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BQ25700A: Charge current oscillated with light load

Jelly li
Intellectual 290 points
Part Number: BQ25700A

charge current oscillated enter into CV mode; oscillated caused by switch to PFM mode?

charge current decrease to 0.13A, start oscillated;

add the load to system higher than 0.25A, charge current not oscillated.

is there some registers disable the PFM mode or how to fix the issue?

thanks

BR

JellyBQ25700 issue (2).docxBQ25700 issue (2).docx

  • 0
    TI__Mastermind 25800 points
    Hey Jelly,

    You cannot disable PFM mode. You may try using the EN_OOA bit in ChargeOption0 - 12h[10] to increase the minimum PFM frequency to 25kHz. However, this may not completely eliminate the ripple you are seeing.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    TI__Genius 13625 points

    Jelly,

    For me your observation is normal for me because charger is operating in PFM mode. If you concern the ripple voltage, you may add more bulk caps in system side to reduce ripple voltage.

    Thanks

  • 0 in reply to Paul Yeh
    Intellectual 290 points
    disable the 25kHZ limit, the charge current still existed at light load, keep the oscillation frequency higher than 25kHz
  • 0 in reply to Jelly li
    TI__Expert 5330 points

    Hello Jelly,

    I took some scope captures to compare your results to EVM.  I also see a current ripple of 0-300 mA when in very light-load PFM.  The following was taken with VSYS = 0, in CV mode, gradually increasing VBATT to decrease IBATT

    As with your example, current ranges between 0 --> 300 mA at very light load.  When I applied OOA, it had very little effect. 

    As said by Joel and Paul, this is due to the increased ripple when operating in PFM mode.  Here is the ripple with 1A charging, (0 A VSYS)

    (the red math is VSYS - VBATT).  You can see about 50 mA of ripple. 

    With 0.1 A charging current, PFM pulses are much farther spread (notice change in timebase), resulting in the increased ripple.  Also, because the charging current is itself decreasing, this ripple becomes more apparent.

    In this zoom you see the 0 to 300 mA. 

    I believe that your results are inline with both the EVM and the expected behavior of the device in such extremely light load.

    Regards,

    Steve