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BQ76920: Cell balancing for 70mA

Eohan George22
Intellectual 620 points
Part Number: BQ76920

On page 40 of http://www.ti.com/lit/ug/slvu924d/slvu924d.pdf , the circuit shows R4-R9 having a 100ohm resistance. This means that between the + and - of a cell the resistance is 200 ohm. This means that the max current for a cell of ~3.5v is 17ma. 

But in page 10 of https://www.ti.com/lit/ds/symlink/bq76940.pdf , the cell balancing current is 70mA for bq76920. How is this achievable with the reference design? Is it advisable to go lower resistance for R4-R9 ? 

Please suggest why 70mA was specified for this chip. 

  • 0
    TI__Guru* 81034 points

    Hi Eohan,

    If you see Page 11 of the datasheet, the maximum cell balancing current for the BQ76920 is 50 mA. You can select resistor values to give you the desired balance current within the specified range. 70mA is the value specified in the Abs. Max tables (not the recommended operating range).

    If higher current is needed, external FETs are required. There are multiple reference designs using external FETs if you need examples.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 620 points

    Hi Matt, 

    1. Is there a downside from moving the resistance to the lowest value (40 ohm) suggested for the BQ76920 series ? 
    2. Could you provide the reference designs using external FETs for the BQ76920 series ? 

  • 0 in reply to Eohan George22
    TI__Guru 54715 points

    Hi Eohan,

    1. With smaller resistor values the cell balance current is higher but more transients can get to the IC.  If the design has low transients this should not be a concern.  

    2. As a reference design using external FETs with the BQ76920 see figure 5 of http://www.ti.com/lit/an/slua810/slua810.pdf Note however that the bottom of C5 should connect to GND as shown in the data sheet rather than VC0 as shown in the figure.  This is an oversight which is waiting for correction.