using a capacitor with an actual capacitance around 2 µF on VDDA and VDDG and a value of 4 µF on VDDD?

    I think it would be a good idea to update the datasheet with min and max values for these capacitors and a clear statement that the capacitances listed are actual capacitances and not nominal component values. And also a description of what the effect of higher and lower capacitances on the pins are (like risk for instability with too much or too little capacitance, slow startup with high capacitance, higher noise with low capacitance or whatever the case might be).

    Per

  • 0 in reply to Per Magnusson
    TI__Expert 7500 points

    Hi Per,

    The VDDA, VDDG and VDDD are internal regulator output. If the output cap is small, maybe smaller than 1uF, the internal regulator will be not stable. So with some margin, I recommend the actual cap no lower than 2uF.

    So the cap you selected Taiyo Yuden LMK107BBJ106MALT. 10 µF, 10 V, X5R, 0603, can be used for three regulator. 

    We will consider your advises. 

    Thanks,

    Lishuang

  • 0 in reply to Lishuang Zhao
    Intellectual 500 points

    Ok. Thanks. Now I feel more confident in using the component I suggested,

    Regards

    Per