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BQ25756: BQ25756 Buzzing Noise When VIN ≈ VBAT

Umut Öznar
Prodigy 140 points
Part Number: BQ25756
Other Parts Discussed in Thread: , BQ25798

Tool/software:

Hello,

I am using the BQ25756 as a 4S charger. Especially when the input voltage is close to the battery voltage, I hear a buzzing sound. When I observe this with an oscilloscope, I see switching activity on the bootstrap capacitors of both the buck and boost sides. Therefore, I suspect the noise is caused by both buck and boost stages trying to switch at the same time when the input and output voltages are close.

I have observed the same issue on the EVM as well. Although the schematic I attached shows a different inductor, I am actually using the same inductor as on the EVM. The MOSFETs and bootstrap capacitors are also the same as on the EVM. Could you please help me with this issue?

Best regards.

  • 0
    Prodigy 140 points

    Fullscreen bq25756_config.txt Download 
    * Created: Fri Apr 11 14:34:57 EEST 2025
*
* Format: Register Name  tab Character,\t  Register Address  tab Character,\t  Hexadecimal register value.
* BQZ Container: Charger_1_00-bq25756.bqz
*
Charge Voltage Limit	00	1D
Charge Current Limit	02	1F0
Output Voltage Limit	04	00
Input Current DPM Limit	06	104
Input Voltage DPM Limit	08	348
Reverse Mode Input Current Limit	0A	640
Reverse Mode System Voltage Limit	0C	3E8
Precharge Current Limit	10	28
Termination Current Limit	12	20
Precharge and Termination Control	14	0C
Timer Control	15	00
Three-Stage Charge Control	16	00
Charger Control	17	00
Pin Control	18	60
Power Path and Reverse Mode Control	19	20
MPPT Control	1A	20
TS Charging Threshold Control	1B	96
TS Charging Region Behavior Control	1C	47
TS Reverse Mode Threshold Control	1D	40
Reverse Undervoltage Control	1E	00
VAC Max Power Point Detected	1F	00
Charger Status 1	21	00
Charger Status 2	22	80
Charger Status 3	23	00
Fault Status	24	00
Charger Flag 1	25	00
Charger Flag 2	26	00
Fault Flag	27	00
Charger Mask 1	28	00
Charger Mask 2	29	00
Fault Mask	2A	00
ADC Control	2B	80
ADC Channel Control	2C	0E
IAC ADC - Use Field View	2D	00
IBAT ADC - Use Field View	2F	00
VAC ADC	31	26AD
VBAT ADC	33	107F
TS ADC	37	00
VFB ADC	39	00
Gate Driver Strength Control	3B	00
Gate Driver Dead Time Control	3C	00
Part Information	3D	12
Reverse Mode Battery Discharge Current	62	02

  • 0 in reply to Umut Öznar
    TI__Mastermind 31155 points

    Hello Umut,

    Thanks for being patient with this. We'll get back to you later this week.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 31155 points

    Hello Umut,

    I've got a few questions to ask about your test conditions:

    • What's the input and output voltage this behavior at?
    • What's the charge current for the battery?
    • Is a real battery or a CV load being used for the BQ25756's output?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 140 points

    Hello Ethan,

    • This issue becomes more noticeable when the charger enters the CV phase. The output voltage is 16.8V, and I can hear a noise when I vary the input voltage between 15V and 18V.
    • The charging current is set to 6A, but the noise begins when the current drops the around 1.5A in CV mode and continues until the battery is fully charged.
    • I am charging a real battery. I have two different batteries in a 4S configuration, and I observed the same noise with both of them.

    Apart from this, I also noticed another issue:

    • The ITERM value is set to 400mA, but sometimes the charger terminates the charging process as expected, while other times is doesn't and continues charging.
    • When the charging current drops to around 400mA, it starts to fluctuate unstably. For example, the current suddenly rises from 400mA to 1.5-2A. I don't think this is caused by the battery, as it is a well-tested battery we've used for years. Also, when I charge the same battery using a different charger or power supply, I don't observe such fluctuations in the CV phase. We previously developed a product using the BQ25756, and we did not encounter this kind of issue in that design.

    Thanks in advance for your support.

    Best Regards,

    Umut Öznar

  • 0 in reply to Umut Öznar
    TI__Mastermind 31155 points

    Hello Umut,

    Thanks for the new information

    Umut Öznar said:
    • This issue becomes more noticeable when the charger enters the CV phase. The output voltage is 16.8V, and I can hear a noise when I vary the input voltage between 15V and 18V.
    • The charging current is set to 6A, but the noise begins when the current drops the around 1.5A in CV mode and continues until the battery is fully charged.

    Thanks for these test conditions.

    Umut Öznar said:
    • The ITERM value is set to 400mA, but sometimes the charger terminates the charging process as expected, while other times is doesn't and continues charging.
    • When the charging current drops to around 400mA, it starts to fluctuate unstably. For example, the current suddenly rises from 400mA to 1.5-2A. I don't think this is caused by the battery, as it is a well-tested battery we've used for years. Also, when I charge the same battery using a different charger or power supply, I don't observe such fluctuations in the CV phase. We previously developed a product using the BQ25756, and we did not encounter this kind of issue in that design.

    Does this behavior change if you set EN_PFM=0? We recommend setting EN_PFM=0 when the termination current is below 2A. You can also use an external clock source on FSW to disable PFM as well.

    Friday is TI holiday. I'll look more into this next week.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 140 points

    Hello Ethan,

    Thank you for your support.

    I will set the ENFM register to 0 and proceed with testing. I will continue testing and keep you updated until I hear back from you.

    Happy holidays.

    Best Regards,

    Umut Öznar

  • +1 in reply to Umut Öznar
    TI__Mastermind 31155 points

    Hello Umut,

    Thanks for being patient with this.

    I have confirmed that the BQ25756EVM does produce a faint buzzing sound when in PFM mode. I don't think this will be an issue of the part. Can your system work with this buzzing sound?

    If you don't want this noise, you can use a part like the BQ25798. The BQ25798 has out-of-audio feature to prevent audible noise while in PFM mode.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 140 points

    Hello Ethan,
    Thank you for your response. The BQ25798 is not suitable for my application, so I will proceed with the BQ25756. If this noise doesn't cause any issues for the IC, then it's not a problem for me either.
    Thanks again for your help.

    Best Regards,

    Umut Öznar