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BQ76940: Companion ICs and schematic

Robert Nunez
Intellectual 570 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ76200, BQ78350, TIDA-00792, TIDA-00255, BQ78350-R1

Hi everyone,

I'm trying to build a BMS for an 9S4P 18650 Li-Ion battery pack, for an E-Bike. I plan on using BQ76940 as my AFE device, PIC24FJ64GB002 as my microcontroller, and I2C communication protocol . However, the sample schematic from (www.ti.com/.../tidrqy7.pdf) which I plan to use for reference, has an extra device called BQ76200, which is not present in the schematic from the BQ76940 datasheet (http://www.ti.com/lit/ds/symlink/bq76940.pdf). Is the BQ76200 essential? If not, can I use the sample schematic as reference but without the BQ76200 and replacing the BQ78350 with my PIC24?

  • 0
    TI__Guru 54715 points
    Hi Robert,
    The bq76200 provides high side protection switching which allows PACK- referenced communication outside the battery when the battery is protected. Without this the communication reference must be the battery electronics GND which if brought outside the battery case in many cases will bypass the low side protection switching.
    Yes, you can replace the bq78350-R1 gauge with a MCU in either the EVM design such as TIDA-00255 or the TIDA-00792 design. You will need to provide appropriate fault monitoring & recovery, gauging and balancing for your system of course, and also remember the communication reference. If your output from the battery is +, -, and lights, or if you have an isolated interface, you don't need the high side driver.
  • 0 in reply to WM5295
    Intellectual 570 points

    Thanks for responding! 

    WM5295 said:

    Yes, you can replace the bq78350-R1 gauge with a MCU in either the EVM design such as TIDA-00255 or the TIDA-00792 design.

    Suppose I was to use the TIDA-00255  design as reference- does that mean I can scratch sheet 6, design a schematic for my own MCU(PIC24), and just connect the SCL, SDA, and ALERT signals?

    WM5295 said:

    You will need to provide appropriate fault monitoring & recovery, gauging and balancing for your system of course, and also remember the communication reference.



    All of this is on the software side of the BMS, yes? (I mean, no other additional circuitry for these)

  • 0 in reply to Robert Nunez
    TI__Guru 54715 points
    Hi Robert,
    "Suppose I was to use the TIDA-00255 design as reference- does that mean I can scratch sheet 6, design a schematic for my own MCU(PIC24), and just connect the SCL, SDA, and ALERT signals?"
    - Yes, the "Gauge" sheet 5. Sheet 6 is the resistor divider for the EVM, it is would load and drain the cells if used with a battery. See this idea in www.ti.com/.../slua810.pdf where a user might select the monitor and switching configuration then pick from figure 10 or 11 for a controller.

    "All of this is on the software side of the BMS, yes? (I mean, no other additional circuitry for these)"
    - Yes
  • 0 in reply to WM5295
    Intellectual 570 points
    Will read on it, thank you very much.

    Also just wanted to clarify some things about BAT and PACK, is this correct?
    BAT+ is the positive side of the top battery
    BAT- is the negative side of the bottom battery. Connected to the system ground.
    PACK+ is the positive terminal of the charger
    PACK- is the negative terminal of the charger. Not connected to the system ground.

    And why does the sample schematics only have PACK-, but no PACK+?
  • 0 in reply to Robert Nunez
    TI__Guru 54715 points
    Hi Robert,
    Some identification depends on your perspective. The names used on the board are generally from inside the battery circuit.
    Yes, BAT+ is the positive side of the top battery cell.
    BAT- is similarly the negative side of the bottom battery cell.
    PACK+ is the positive connection to the charger or load outside the battery electronics assembly. From outside the battery assembly it might be considered battery+.
    PACK- is the negative connection to the charger or load outside the battery electronics assembly. From outside the battery assembly it might be considered battery-.
    With low side switching the BAT+ is the same potential and connection as PACK+. Some design tools complain when the same net is given more than 1 name. Some have a net-tie or short component to connect 2 nets, but this will often result in a small metal feature between the 2 nets. In the case of the example schematics/boards for the bq76940 BAT+ (also used as PACK+) was a high current net and a large connection was desired. The same net name was used for the net, there may be text placed next to the terminal on some drawings and the board to indicate the other use.