BQ24725A: BQ24725A – Audible switching noise

Hello Sergii Zavgorodnii,

Could you provide me a waveform of the switch node while the noise is occurring? I'll be able to get a better idea of what could be causing the issue. 

Best regards,

Aaron 

Hi Aaron,

I'm Sergii's colleague working on this issue. Sorry for delay with reply.

Here are requested waveforms of the switch node when noise is occurring (mode described above). Same point at same time in different timeframes and scale for clarity:

And this is audio spectrum of charger noise and an ambient noise (charger off) for reference:

Thank you for this information, I will be looking more into this. 

Dear Elman,

I have two suggestions, 

1. Try adding a small gate resistor to the external FET's (5-10ohms).

2. Switch your output capacitors to tantalum capacitors. 

Let me know if this helps.

Best regards,

Aaron 

Thanks for recommendations.

I have tried 4R7 and 9R1 gate resistors with current 2x10uF ceramic and with 2x10uF tantalum (all combinations). Unfortunately no noticeable difference.

Dear Elman,

I have two questions that may help me further diagnose this issue. Under no system load does this noise still occur? As system load increases do you observe changes in the 8.4kHz peak? 

Best Regards,

Aaron 

Aaron,

Noise present when charging current is about 2-2.5A with battery voltage about 13V. It disappears when load or voltage is significantly different, including no load and max load.

I have also tried to increase output capacitance from 20uF to 40uF. In this case noise is present when battery voltage is 12.6V, but disappears when battery voltage in 13V. So it looks like the point where noise is present moves to lower voltage with output capacitance increase.

Battery is 12V 10AH SLA

Thank you for this, I will investigate this further.