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BQ40Z80: Use without protection FETs

Seth Johnson
Intellectual 380 points
Part Number: BQ40Z80
Other Parts Discussed in Thread: BQ24600, , BQ4050

Can this device be used without the precharge, charge or discharge FETs? I'm looking for a device that supports 6 Li-Ion cells in series and provides fuel gauging, balancing and charging. Unfortunately the closest I could find would be a bq40z80 and a secondary charging device. The secondary charging device (bq24600 in my case) will take care of charge and precharge so I don't need those FETs to be controlled by the bq40z80. Discharge control is handled by a separate device. I'm planning on connecting the CHG pin of the bq40z80 to the CE pin of the bq24600 so it can control charging that way and I'd only need a single thermistor. I realize the voltage of this signal can be up to 12.5V, but that should be fine as the bq24600 CE input voltage can be up to 28V.

Are there any issues with this approach? Have I missed another device which handles all three of the functions mentioned above so I don't need to also include a bq24600? Thanks.

  • 0
    TI__Mastermind 42060 points

    Hello Seth,

    Yes, you can just leave the Protection FETs pins open and use the BQ40Z80 without the protection by directly connecting BAT+ to PACK+. The BQ40Z80 detects if it's charging/discharging by measuring the sense-resistor voltage(See Section 8.3.7.1 of datasheet), so I don't believe you'd be able to control charging that way. 

    I don't believe we provide a device that can do all of these functions in a single chip. I would advice you see this BMS Reference Design in order to get a better idea in how you can use these devices together. The design uses a BQ24600 and BQ4050, however it can easily be adjusted to use the BQ40Z80 instead. I would also advise you see the BQ40Z80-EVM to see how the BQ40Z80 is usually configured.

    For more reference designs using the BQ24600, you can check under Reference Designs in the BQ24600 Product Page.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 380 points

    Thanks Luis. The EVM schematic in the datasheet shows the PACK pin connected to the positive battery terminal (after the FETs and fuse) through a 1.1K resistor, and the BAT pin to the positive battery terminal through a 100 ohm resistor and a diode. If I can connect the BAT and PACK pins together, should I then connect the pair to the positive terminal through a 1.1K resistor, or a 100 ohm resistor and a diode?

    My intent was to be able to use only a single thermistor to both control charging (disable if over-temp) and to read via the SMBus interface. I was planning on letting the BQ40Z80 do the temp sensing and letting it disable its CHG pin like normal if the pack gets too hot. Bringing CHG low would then also disable the CE pin on the BQ24600, thereby disabling charging. Would this be okay?

  • 0 in reply to Seth Johnson
    TI__Mastermind 42060 points

    Hello Seth,

    Yes, connect the PACK pin through the 1.1-kOhm resistor and the BAT pin through the 100-Ohm resistor and Diode. You can read more information information about these on the datasheet(Section 10). 

    I think it would work if that's what you're trying to do. However, I would advice you read the Technical Reference Manual to make sure there aren't any issues with your application of it.

    Best Regards,

    Luis Hernandez Salomon