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BQ76940EVM: Schematic CHG & DSG mosfet

Rajaneesh Datta
Intellectual 275 points
Part Number: BQ76940EVM

Hi,

As i seen in BMS that in TI design & other/China,CHG & DSG will be connected to the mosfet and source of both the mosfet will be connected to Pack-,BATT-.my question is,what's the reason behind that? why dont we switch mosfet to Pack+,BATT+?CHG & DSG)

1)Can i connect CHG signal without Q5? Please confirm

  • 0
    TI__Guru 54715 points
    Hi Rajaneesh,
    The part design supports the series connection of the FETs with common drain. Since DSG and CHG pins do not go below VSS Q5 allows the charge FET gate to go below ground. The charge FET turns off slowly with Q5 so the charge FET is closest to the pack since the slower turn off of charge FET is tolerated better due to lower charge currents.
    Q5 is needed to allow the charge FET gate to go below VSS and keep the FET off. If you have a 40V charger attached to a battery at 30V which should not charge, without Q5 charge would continue because the charge FET would operate as a source follower allowing current to continue.