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BQ76940EVM: BQ76940EVM

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

Hello,

I am starting to work on evm bq76940, but I want to use only the front end with 10 cells, so I have shorted the test points: vc3 and vc4, vc7 vc8 and vc9, vc12 and vc14.
Then I connected the batteries and measured the voltage between bat + and bat- and read 37v, then measured the voltage between pack + and pack- and read 31v ... why is there this 6v difference?

Another question is if the bq76940 supports i2c communication with another microcontroller that has 3.3v voltage level?

Thanks!

  • 0
    TI__Expert 6085 points

    1)seems the 10cells are connected wrong on the  15S systems( bq76940). please follow Table 8-3 in the d/s.

    there should be a minimum of 3 cells/group.

    2)  depending on the version of the bq, there is 3.3V or 2.5V of the REGOUT. refer to table 4 of the d/s.

    typically the  REGOUT of the bq76940 is used to power the uC and also to tie the pullups ( 5K-7k) on SCLK,SDATA lines

  • 0 in reply to Vish Nadarajah
    Intellectual 370 points

    Above is the EVM and the connections I did as specified on the first message I sent.

    The shorts I did I've followed the Table 8-3.

    The REGOUT i've measured 2.5v but I won't use it to power the host uC and I won't use the bq78350. I'd like to test using other uC, but this is powerd from another source and is 3.3v.

    My doubt is if I connect the i2c from this host that is 3.3v powerd wil I damage the bq76940 or the bq is 3.3v tolerant?

    the bq version is BQ7694000.

    Thanks

  • 0 in reply to Joao Paulo Teixeira
    TI__Guru 54715 points
    Hi Joao,
    The bq76940 SCL and SDA pins can work at 3.3V and will not pull up the REGOUT voltage.
    The EVM has pull up shunts J6 and J7 for the pull ups to REGOUT. If your 3.3V board does not have pull ups for SDA and SCL and will work with 2.5V signaling on SDA & SCL you could operate the EVM with the shunts. If your 3.3V board has pull ups to 3.3V, remove the J6 and J7 shunts so that the pull up resistors do not pull up REGOUT.
    ALERT is different, it will pull up the REGOUT voltage. You may need to use a limiter between the 3.3V and 2.5V.
    The bq76940 can operate with a 3.3V REGOUT, but modulating the voltage on a bq7694000 from 2.5 to something higher may produce undesired results.
  • 0 in reply to WM5295
    Intellectual 370 points
    Hello,

    About I2C I understood, but about the ALERT I didn't understand...
    If the host uC is powerd with 3.3V but recognize the 2.5V as high level I can connect the ALERT pin direct to de host, am I right?
    I need to know this because I want to test the AFE with other uC being the host, and this host is 3V3 based. When I customize the board i'll chose the BQ76940 version that has a REGOUT of 3.3, but for now what I have is the EVM that works with 2.5V.

    And another problem that I had is that I connected the batteries and measured the voltage between bat + and bat- and read 37v, then measured the voltage between pack + and pack- and read 31v ... why is there this 6v difference?

    Thanks
  • 0 in reply to Joao Paulo Teixeira
    TI__Guru 54715 points
    Hi Joao,
    Yes, if your 3.3V device recognizes 2.5V as a high connecting the 2.5V ALERT to the 3.3V input should be fine. The possible issue comes if the 3.3V device wants to drive ALERT high to cause the alert-override condition. Applying 3.3V onto ALERT can raise the REGOUT voltage above 2.5V and while not damaging may result in unexpected operation due to the voltage change.

    For the pack voltage I expect the FETs are off, this is the reset condition. With the CHG FET off there is a 1M pull down by the CHG pin and the load present comparator is enabled. There is also the 1M current limiting pack- voltage sensing resistor R8 on the EVM. Measuring the pack voltage with a standard 10M ohm input volt meter will pull up PACK- and create a voltage divider. Voltage across D3 is small since the current is low. The expected voltage in this condition is approximately 10M/(10+2)M x Vbattery. In the case of 37V, this calculates to 30.8 V. A residual load on pack or volt meter with a different input impedance will show a different value.
  • 0 in reply to WM5295
    Intellectual 370 points

    Hello,

    One more question that i have is about the power consumption, as the BQ76940 divides the cells in groups, do it causes an imbalance of cell of different groups?
    For example if the Alert pin is set it will cause a higher consumption from the bottom group.. are there other conditions that may cause this imbalance?

    And what would be the total consumption of the bq76940 if all the features are on? (ADC, CC, etc)

    Thanks very much

    Regards

  • 0 in reply to Joao Paulo Teixeira
    TI__Expert 6085 points

    Please refer to group currents( dInom,dIship, dAlert, dIcell, listed in the d/s table. page 14.

    the illustrations below will give an explain the branch currents.

    2) typical NORMAL mode( ADC ON , CC ON) =195uA MAX. Likewise other modes like normal mode currents( ADC on/off, CC on /Off are given in table 6.5 of the d/s.

  • 0 in reply to Vish Nadarajah
    Intellectual 370 points
    Hello,

    Ok I got it, but what about the imbalance that the bq76940 causes that I told above?
    Is there a higher consumption of the bottom group? Is it significant?

    And another question is what is the maximum current that the REGOUT can provide?

    Thanks