Tool/software:
Hi,
I am currently working with the BQ25756 and experiencing an issue when operating in reverse mode. When the device is configured to provide 20 V at higher load currents (above ~3 A), I notice audible noise coming from the system. The noise is quite loud and unacceptable for my application.
I have attempted several hardware and software changes to address this issue (schematic attached), including:
-
Increasing both input and output capacitance by 100 µF (100 µF in parallel with C34 and 100 µF in parallel with C39),
-
Adjusting the dead-time setting to the maximum (register 0x3C set to
0b0001111), -
Reducing the driver strength (register 0x3B set to
0b11111111) to achieve smoother transitions, -
Increasing the switching frequency by using a 52 kΩ resistor on the FSW_SYNC pin (R37 in the schematic),
-
Removing R33, R34, C49, and C48,
-
Adding a 1 MΩ resistor at R32.
For reference, the inductor’s saturation current is 24 A, and its DC maximum current is 16.5 A.
These adjustments have helped mitigate the problem to some extent, but the audible noise is still present.
I'm using this converter for all types of conversions—from VAC to VBAT—and everything works fine. I can achieve high power delivery without issues, whether the input voltage is 5 V or 20 V, validating both buck and boost operation. In reverse mode, when operating as a buck converter (i.e., reverse voltage is lower than battery voltage), it works well. However, when I try to sink 20 V at more than 3 A in reverse mode, the system begins to emit audible noise.
Therefore, I would like to know:
-
Is this behavior an expected characteristic of the BQ25756 in reverse mode?
-
Are there any additional changes I can apply to fully eliminate the audible noise—such as increasing inductance?
Thank you in advance for your support.
