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BQ25886: BQ25886 ILIM & CHARGE CUURENT SETTING

SO KWOK CHOI
Prodigy 100 points
Part Number: BQ25886

BQ25886-SCH.pdf

I use BQ25886 to charge a 2cell 7.4V 2500mah battery. The setting as below :

ILIM = 2.36A ,    RILIM = 470R

ICHG = 0.98A , RICHG = 3K9

VBAT = 8.4V ,    RVBAT = 180K

VBUS = 6V 2.5A , OTG DISABLE , D+ & D- SHORT TOGETHER

Why the charging current is only 250mA ? Please verify the attached scematic having any mistake.

Best regards

KC So

  • 0
    TI__Guru**** 165766 points

    Hi KC,

    As a reminder, unlike a buck converter, a boost converter's input current must be much larger than its output current.  The input current can be approximated using an efficiency balance:  eff = Pout/Pin=(VSYS*(ISYS+ICHRG)+(VSYS-VBAT)*ICHG)/(VBUS*IBUS) and solve for IBUS.  You can estimate efficiency from the datasheet efficiency curves.

    On the schematic, D+ and D- are open, which the charger interprets as unknown adapter and sets input current limit to 500mA.   This would limit your charge current.

    If D+ and D- are shorted, then the charger enables its ICO routine to find the input current limit that doesn't pull the input voltage below VINDPM=4.3V typical.  If your input source, board traces or connections are highly resistive, the ICO routine might set input current limit very low.  If that is not the issue, then input current limit is set by the RILIM.

    Lastly, its possible that the board traces, cabling or connectors from the charger to the battery are highly resistive, causing the voltage sensed at the charge BAT pin to be at VBATREG=8.4V with only 250mA charge current flowing.

    Regards,

    Jeff 

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi Jeff,

    After I use resistor 180R to short D+ & D-, the result is same. Would you think that is caused by highiy resistive on the input source, board traces or others?

    Best regards

    KC So

  • 0 in reply to SO KWOK CHOI
    TI__Guru**** 165766 points

    Hi KC,

    Is the voltage at VBUS being regulated to slightly above VINDPM=4.3V?  If so, then the problem is high resistance from source to VBUS pin.  If the voltage measured at the IC's BAT pin 8.4V even the battery pack voltage is lower?  Then the problem is too much resistance from BAT pin to pack.

    Regards,
    Jeff

  • 0 in reply to Jeff F
    TI__Guru**** 165766 points

    Hi KC,

    Any update on this?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi Jeff,

    Sorry for late reply as the vacation of Chinese New Year. I think the issue is caused by thin copper track of supply input pin.

    I will modify it and check again. Thank you very much!

    Best regards,

    KC So

  • 0 in reply to SO KWOK CHOI
    Prodigy 100 points

    Hi Jeff,

    I made one more PCBa for testing. The charging current & STAT LED flash very fast.

    They will go to stable after I placed the solder iron on the chip for a few seconds. The issue will be repeated  after the supply is reconnected.

    What is happening?

    Best regards,

    KC So

  • 0 in reply to SO KWOK CHOI
    TI__Guru**** 165766 points

    Hi KC,

    Are you lightly touching or pushing hard with the iron?  Is the IC GND pins and/or powerpad not making a good connection? 

    Regards,
    Jeff

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi Jeff,

    I just lightly touch with the iron. The IC GND & powerpad are well connected.

    The phenomenon can be repeated by using a Hair Dryer with heat air on the chip.

    Is the chip working temperature not enough? Our room temperature is about 20°c.

    Best regards,

    KC So

  • 0 in reply to SO KWOK CHOI
    TI__Guru**** 165766 points

    Hi KC,

    I do not understand the solder iron effect.

    In order to help to help further, I need to see oscilloscope plots off VBUS, REGN, IBUS, and VSYS, on the same plot.  If you add additional capacitance at PMID, so the chip work correctly?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi Jeff,

    This issue is still happen after I apply additional cap. at PMID.

     I found this issue is appeared when loading current of SYS pin is more than 100mA. The chip works stable without any  sys load.

    What do you think about it?

    Best regards,

    KC So

  • 0 in reply to SO KWOK CHOI
    TI__Guru**** 165766 points

    Hi KC,

    In order to help to help further, I need to see oscilloscope plots off VBUS, REGN, IBUS, and VSYS, on the same plot

    Regards,

    Jeff