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BQ76920: Application and communication issue

Akarsh Gopal
Prodigy 80 points

Part Number: BQ76920
Other Parts Discussed in Thread: ENERGIA

Hello

I am working with the bq76920 AFE for a BMS project, and have a problem communicating with it. 

I have followed the typical application given in the datasheet for the said AFE ,  and am using an MSP432P401R(launchpad) to communicate with it on I2C ( However I have tried using Energia(for MSP432 family) to do this and not Code Composer. Additionally, I have also tried using Arduino for the same.) .When I scan the I2C bus for devices, there is no hit on the IC( I specifically tried scanning addresses 8 and 18  as well ).  Also, I am not sure the IC is booting up on application of Vboot on TS1. I can read 3.3 V on CAP1 , but get only 0.3V on REGOUT , and the voltage on CAP1 falls  randomly.

My objective: Use the bq76920 sample to measure the parameters of 5 cells, and communicate with MSP432P401R.

Could someone please walk me through the steps to achieve my objective if I have just the bq76920 and the MSP432P401R with me (and not the evm).

  • 0
    TI__Guru 54715 points

    Hi Akarsh,

    It sounds like you may have a missing supply to the part, a shorted REGOUT or a damaged IC.  A process to check might be the following:

    • With power off, ohm REGOUT to GND with a DMM suitable for electronics (since your ohm meter applies a voltage, you want to use one which uses a low voltage).  If it is several kOhm it is likely not shorted.  If it is a few ohms you likely need to find and fix the low resistance.
    • Apply power to the board, confirm you have a suitable supply voltage at the BAT and REGSRC pins.  The part is in a low current state and the voltages should be near the board input voltage.
    • Apply your boot voltage to TS1, confirm it is above the VBOOT threshold and below abs max limits.
    • Measure CAP1, it should be 3.3V.
    • Measure REGOUT, it should be 2.5 or 3.3 depending on the version part you have. 
    • If the REGOUT voltage is low as you observed, you need to determine if the part is not turning on the regulator or driving maximum current into some dynamic short.  You might measure the REGSRC pin again to see if the voltage is similar to before.  There should be a voltage drop depending on the load current.  The maximum current the part can provide with the datasheet specifications is 20 mA, short circuit current will be higher.   Calculate your current from the filter resistor to see if
    • If your REGSRC and BAT pins share the filter, pulling down REGSRC will pull down BAT and if below VSHUT the part will turn off causing CAP1 voltage to drop.
    • If REGSRC is drawing high current you might try lifting the REGOUT pin to see if the load is in the part or the net.  With the fine pitch this may be difficult. Removing the part and inspecting for shorts under the part or powering the REGOUT net with a supply to check the load current.
    • REGOUT is used internal to the part to power the I2C buffers (and others), so it will not communicate until REGOUT works.

    There are many troubleshooting techniques, hopefully this suggestion helps.

  • 0 in reply to WM5295
    Prodigy 80 points
    Thank you for the above, I'll follow the process and get back to you.
  • 0 in reply to WM5295
    Intellectual 340 points
    I was having a similar issue and desoldered the BQ IC from my board. I measured the resistance between REGOUT and VSS and it was only ~10 ohms. The low resistance in my case was the IC itself. What could have caused this issue?

    Thanks!
  • 0 in reply to Scott Nelson
    TI__Guru 54715 points

    Hi Scott,

    The system failed to notify us or we overlooked your post.  Hope you issue has been resolved.

    ESD is a common damage cause to semiconductor devices.  Other causes for damaged REGOUT may be excessive voltage applied to the output or excessive power in the part.