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BQ76920: Review and EVM suggestion

Gaurav Agrawal
Intellectual 735 points
Part Number: BQ76920
Other Parts Discussed in Thread: ISO1540, BQSTUDIO, BQ78350, EV2400, GPCCEDV, BQ40Z50, BQ4050,

Hi,

1) We have implemented the attached circuit for monitoring, balancing and protection of 4 cells of LiFEPO4, with a charge and discharge current of 10A. We request TI for a circuit review.

2) We would like to interface this PCB through an isolator circuit (built around ISO1540) and an I2C-USB converter to a PC running BQSTUDIO tool. Is this possible, or we need to use a TI EVM?

3) Can BQSTUDIO support the LiFePO4 chemistry and help us do a quick program of the BQ78350 parameters as per our application?

Regards

Gaurav

PCM.pdf

  • 0
    TI__Guru 54715 points
    Hi Gaurav,
    1. The schematic generally looks OK. A suggestion, TS1 must see an edge to boot, a pull down should be used for R21 to be sure the voltage goes low, it could be approximately 100k

    2. The ISO1540 should interface the signals, but it requires a 3V supply from the battery voltage. The power for the other side of the isolator somes from outside of course, bqStudio is a setup and debug tool, it only understands the EV2400 and EV2300. If you want your product to have a USB interface it would need a hardware design and your own PC tool.

    3. Yes, bqStudio has a variety of LiFePO4 chemistries. The bq78350 loads chemistry with the chemistry tab, and it is used to estimate capacity at reset only. Gauging for a CEDV part is based on coulomb counting and is dependent on the parameters loaded. The CEDV parameters need to be calculated from the GPCCEDV calculator. If fixed EDV is suitable you can determine the fixed EDV voltages from a log.

    Note that 4 cells the bq40z50 offers a more compact solution and IT gauging. If CEDV is preferred the bq4050 also supports 4 cells.
  • 0 in reply to WM5295
    Intellectual 735 points
    Hello,

    Thanks for the inputs.

    1) We shall make sure of the TS1 edge, thanks for pointing this out.

    2) Yes the ISO1540 has been powered through an LDO driven from main battery power. We dont need a USB interface as such, just want to have it to be useful during development with BQSTUDIO. For our particular case which one is more suited - EV2400 or EV2300, as I see them costing the same?

    3) Do EV2X00 offer isolation of the battery pack from the PC? If not, do you see any potential issue going ahead without isolation in case we also do away with ISO1540 to keep the BoM cost low.

    3) Our main idea is to have a quick start up with the LifePO4 chemistry and program BQ78350 with monitoring, protection and balancing parameters. Please confirm that is doable with BQSTUDIO.

    4) Cell capacity is 89Ah for each of the 4 cells being used. So BQ4050 may not be suited, and that was the reason for using BQ76920. Please let us know if any ither part is better suited for this capacity.

    Regards

    Gaurav
  • 0 in reply to Gaurav Agrawal
    TI__Guru 54715 points
    Hi Gaurav,
    2. The EV2400 works with HID drivers, so there is no special installation of drivers. Driver installation can be a problem for EV2300 and can be more difficult with 64 bit OS versions. The EV2400 is the recommended interface.

    3.1 The EV2x00 do not have isolation. You may have noticed the EVMs for bq769x0 do not include isolation. This can work OK depending on the environment, equipment must be kept isolated, connecting negatives can bypass protection FETs or put high current through equipment. Follow all applicable safety guidelines for your lab, industry and equipment.

    3.2 Yes, that is the intent. Of course gauges are complex systems. It can be helpful to move in steps from an EVM with the development tools to a cell setup to custom board to custom firmware/host system.

    4. Good point. FET drive sometimes also a consideration if high capacity also gives high current. The bq78350-R2 firmware download has the IPScale function working. Manual scaling is sometimes used for higher current, voltage, or capacity. If you are interested in scaling you might see the presentation in this post e2e.ti.com/.../1854920