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BQ25895: VSYS and I2C problems without battery hooked up.

user4053363
Prodigy 210 points
Part Number: BQ25895

Hello all,

I have a custom charger design that has two problems without a battery hooked up.

1) Without a battery the VBUS voltage will not pass through to VSYS. 5V in gets me 0.33V on VSYS.

2) Without a battery the I2C is not functional. The I2C bus has the required pull-ups going to 3.3V and the 3.3V is present.

Once I plug the battery, I can remove it and still have VSYS and I2C functional.

Thanks.

-Jeff

  • 0
    TI__Mastermind 46551 points

    Jeff,

    1.Referring to page 8 of the d/s, the IINDPM is set to 200mA if VSYS is less than 2.2V.

    If the charger is powered up without the battery, VSYS will start from 0V and may stuck in 200mA IINDPM if SYS tries to pull too much current during start-up.

    2. Below are the charger specs for active I2C. Both I2C pull-up source and pull-up resistors are needed for I2C communication.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Prodigy 210 points

    Ning,

    Thanks for your response.

    With R426 across D+/D-, I thought the BQ25895 would negotiate to USB DCP = 3.25A. Therefore, the input current would be the lower of I2C or the ILIM pin and that is 2A (ILIM pin). Why is that not occurring?

    I'm kind of stuck in a loop then. I need vsys to come up in order to change the registers to get more power. Page 32 of the datasheet indicates that the default is 500mA and that would be enough get the system up and running to change the register. Its troubling that the max is 200mA when the default should be 500mA. I need to negotiate more power without a processor.

    Any other ideas?

    -Jeff

  • 0 in reply to Ning T
    Prodigy 210 points

    Ning,

    Thanks for your response.

    With R426 across D+/D-, I thought the BQ25895 would negotiate to USB DCP = 3.25A. Therefore, the input current would be the lower of I2C or the ILIM pin and that is 2A (ILIM pin). Why is that not occurring?

    I'm kind of stuck in a loop then. I need vsys to come up in order to change the registers to get more power. Page 32 of the datasheet indicates that the default is 500mA and that would be enough get the system up and running to change the register. Its troubling that the max is 200mA when the default should be 500mA. I need to negotiate more power without a processor.

    Any other ideas?

    -Jeff

  • 0 in reply to user4053363
    TI__Mastermind 46551 points

    Jeff,

    As mentioned earlier, when VSYS<2.2V, IINDPM is set to 200mA which overwrites any other IINDPM default values. So when a battery is not present, please limit SYS load to less than 200mA before VSYS reaches 2.2V so that VSYS may start up normally.

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Prodigy 210 points

    Yes, with no load the BQ25895 operates as expected. I have even modified the PCB to allow me to bring up the BQ25895 and then apply the load afterwords. VSYS immediately collapses.

    It would appear that the BQ25895 needs to be replaced. Reducing the current below 200mA is not an option.

    Whats the point of advertising that this part does not need processor intervention when its preprogrammed for 200mA and ignores all of the hardware settings?

    -Jeff

     

  • 0 in reply to user4053363
    TI__Mastermind 25800 points

    Hey Jeff,

    In our customers' applications, the system loading (non-processor) is limited or load switched-in until after the system rail has come up to full regulation. In most cases, a system voltage below 2.2V will not enable any downstream components, and at the very least, will have load synchronization or switching in order to avoid pulling power before the system rail has come up to regulation. Also common is the dead battery condition where customers will allow the charging converter to startup in order to close the protection FETs of the battery and then allow the system to perform its functions, before pulling power. 

    It is also more commonly required for power converters to have soft-start functionality in order to avoid a large amount of inrush current, which can trigger overvoltage events, faults, and damage sensitive components. 

    Altogether, the soft-start functionality of the charger is meant to limit the inrush current to the output VSYS until the rail has risen up to 2.2V. After this threshold, the system can draw beyond the 200mA current limit up to the IINDPM threshold or the ILIM resistor setting.

    Therefore, to your comment, yes the charger does not need processor intervention and a sequenced loading system design will allow the charger to regulate.

    Regards,

    Joel H