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BQ27421-G1: Hooking up USB to BQ27421-G1.

Jon Ciccozzi17
Prodigy 140 points
Part Number: BQ27421-G1

I am seeing an issue when I hook up a USB cable to the battery gauge.  In this state the microprocessor is unable to communicate with the device.  

This is not a consistence issue but happen enough.  The microprocessor is powered up and has the correct voltage, how ever we are not able to communicate with the chip to get the voltage and current.

I2C bus receive no response from the part.    

The device works correctly when the battery is inserted.  

Below is the diagram.  Ex+ and Ex- are connect to V+ and Gnd on the USB cable.   

  • 0
    TI__Guru** 105600 points

    Hello Jon,

    I think there was an issue with your image upload.

    You are testing using a USB power to simulate the battery? 

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 140 points

    When you take out the battery and put in our custom USB cable, the device is a mass storage class device.  So the USB power and gnd will be connected to the BQ part. 

  • 0 in reply to Jon Ciccozzi17
    TI__Guru** 105600 points

    Hello Jon,

    As long as there is a good power connection the gauge it should communicate, we use power supplies for the gauge inputs often for testing.

    I would double check all your connections using the USB to make sure the gauge is really getting power.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 140 points

    Wyatt,

    What I found that this issue only happens when connected to Laptop not to desktop.  I also found a solution where I put a 1 ohm resistor in line with the USB V+.

    I think it has something to do with the inrush of current and the Laptop USB power maintaining voltage.  Perhaps the volts dips to low momentarily and locks the LDO on the BQ27421.  But because the SRX output is powered directly by the battery through a internal resisitor, it still provides power to rest of the system.  

  • 0 in reply to Jon Ciccozzi17
    TI__Guru** 105600 points

    Hello Jon,

    Yes an in series resistor will help filter out any spikes, you may need to include this in your system is your supplies are irregular.

    It could have triggered a safety event and shutdown the gauge.

    Sincerely,

    Wyatt Keller