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Original question:

BQ24715: bq24715 could not loading above 1.5A on 1MHZ frequence

BQ24715: Charging noise occurs in light load mode

Lumina Gui
Genius 3740 points
Part Number: BQ24715


Hi TI Teams,

Our customer feedback a new problem when using the BQ24715. That is, when the adapter is connected and the battery is fully charged to13.8V. When the computer was shut down, they measured the output voltage of the adapter was 19.9V and the output current was 45mA. At this time, the charger will not charging the battery and just supply the power to system, and the switching frequency is jumped between 8khz to 600khz, which results a charging noise occurs. But when the notebook stars up and the load current is large, the switching frequency is 600KHz and the noise disappears.

The wavefrom is as below:

We wondering that the cause of this problem is that BQ24715 have entered PFM mode with light load, can u confirm it? Can we solve this problem by setting BQ24715 in PWM mode all the time? If yes, how to realize it? Otherwise, can u give us some suggestion for this problem?

Thanks,

Kind Regards

  • 0
    TI__Mastermind 36106 points

    Hello Lumina,

    Yes, In order to improve converter light-load efficiency, the bq24715 switches to PFM control at light loads with charge disable or charge in LDO. No PFM mode can not be disabled. Since the charging noise only occurs when the battery is at full charge. I recommend to disable charging when the battery is at full charge to see if that reduces the charging noise.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Genius 3740 points

    Hello Christian,

    Our customer feedback tha the current noise is still exists when they disabled the charging after the battery is full charged. They also found that under light loads, the noise is exists even the adapter was not plugged in.

    Thanks,

    Kind Regards

  • 0 in reply to Lumina Gui
    TI__Mastermind 36106 points

    Hello Lumina,

    The minimum frequency can be limit to 40kHz to avoid audio noise (ChargeOption() bit[10]=1). I recommend enabling this bit to check if this reduces the noise that the customer is seeing. If that doesn't work my next recommendation would be to order the BQ24715 EVM board to see if they replicate the issue to determine if there is an issue with the customers design or the actual charger.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Genius 3740 points

    Hello Christian,

    We have set ChargeOption() bit[10]=1, but the problem still exists. We also tried to modify the size of the output inductor and capacitor to exclude the resonance interference problem, and found that we need to add a very large capacitor to modify the capacitance, and the charging noise was disappears until to add more than 220uF electrolytic capacitor. But at this time we can still hear a little sound from the MOSFET, no sound like 2KHz ~ 3KHz charging noise.

    We captured the output current and voltage waveforms as follows:

    1)Waveform after VSYS inductor output

    2) VSYS waveform after adding 220uF electrolytic capacitor 

    3) The waveform of SW

    Thanks,

    Kind Regards

  • 0 in reply to Lumina Gui
    TI__Mastermind 36106 points

    Hello Lumina,

    My recommendation would be to order the BQ24715 EVM board to see if they replicate the issue to determine if there is an issue with the customers design or the actual charger. I also recommend that the customer looks at the schematic in the BQ24715 EVM user guild, to ensure that they are not missing anything in their design.

    Best Regards,

    Christian