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multiple bq77pl900's

Jeran Cloete
Prodigy 75 points
Other Parts Discussed in Thread: BQ77PL900, BQ76930

Hello, 
I am wondering if ithere is a problem with connecting the outputs of multiple BQ77PL900 circuits?

Example: Take the BQ77PL900 evm-001 evaluation module. would it be possible to attach 2 modules outputs in series or parallel? (with externel FET's to handle the current in series). So the IC's would be operating separately on separate 10S battery packs but the outputs would add as needed (series or parallel).

The only thing that occurs to me is if the current monitor input to the bq77pl900 returns the current which is flowing through both modules it may give overcurrent warnings and shutdown?

Thanks for the assistance.

~J

  • 0
    TI__Guru 54715 points

    Connecting modules in series such as 10 cells each with each module having its own FETs typically will not work.  When one module opens the FETs with a load present, the voltage on PACK of the open device will go to a negative voltage.  This is typically either damaging or at least an undesirable leakage.  If you design with a single set of FETs, you will need to level shift the outputs to a common voltage, combine the controls with appropriate logic and drive the common FET.  A wake up signal will need to be level shifted to the 2 devices.

    When operating modules in parallel it may take some system design to determine how to connect the modules to avoid rapid balancing between modules causing high discharge on one module and charge of the other.  Once this was solved, to know the system load you would need to measure and sum current from both modules. certainly a protection event on one parallel module would suddenly increase the load on the second module.  How the modules share current may depend on the FETs. 

    If the devices are operated in parallel on the same set of cells (redundant devices), no significant issue is expected.  Each device should have separate filters. The FET control outputs would need to be combined with logic and decide how to react to the different calibration points of the safety limits.  If you monitor both devices with a host it would need to resolve calibration differences as well.  Some failures of a redundant device may still cause the battery to fail such as if balancing fails on in one device it will pull down the cell voltage.  The other device could see the effect but could not "unbalance" the first part.

  • 0 in reply to WM5295
    Prodigy 75 points

    Thank you for your reply. That is what I suspected.

    I am investigating a way to utilize 4 bq77pl900's in parallel on a 10s4p pack. 
    I will use an arduino to operate them in host mode to ensure they the outputs are always at a similar voltage.
    high power schottky diodes to protect the 10S stacks when unloaded.

    Before I design a PCB are there any other TI products which could be used to perform the same functions as the BQ77PL900 on a 40 cell 10S4P pack?

    Do you know where I can find an eagleCAD library that contains the BQ77PL900?

    Thanks again

    ~J

  • 0 in reply to Jeran Cloete
    TI__Guru 54715 points

    If you are using the bq77PL900 in host mode, the bq76930 might be considered similar in that it supports 10 cells.   Some obvious differences is that it does not do standalone operation, it has digital voltage output and has signals for FET control referenced to the VSS which could be used to drive low side FETs or interfaced to a driver for high side or other switching. Check the datasheet for more details.

    The bq77PL900 has a CAD package available which should convert to Eagle.  See http://webench.ti.com/cad search for bq77PL900 or go to http://webench.ti.com/cad/dlbxl.cgi/TI_BXL/BQ77PL900_DL_48.bxl