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BQ28Z610: Measurement shunt position in design

John Valentine Major
Prodigy 20 points
Part Number: BQ28Z610

Hi all,

at my company we are designing BMS using BQ28Z610 which is currently different from the application example found in SLUSAS3D, but we would not change it for layout simplicity.
In our BMS the current measurement counts in the balancing current and other consumptions of BQ, because its current flows through the shunt resistor. On the picture you can see the measurement is placed before the BQ so its current can flow through the shunt. Load connects to LGND, but not shown on the schematic.

What kind of problem or consideration would we take if we designed it that way? Sensing the extra current does not effect to the BQ state machine and cause error?

Thank you in advance!

  • 0
    TI__Mastermind 27321 points

    Hello,

    Can you please explain a little bit more on what you are trying to do?

    It is important that the sense resistor R14 uses a different ground from IC ground to add additional noise immunity. The bq28z610 has been tested using our layout guidelines. 



    The bq28z610 comes with an integrated ADC for current measurements through the sense resistor (shunt resistor), I am confused by "In our BMS the current measurement counts in the balancing current and other consumptions of BQ, because its current flows through the shunt resistor"

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 20 points

    Thanks for the fast reply, and sorry for my careless clarification.
    Our logical design differs from the typical application circuit, because the VSS and the ground of the filters are connected to the PACK- instead of BAT-.
    Shown on the picture below:



    We cannot experience noise but we are afraid of the BQ will go to an error state or detect an anomaly, because the power of the IC (including balancing current) is going to flow through the shunt resistor in our design.

    Do we need to change the design at any cost? What is your opinion?

  • 0 in reply to John Valentine Major
    TI__Mastermind 27321 points

    Hello John,

    The only reason why I wouldn't connect IC's VSS to PACK- is because what happens when the charger is not attached, but the cell is still attached? Will the BQ IC still be powered? Normally you want the gauge to operate as long as the cells is attached, even if the charger is not attached.

    I recommend you keep VSS and IC's filters connected with Cell-. You can use a Net-Tie to short GND and Cell- and use GND for BQ IC and filters. This way keep Cell- only for Rsense.

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 20 points

    BQ remains powered because its VSS is connected through the shunt to CELL-. So do not we have anything else to worry about?
    Thanks!

  • +1 in reply to John Valentine Major
    TI__Mastermind 27321 points

    Hello John,

    It depends on the application max current. For a low current application it’s less of a concern. The issue is that our ground reference for our ADC measures changes depending on the amount of current flowing through the sense resistor, if referenced to PACK-. This introduces measurement and gauging errors.

    Regards,
    Jose Couso