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BQ25886: Audible Noise While Battery is not charging.

Jacob S
Prodigy 90 points
Part Number: BQ25886

Tool/software:

Hi Team, 

We are getting audible noise while we disconnect the battery from the charger IC(Charging Enabled) but the Load is connected in SYS pins.

If the battery is charging and Load current also consumed from SYS pin, There is no audible noise. But when we disconnect only battery without disturbing the SYS load we could hear the audible noise. 

Load current is around 300mA

When charger current and Load current are consumed the switching frequency is 1.5MHz. Below waveform is from PMID pin.

When we disconnect only battery the wave form in PMID pin as in the image. (Charger IC is trying to charge battery in 9.3KHz). In this stage we are having audible noise. 



More Details, 

VSET- set for 8.4V

ILIM- 0Ohm mounted

Icharge- 2.49KOhm mounted

Thanks in advance.. 

  • 0
    TI__Mastermind 40485 points

    Hello,

    Can you please see what happens when the CE/ pin is pulled high? This is likely caused by the charging/discharging of the output capacitor due to battery connection looking like an extremely small battery.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 90 points

    Hi Mike, 

    Thanks for the immediate response. 

    If the CE pin is High. We won't hear any noise with or without battery. I have verified that. 

    One more observation, If we remove Inductor which is available in PMID pin, There is no audible noise. 

    Below is the circuit. 

    Is there any recommendation to reduce the audible noise? f

  • 0 in reply to Jacob S
    Prodigy 90 points

    Hi Mike, 

    One more recent observation. 

    Based on the datasheet, Since we are not connected the battery, charger is going to PFM mode. Below is the wave form of PFM mode. 

    I hope, Because of battery is not present we are getting PFM mode frequency of 9.3KHz. Please confirm. 

    And Based on what criteria this PFM frequency getting fixed? Can we fix it above 20KHz, So we will not have the audible noise. 

    Looking forward to your response. 

    Thanks 

  • 0 in reply to Jacob S
    TI__Mastermind 40485 points

    Hi Jacob,

    As said before, this is likely caused by the charging and discharging of the output capacitor with no battery. To the charger it looks like a very small battery that it charges up, and then discharges from leakage. When you disable charge this doesn't happen. Please disable charge when no battery is present.

    You cannot remove the PMID SW inductor, the converter will not work.

    Also, we require a 47 nF BTST capacitor. We also require 44 uF close to SYS pin.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 90 points

    Hi Mike, 

    Yes, This audible noise created from the 10uF cap which was available in the battery path.

    Thanks for your support

  • 0 in reply to Jacob S
    TI__Mastermind 40485 points

    Hi Jacob,

    Glad we could resolve the issue. Please click "Resolved" if your question was answered.

    Regards,

    Mike Emanuel