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BQ76200: SLVA729A Test results

Jeffrey Wright
Expert 2875 points
Part Number: BQ76200

Hello;

I'm considering using the BQ76200 in a new design and after reviewing the Test results in LSVA729A, I'm curious as to what cells (if any) were used for these tests, or whether an external supply was used.  If real cells,  I'm assuming that their configuration was nS1P (n = 4,5,.etc.)?.

I'm curious because Fig.25 appears to indicate the short circuit current was only ~10A (SRP = 0.1 V and Rsense = 10mOhm?)... did you limit the short circuit current to 10A? 

Also, BATT+ dropped to zero within a few microseconds and then rebounded to ~10V after ~50us during the short, which (and please correct me if I'm wrong) is due to the cells internal impedance/resistance (?)   Rebounding to 10V would then be sufficient to run the system until it opened the FET.  Assuming of course, that you used real cells.

My application will use 4S25P yielding ~90AH capacity, and so the short circuit current will be much larger, but I'm "assuming" that the rebound voltage will still be about the same?.

Thanks in advance 

  • 0
    TI__Mastermind 24870 points
    What is the cell spec sheet, can you help attach?
  • 0 in reply to Kang Kang
    TI__Guru 54715 points
    Hi Jeffrey,
    In www.ti.com/.../slva729a.pdf the test conditions varies with the configuration. Note table 1.
    For OV and UV tests, a power supply may have been used to avoid stress to cells, and currents may be nominal, see the appropriate figure. Example for UV is figure 3, current was about 1 A.
    Short circuit would use cells to achieve the higher current and transient response. Note in figure 5 the current is represented by the SRP voltage, at 1 mOhm about 100A was used.
    In figure 25 the BMS circuit was similar with 1 mOhm, the current waveform seems to have a bit of offset, again in the 90 to 100 A range.
    As you parallel cells the overall inductance should go down. Depending on the physical construction though you may add some inductance with the interconnect and the short circuit current will not increase per cell due to the resistance of the interconnect. In our observation a cell which delivers a 100A short circuit from a nS1P construction will not likely give 200A short circuit in a nS2P construction, but your results may vary. In general the response/rebound of the cell will be the same, but as show in the various figures, as more FETs are driven the switching speed of the bq76200 slows down because gate capacitance load increases but the internal and external resistances for the driver remain the same, so switching is slower. The slower switching results in a smaller dI/dt (dt increases more than dI). But your situation may be unique, be sure to evaluate thoroughly.