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BQ24735: ACOK not getting high

Thorsten Dalkmann
Prodigy 30 points
Part Number: BQ24735

Dear Sir or Madam,

we use the BQ24735 as a charger for our battery and it works so far. We can set the charger over SMBus and charge the battery.

Because we find no information in the registers for reading out the state of the charger (charging, charge complete, abnormal state..), we want to use the ACOK signal (together with the IOUT) to determine the actual state. Did we miss something?

The ACOK pin goes directly to an input of a microcontroller, pulled up to 3.3V with a 10k resistor. 

Now the problem: The BQ24735 pulls the ACOK low even though the BQ24735 is charging and working fine. When I measure the voltage at this pin it is between 40mV and 285mV depending on the PCB.

I also measured this conditions:

ACDET = 2.7V

REGN = 6V

VCC = 18.9V

SRN = 14.5V

Does anyone has an idea what's going on with the ACOK?

I would appreciate any ideas.

Thank you and best regards,

Thorsten

  • 0
    Prodigy 30 points

    Attached you'll find the schematic:

    Best regards,

    Thorsten

  • 0 in reply to Thorsten Dalkmann
    TI__Genius 11676 points

    Hello Thorsten,

    This is a stange issue.

    Can you determine if the MCU is actually outputing the 3.3V rail?

    Can you also check to see if the pull up resistor is actually 10k ohms?

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Hello Christian,

    yes, I measured the 3.3V and checked the 10k resistor.

    Both is ok on the different PCBs.

    Any thoughts about this? ->

    Thorsten Dalkmann said:
    Because we find no information in the registers for reading out the state of the charger (charging, charge complete, abnormal state..), we want to use the ACOK signal (together with the IOUT) to determine the actual state. Did we miss something?

    Thank you so far and best regards,

    Thorsten

  • 0 in reply to Thorsten Dalkmann
    TI__Genius 11676 points

    Hello Thorsten,

    Thorsten Dalkmann said:
    Because we find no information in the registers for reading out the state of the charger (charging, charge complete, abnormal state..), we want to use the ACOK signal (together with the IOUT) to determine the actual state. Did we miss something?

    I don't see any obvious issues with using this method to determine the state of charge of the battery.

    Thorsten Dalkmann said:
    Now the problem: The BQ24735 pulls the ACOK low even though the BQ24735 is charging and working fine. When I measure the voltage at this pin it is between 40mV and 285mV depending on the PCB.

    I recommend running a test using REGN as the pull up rail for ACOK to check if that resolves the issue.

    Please refer to section 9.3.1 Adapter Detect and ACOK Output:

    Based on the information that you have given, and the requirements for ACOK to operate. The IC may be damage, but I don't think this is the issuse since this is occuring on mulitple boards, or the pull up rail may be the issue.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Hello Christian,

    Christian Moyer said:
    I don't see any obvious issues with using this method to determine the state of charge of the battery.

    Okay. We will see which possibilities we have.

    Christian Moyer said:
    I recommend running a test using REGN as the pull up rail for ACOK to check if that resolves the issue.

    I tested it as in the EVM, without great expectations. Unfortunately, there is no difference. Still pulled low (40mV).

    Maybe something else in the schematic you would check?

    Thank you and best regards,

    Thorsten

  • 0 in reply to Thorsten Dalkmann
    TI__Genius 11676 points

    Hello Thorsten,

    I recommend replacing R14 with a 10ohm resistor and R15 with a 7.5ohm resistor to see if that resolves the issue. I also recommend reviewing the schematic in the EVM USER GUIDE.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Hi Christian,

    I did not expect any change, because this resistors are for better reversed battery polarity protection. And unfortunately the problem still exists. The only difference is that the ACOK voltage is now a bit higher (400mV).

    We have a different phenomenon. When everything is powered on and we unplug the (I2C) battery (which only communicates with the µC) during operation, only sometimes the charger seems to hung up and switches on and off the Mosfets Q1/Q2. Then we have to unplug the charger. (Which is not a big problem for us, because the battery normally will not be unplugged during operation.) Can this indicate something?

    Thank you and best regards,

    Thorsten.

  • +1 in reply to Thorsten Dalkmann
    TI__Genius 11676 points

    Hello Thorsten,

    Thorsten Dalkmann said:
    I did not expect any change, because this resistors are for better reversed battery polarity protection. And unfortunately the problem still exists. The only difference is that the ACOK voltage is now a bit higher (400mV).

    This is strange. They ACOK has to meet the following conditions to be pulled to an external pull up rail. 

    Based on the information that you gave, your charger meets all of the requirements. I also don't see any obvious issue with your schematic that could cause this problem. I suspect there is a damangend component on your board either the IC or different component. I recommend replacing the IC to see if that resolves your issue. If that doesn't work I recommend ordering the BQ24735 EVM to see if you replicate your test and see if you run into the same fault.

    Thorsten Dalkmann said:
    We have a different phenomenon. When everything is powered on and we unplug the (I2C) battery

    Are you unplugging the battery and the I2C, or just the I2C.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Hello Christian,

    thank you. We will check that.

    Christian Moyer said:
    Are you unplugging the battery and the I2C, or just the I2C.

    Both at the same time.

    Best regards, Thorsten

  • 0 in reply to Thorsten Dalkmann
    TI__Genius 11676 points

    Hello Thorsten,

    Thorsten Dalkmann said:
    We have a different phenomenon. When everything is powered on and we unplug the (I2C) battery (which only communicates with the µC) during operation, only sometimes the charger seems to hung up and switches on and off the Mosfets Q1/Q2. Then we have to unplug the charger. (Which is not a big problem for us, because the battery normally will not be unplugged during operation.) Can this indicate something?

    This problem might be in relation with the ACOK not going high. I recommend folllowing the steps in my previous reply, resloving the ACOK issue, then coming back to see if this is still a problem.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Hello Christian,

    I replaced the BQ24735 and the Microcontroller on all four PCBs and now the ACOK pin is getting high on every board. Im sorry for the confusion. I didn't expect this, because everything else worked fine. It could be good that the other phenomenon is also settled.

    I'll observe it, but I think we got it now. Thanks a lot for your effort.

    Best regards, Thorsten.