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BQ77915: Could you review schematic for BQ7791500?

Downey Kim
Expert 2675 points
Part Number: BQ77915

Hi TI experts,

Could you review schematic for BQ7791500?

5751.BMS.pdf

There is no signal at the gate pin of DCHG, CHG FET. Is there a problem with the circuit? Or is it a protection action?

Please comment.

Thanks,

Downey Kim.

  • 0
    TI__Guru 54715 points

    Hi Downey,

    I don't see a problem with the schematic.  It is unusual not to see a diode across R35 (such as figure 24 of the BQ77915 data sheet) since many designers want the maximum possible gate voltage, but at UV you will still have about 8V for Vgs, so it seems fine.  Be sure the FET can handle any transient from the load.

    Understanding the fault on a protector can be difficult since it does not communicate a status. It becomes an investigation of the possible faults from the states shown in table 5 of the data sheet.  You mention DCHG, I'm not sure if you might DSG as well as the CHG FET. 

    If neither FET output comes on (DSG or CHG):

    • Check for the PRES going high
    • Check the voltage at AVDD, it should be about 3V.
    • Be sure the cells are all attached at all solder joints from the connector to the IC pins.
    • Check that the RVTB (R32) and the thermistor are not open or shorted.  If you inspect the thermistor with an oscilloscope there should be 4 pulses repeating at about 1/2 of AVDD
    • Apply and remove the charger to clear a possible OCD or OCC fault.

    If it is only the CHG pin that is not high, then power is likely fine and the inputs are ok.

    • Check the applied cell voltages, both at the cell and at the VCn pins to be sure there is not some leakage causing the part to see a different voltage than the cell:
      • If they are OV CHG will not go high
      • When the part wakes from hibernate it assumes fault,  see the data sheet section 9.3.2.1 Operation in OV. Check if the cell voltages are above OV - hysteresis (4V for BQ7791500).  When a load is applied the battery will discharge through the body diode of the charge FET, when the state comparator threshold is met it will turn on the charge FET to avoid heating of the charge FET.  When the cell voltage drops it will recover from OV.
    • Check the TS amplitude as noted above, a more careful check may be needed to be sure the pulses are not above UTC or below OTC thresholds. 
    • Be sure CTRC is low (R45 & solder)

    For other future readers DSG low only can be checked by similar analysis of the table 5 conditions.

    Hope this helps identify the issue.