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Original question:

BQ25887: Audible noise can be heard near the IC when taper charging.

BQ25887: Audible noise can be heard near the IC when taper charging(re-post)

kuro
Prodigy 90 points
Part Number: BQ25887

I measured the voltages at each node and compiled them into a document.
The post is locked on E2E, I couldn't reply directly, so I'll send you the material here.
Please check it out.

When the noise is audible, it is at least taper-charged, but the conditions under which the noise is audible are more complex and less reproducible.
I thought that audible noise also occurs when there is oscillation in the VSW as shown in fig.2B and fig.3B, but this does not seem to be the case.
The audible noise was also generated when there was no oscillation in the VSW as shown in fig.4.

The oscillation of VSW is probably due to the parasitic capacitance of the inductor, which is a behavior specific to DCM step-up converters, and the schematic of the BQ25887 evaluation board (SLUUC12) shows that a snubber circuit can be mounted on the SW pin, which may be a solution to this problem. It seems to be the case.


Since the mounting area is small and there is no microphone for measurement, I can't determine which component is emitting the sound, but it could be C19 or L1 in the schematic.

Can you suggest a hardware or software solution?

Sorry for the long wait for your reply.

200727_BQ25887_audible_noise_issue.pdf

  • 0
    TI__Genius 10385 points

    Kuro-san,

    In software please make sure PFM_OOA_DIS is set 0. In hardware the audible sound can be reduced with mechanical dampening or by choosing a different inductor. We always recommend following the BOM from the EVM for optimal performance. Are you observing this audible phenomenon on the BQ25887EVM?

    I hope this resolves your issue,

    Ricardo 

  • 0 in reply to Ricardo
    Prodigy 90 points

    Hello Recardo,

    The Audible phenomenon is not occurring on BQ25887EVM, but on the PCB I created.

    PFM_OOA_DIS has been set to 0 for some time now.
    I also tried PFM_DIS, but it didn't work, so I've enabled PFM.

    I changed the inductor to the one of the same series with different value (1.2uH, 2.2uH, 4.7uH) and confirmed the audible noise.
    The higher the value of the inductor, the lower the frequency of audible noise.
    But even at 4.7uH, the sound keeps coming out.

    The inductor in the BOM of the BQ25887EVM is too large for my current PCB footprint, so I need to use a different inductor.
    I'd like to do some more experimentation with inductor replacement in the future, but could you show me the current waveforms of the BQ25887EVM so that I can determine the current rating of the inductor I'm purchasing?

    Thank you,

    Kurokawa

  • 0 in reply to kuro
    Prodigy 90 points

    It seems that the sound was coming from the decoupling capacitor attached to the MID terminal.

    I removed the capacitor or replaced it with a 10uF and the noise went away.

  • 0 in reply to kuro
    TI__Genius 10385 points

    Kuro-san,

    Please refer to Figure 70 through Figure 80 of the datasheet. These oscilloscope shot were taken using the EVM and the inductor described.

    Could you please provide the voltage rating for the 0.1uF capacitor and the 10uF capacitor you used?

    Thanks,

    RIcardo