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BQ76940: BQ76940 queries

Shruti Pandey
Prodigy 10 points
Part Number: BQ76940

Tool/software:

Dear Texas Instruments Support Team,

I am reaching out to seek clarification and assistance regarding the BQ79640 series AFE, specifically in its application within our existing off-the-shelf (OTS) product. This is my first time contacting your support team, and I would greatly appreciate your guidance on the following technical queries:

  1. Cell Group Operation and LDOs:

    • The datasheet mentions that each cell group (consisting of 5 cells) requires a minimum of 6V to operate properly.
    • Could you please confirm if the BQ79640 series AFE has three LDOs, one for each cell group, which are utilized to power up the NTCs connected to each cell group?

  2. Effect of Insufficient Voltage on AFE Functioning:

    • What would be the impact on the AFE’s performance if the required 6V supply is not provided through the cell groups?
    • How would this voltage insufficiency affect the behavior of the NTCs connected to the AFE?
    • Would the AFE still be capable of accurate voltage acquisition for the specific cell group if the voltage requirement is not met?

  3. Observation on Temperature Fluctuation:

    • We have observed that the temperature measured by NTC3(TS3_FIL) tends to fluctuate when the voltage is low and the temperature is in the negative range.
    • This fluctuation is not as pronounced when the voltage is low and the temperature is positive, or as the temperature increases to higher values.
    • Could you help us understand the possible reasons behind this behavior?

We would appreciate any insights or recommendations that you could provide regarding the above points. Your support in this matter will be highly valuable to us in optimizing the performance of our product.

Thank you for your time and assistance. We look forward to your guidance.

  • 0
    TI__Mastermind 42060 points

    Hello Shruti,

    Shruti Pandey said:
    Could you please confirm if the BQ79640 series AFE has three LDOs, one for each cell group, which are utilized to power up the NTCs connected to each cell group?

    Correct.

    Shruti Pandey said:
    • What would be the impact on the AFE’s performance if the required 6V supply is not provided through the cell groups?
    • How would this voltage insufficiency affect the behavior of the NTCs connected to the AFE?
    • Would the AFE still be capable of accurate voltage acquisition for the specific cell group if the voltage requirement is not met?

    There will be an XREADY error, cell voltage measurements of the respective die group and above groups may not measure correct cell voltages.

    Unaffected cell groups may still measure correctly.

    Shruti Pandey said:
      • We have observed that the temperature measured by NTC3(TS3_FIL) tends to fluctuate when the voltage is low and the temperature is in the negative range.
      • This fluctuation is not as pronounced when the voltage is low and the temperature is positive, or as the temperature increases to higher values.
      • Could you help us understand the possible reasons behind this behavior?

    Do you have any more information on this behavior? Any data logs? What is the battery voltages at this time and the schematic of the board?

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 10 points

    Hey Luis,

    Thank you for the response.

    Please find the snapshots of the schematics of TS3 connection.

    Additionally, please find some more details below.

    - 9 Cells are connected to the AFE such that only 2 cells are connected to the 3rd cell group.
    -  According to the tests conducted, it could be seen that the NTC starts fluctuating cell voltages are below 2.9V.

  • 0 in reply to Shruti Pandey
    TI__Genius 17875 points

    Hello Shruti,

    The engineer will get back to you tomorrow regarding your question. 

    Best Regards,
    Alexis

  • 0 in reply to Shruti Pandey
    TI__Mastermind 42060 points

    Hello Shruti,

    If I am understanding correctly, you see this issue on the third thermistor when the third cell group drops to below 9-V?

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 10 points

    Hi Luis,

    This issue occurs when the voltage of the cell group falls below 5.9V (this is an observation). As per our customer's harness design, only two cells are connected to the third cell group, which makes this scenario quite likely.

  • 0 in reply to Shruti Pandey
    TI__Mastermind 42060 points

    Hello Shruti,

    That would make sense. This is below the cell group's supply voltage. You would want the cell voltage to remain above the minimum recommended supply voltage.

    Additionally, we do recommend to have at least three cells on each cell group.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 10 points

    Hello Luis, 

    I have a few specific questions about this:


    - Unexpected behavior(jump in NTC3 reading) is being observed at negative temperatures when the cell voltages are low.
    - There is a need to understand why this particular behavior occurs at temperatures below zero or why the effect is more pronounced as we move to lower temperatures.
    - Is the output voltage of the LDO influenced by the voltage from the NTC sensor?
    - An explanation is needed regarding how the interaction between the LDO and the NTC sensor voltage may be contributing to this behavior in low-temperature conditions.

  • 0 in reply to Shruti Pandey
    TI__Mastermind 42060 points

    Hello Shruti,

    Shruti Pandey said:
    - Unexpected behavior(jump in NTC3 reading) is being observed at negative temperatures when the cell voltages are low.
    - There is a need to understand why this particular behavior occurs at temperatures below zero or why the effect is more pronounced as we move to lower temperatures.

    It may be that the internal supply threshold may change slightly at different temperatures, when you are below the recommended voltage, it may be that it works somewhat okay at first. But the change in temperature changes just enough that it starts to affect function of the device. 

    Shruti Pandey said:
    - Is the output voltage of the LDO influenced by the voltage from the NTC sensor?

    The NTC pull-up comes from the CAPx pins, which is the internal LDO rail used to power internal circuitry.

    Shruti Pandey said:
    - An explanation is needed regarding how the interaction between the LDO and the NTC sensor voltage may be contributing to this behavior in low-temperature conditions.

    The internal rail LDO is used for thermistor pull-ups. So any disturbances on that rail may cause issues with thermistor measurements.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 10 points

    Hi Luis, thanks for the response. Is it possible for you give me a transfer function for the behaviour of the output  voltage of the LDO as the voltage of the cell group goes from 6V-4.2V? 

  • 0 in reply to Shruti Pandey
    TI__Mastermind 42060 points

    Hello Shruti,

    We unfortunately do not have such transfer function to provide.

    Best Regards,

    Luis Hernandez Salomon