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BQ76940EVM: BQ76940EVM

Joao Paulo Teixeira
Intellectual 370 points
Part Number: BQ76940EVM
Other Parts Discussed in Thread: BQ78350, , BQ76940

Hello,

The I2C communication stopped working and I do not know what might have happened to cause this, so let me try to explain to you how things are connected so that you can try to help me.

- I'm using BQ76940EVM but only the bq76940 chip. (I'm not using the BQ78350);

- The host is other uC and it's power level is 3v3;

- I'm using a pack of 10 batteries, so I followed the "Table 8-3. Cell Connections for bq76940";

- I2C was working... read and write messasges was working, but i don't know why it stopped;

- so I turned all off and checked if everything was fine, but I discovered that the SCL and ALERT pin was shorted to ground;

- But the bq76940 still enters in shipmode if we remove all the batteries and get out shipmode if we click the button, the regout is working, so apparently the bq is working...

So the question is what could have happend to cause such behavior? 

Is there an especific sequence to connect the batteries?

Thank you

  • 0
    TI__Expert 6085 points
    Hi Joao,
    The ALERT pin(active when HIGH) internally is tapped off the REGOUT. There is typically a filter network on the ALERT pin. 470pF || 470K. If any of these component are shorted, there could be condition to pull the > 20mA of the REGOUT, which can droop the REGSRC and BAT. The SCL and SDA externally is tied to the REGOUT via 2K-7K Ohm pull-up. If REGOUT gets out of regulation, I2C would be affected too. When the device below Vshut threshold, it will be forced to enter SHIPMODE.
  • 0 in reply to Vish Nadarajah
    Intellectual 370 points
    Hello Vish,
    I think that there isn't a filter, just a pull down resistor that is the R42 and I removed this resistor and the problem isn't the resistor, but even when i removed it the alert and the scl continued shorted to ground. There is nothing connected to REGOUT, and it's working properly (2V5).
    The bq76940 is not entering SHIPMODE, only if I remove all the bateries from the pack, but it gets out SHIPMODE if i click the button attached to TS1. So I still don't know why ALERT and SCL are shorted to ground.

    Regards,
    João Paulo
  • 0 in reply to Joao Paulo Teixeira
    TI__Expert 6085 points
    Check REGSRC and BAT connection. The device needs power from the REGSRC supply. What is the voltage on the VC5X and VC10X, CAP1, CAP2, CAP3?
  • 0 in reply to Vish Nadarajah
    Intellectual 370 points

    Hello,

    The image bellow shows how is my test setup that was working and now isn't.

    - As i'm using a 10 pack battery so I followed the "Table 8-3. Cell Connections for bq76940" to do the connections;

    - Even using the SCL and SDA pullup jumpers (J6 and J7), when I measure the voltage on J14, the SCL, SDA and Alert pins are 0v.

    - REGSCR with jumpers J6 and J7 = 12.09V / REGSCR without jumpers J6 and J7 = 12.59V

    - BAT = 39.67V

    - REGOUT = 2.52V

    - VC5X = 14.27V

    - VC10X = 27.87V

    - CAP1 = 3.29V

    - CAP2 = 17.56V

    - CAP3 = 31.17V

  • 0 in reply to Joao Paulo Teixeira
    TI__Expert 6085 points

    VCAP3 = 31.17-27.87 =3.30V

    VCAP2= 17.56V-14.27v=3.29V

    VCAP1=3.29V

    all seem Ok.

    I cannot find reason why the device SDA/SCK & ALERT are shorted. May be an external overstress on the pin may have damaged the pin and now shorted

    internally. At tis point, I can only suggest changing the device with a new device.

  • 0 in reply to Vish Nadarajah
    Intellectual 370 points
    1) Is there possible that a random cell connection causes it?
    2) Or and external 3v3 pullup on SCL and SDA damaged it?
    It's really strange...
    3) If I buy only the BQ76940 uC and change on the EVM would it fix the problem?

    Thanks,

    Regards,
    João Paulo
  • 0 in reply to Joao Paulo Teixeira
    Intellectual 370 points

    Another question that I have is that if I substitute the bq7694000DBT that is on the EVM, because as far as I understood is damaged, for the bq7694002DBT will it works on the EVM or it's better to buy a new bq7694000DBT?

    What should I do to do not lose my EVM?

    Thanks

  • 0 in reply to Joao Paulo Teixeira
    TI__Expert 6085 points

    Soldering down a known good device should resolve the issue.

    Before you solder down, power the board to note is any of the external components are damages  and  the board

    is OK.