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BQ76905EVM: Cell Balancing

Fabio Gou
Expert 4445 points
Part Number: BQ76905EVM
Other Parts Discussed in Thread: BQ76905

The customer is using BQ76905 to test the cell balancing function.

When TS mode is set 0; the cell balancing function is normal.

When TS mode is set 1; without 100kΩ to ground in TS pin, only 3 diodes to ground. the cell balancing function is normal.

When TS mode is set 1; with 100k to ground in TS pin, the cell balancing can not operate.

The customer need the 100kΩ.

Could you pls tell me why "When TS mode is set 1; with 100k to ground in TS pin, the cell balancing can not operate" ?

Whether this is the internal cell balancing algrithum?

Could you pls help to reproduce this in our EVM board?

  • 0
    TI__Mastermind 42060 points

    Hello Fabio,

    What is the purpose of the diodes? Can you explain this circuit?

    It seems like the TS pin is going below ground when cell balancing is not working. Can you explain why that is happening?

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    TI__Expert 4445 points

    Hi Luis,

    The diodes is to make sure the TS could wake up BQ76905. Because the diode turn-on voltage is higher than 1.2V.

    The TS pin voltage below 0 is because of the capacitor C5，which is used to avoid the unintentional wake up.

    The other picture shows: When TS mode is set 0; with 100kΩ resistor, the cell balancing function is normal.

    So the under 0 voltage is not the reason why cell balancing do not work.

    Could you pls help to check the internal cell balancing algorithm?

    What could be the reason? 

  • 0 in reply to Fabio Gou
    TI__Mastermind 42060 points

    Hello Fabio,

    I will test on an EVM tomorrow to see if I can recreate this. I will let you know the results tomorrow.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    TI__Mastermind 42060 points

    Hello Fabio,

    I appreciate the patience! I was able to recreate the cell balancing stopping on an EVM when ADCIN is configured on the TS pin.

    If the ADC TS Measurement (See image below) reads negative, it will cause cell balancing to stop.

    A work-around to stop this is to add an offset to the TS pin as such the TS Measurement() does not read negative. 

    This can be -10 or -100, depending on how low the measurement is reading. The offset can then be re-adjusted on the MCU-side. 

    Best Regards,

    Luis Hernandez Salomon