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BQ27741 questions

Mike Valvo
Intellectual 470 points
Other Parts Discussed in Thread: BQ27741-G1, BQ34Z100, BQ77908A, TCA9548A, ISO1540, BQ27742-G1, BQ40Z50

hello, I have a few questions about the BQ27741 chip:

1.  what is the best resistance for the sense resistor if my max current is 10A?  The datasheets don't seem to discuss it.  The reference design uses 5m-ohm for 2A max. 

2.  Is the I2C address fixed at 0xAA?  can this be modified?

3.  Can I have a master-slave I2C bus architecture, where I have 5 BQ27741 slaves and 1 master processor?  Are there any design examples of this? if I have 5 cells in series, how do I handle the different voltage levels of each cell with respect to the processor GND? 

thanks

  • 0
    Guru 22270 points

    Hello Mike,

    Welcome to the Forum!

    1. The documentation is a little light on doing the current sense resistor calculation.  Sense resistor calculation is based on a 5 mohm to 20 mohm sense resistor and the default Vocc is 20mV, your resistor value would have to satisfy  those conditions.

    2. The slave address is fixed at  1010101

    3.  Since the i2c address is fixed, you would only be able to have one device on the i2c bus.   To accommodate more   devices you could add an i2c switch.

    You can configure it for HDQ mode which is a single pin communication protocol.  This would allow you to operate with both master and slave.  Details on configuration can be found in the User's Guide http://www.ti.com/lit/pdf/SLUUAA3

    I am not sure about the last part of your question.  The Pack-Side Track looks like it is specifically for monitoring the entire pack and not individual cells.

    Thanks,

  • 0 in reply to Greenja
    Intellectual 470 points

    thank you for responding. 

    If I understand correctly, the minimum resistor I can use is 5 mohm and the maximum current is 4A if Vocc is 20mV?  Can Vocc be changed?

    What we are attempting to do is:

    1.  have 5 Li-ion cells in series with each cell powering a BQ27741 chip

    2.  have 1 FPGA communicating to the 5 BQ chips through 1 communication bus, where the FPGA is powered by either an external power source, or from Battery to DC-DC converter.  

    has anyone here used the BQ chip in a multi-cell configuration? and how do you handle communication?  

    Am I using the correct chip for this application?  Is there a better one?

  • 0
    Genius 3970 points

    (Long-winded response below, sorry!)

    In response to your last question:

    If you're having a 5-series pack there may not be much use in having an individual bq27741-G1 to control each cell. You'll probably want to use a gauge like the bq34z100 (which is a fuel gauge only, but doesn't monitor individual cell voltage) and a standalone Li-Ion protector like the bq77908A.

    In terms of a master-slave setup for the bq27741, there is no means of configuring a bq27741 as a 'master' to control the other gauges. However, if individual cell gauging and protection is what you need (note that this sort of design is really NOT RECOMMENDED), you could use the TCA9548A I2C multiplexer, and one ISO1540 isolator per cell to avoid the issue of different Vss voltages for each gauge. Your host microcontroller/FPGA would then send a channel-switch command to the TCA9548A so it can select an individual fuel gauge. Note that this would be a very inefficient design and also quite power-consuming. Each ISO1540 draws about 3 mA continuously and you'd probably want to have the isolator connected to Pack+ rather than Cell+ (that is, connected after the protection MOSFETs so that it can be powered off in an over-discharge condition). This means that you'll lose communication if the bq27741 ever has to disconnect the power output and it would also likely interfere with the protector's auto-recovery after a fault condition.

    The bq27741's VOCC cannot be modified. TI's brand-new bq27742-G1 does have some support for customizing protection thresholds, and from what I can tell it is pin-compatible with the bq27741.

    If your design is fine with using 4 cells in series, you have a lot more (and better) choices. TI's latest bq30z554 and bq40z50 fuel gauges have an improved Impedance Track algorithm, built-in protection and cell balancing (even when the pack is not in use), and a lot more customizable features like lifetime data and fault logging.

  • 0 in reply to Jason Gin
    Intellectual 470 points

    thank you. 

    Does TI offer any fuel gauge products where the I2C address is not fixed? or are there any fuel gauge products that allow multiple devices on a single communication bus?

  • 0 in reply to Mike Valvo
    Genius 3970 points

    If I recall correctly, there aren't any gauges that can have their addresses reconfigured.