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BQ76930: BQ76930 discharge with heavy load, the balance is weakly turned on

Star Xu
Genius 9645 points
Part Number: BQ76930

Hi 

My customer test with BQ76930 and find a query as below.

The BQ76930 experienced a problem in the discharge of heavy load, the balance circuit  has a weak open state on the balanced circuit. After analysis, the problem is as follows:

1. Under heavy load, battery voltage drops, the capacitance between the BQ chip BAT-VC10X, VC10X-VC5X, and VC5X-VSS will be discharged by sampling the input resistance to the battery

2. A voltage drop of about 50mV on the input resistance Rf. The voltage drop is related to the size of the capacitor. Since the balanced MOS uses a low-voltage NMOS, the 50mV voltage drop between gs will make the MOS have a weak current.

Want to know if there is any solution to solve this problem.

Thanks 

Star

  • 0
    TI__Guru 54715 points

    Hi Star,

    I don't know of a way to solve this.  With N-ch FETs the voltage across the Rin resistor shown will create a voltage which will tend to bias the FETs toward Vgsth at the application of the load.  With P-ch FETs the bias would be on the release of the load. In either case the effect is most likely to occur at the top of the stack where the delta cell voltages are multiplied by the number of cells compared to the capacitors trying to hold the steady voltage at the IC inputs.

    There are tradeoffs which will affect the behavior.  Larger cell capacity for the typical load would provide less voltage deviation, but obviously has a big system impact.  FETs which are less sensitive but still allow good RDSon at the available balancing voltage may be an option, but all have tolerance and there is no ideal FET.  Allowing a less sensitive FET with a high RDSon to control a second FET with a lower RDSon at higher voltage (Vcell available rather than Vcell/2) for the high current path is a possible option, we have not illustrated such a circuit.