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Original question:

BQ77905: Can high-side protection be implemented? Need to maintain SMBus communication with pack

BQ77905: Is the BQ77905 series battery protection chips directly compatible with the BQ76200 high-side driver chip?

THOMAS RANZ
Prodigy 90 points
Part Number: BQ77905
Other Parts Discussed in Thread: BQ76200

Hi:

I am still looking at using the BQ77905 battery protection chip for a pack-side project.  Are the BQ77905 battery protector and 76200 high-side mosfet driver fully compatible?   I need a simple solution with minimum parts count and low power drain.

Thnaks,
Tom

  • 0
    TI__Guru 54715 points

    Hi Thomas,

    Yes and no.

    The BQ77905 CHG and DSG will drive the CHG_EN and DSG_EN inputs, check the voltage levels, a series resistor or even a divider may be desired for the maximum conditions.  The BQ76200 will add quiescent current, as will the divider on the inputs if you use it.  

    Being a simple low power protector the BQ77905 supports low side switching, the LD input is intended for low side switching.  If your configuration needs LD for fault recovery you will need to design some method to control the pin.  That might be a transistor with pull ups, they will take some bias current, or your system may have some other mechanism available for control.

  • 0 in reply to WM5295
    Prodigy 90 points

    Looking further at the BQ76200 specs I see that the minimum Batt voltage is +8.... that could be a problem when a pack is deep discharged.

    I am currently using a ABLIC S-8254AAW chip, it uses p-channel mosfets to control the charging and discharging.  Would it not be possible to use transistors and the BQ77905 to control p-channel fets in a similar fashion?  If anyone has done this please give an example circuit or description of how it was accomplished.

    Tom

  • 0 in reply to THOMAS RANZ
    TI__Guru 54715 points

    Hi Thomas,

    See the application note https://www.ti.com/lit/pdf/slua910 showing an example of a high side P-ch charge FET.  With the range of CHG and DSG from the BQ77905 (N-ch low side driver) you will need to have the external control transistors and a clamp if needed to turn off the power FETs quickly.   Similar to using the BQ76200 you may still need to manipulate LD if not tying it low.

  • 0 in reply to WM5295
    Prodigy 90 points

    Thanks, that is a good article to get me going.  It is interesting there are no examples so far implementing the Discharge control, only the charging.  I will follow the charging example for the discharge circuitry.

    Thank You!
    Tom