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BQ76940: AFE Drive for N-channel mosfets (Chg & Dsg)

smash Micheal
Intellectual 1000 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ76930

Dear All,

Am facing issue of getting my output drive's from Afe to Off, which is because of high current from Mosfet gate which is drawn in case of mosfets going to get damaged.

Kindly help to confirm that Afe makes it's Chg & Dsg Drives to Off in case of Huge current is drawn from it ,is it correct or not ?

Also confirm maximum output current capability of Afe drive of Bq76940 & 30 both.

Also had observed at times that output off Afe drives is 12vdc w.r.t Battery (Gnd) but at same time Mosfet Gate to source voltage (chg side) is 16 volts, has anybody faced such scenario & immediately after some time those mosfets got damaged.?

So please comment on above voltage difference which is observed.

Rohit

Thanks &regards.

Rohit

  • 0
    TI__Guru 54715 points

    Hi Rohit,

    If the BQ76930 or BQ76940 has a sense voltage applied to SRP and SRN from a discharge current which exceeds its set threshold for the delay time, it turns off its DSG output.  See table 1 in http://www.ti.com/document-viewer/BQ76940/datasheet/high-side-fet-driving-t3192268-28#t3192268-28 

    The output drive capability of the BQ76930 and BQ76940 is limited by the driver resistance, see the data sheet specifications for RCHG_OFF and RDSG_OFF.  Usually a series resistance such as 100 ohms is used in a design to isolate the IC pin from the high current path, or a larger value to slow the switching speed.   Common methods to drive the FETs off more quickly are shown in http://www.ti.com/lit/slua618, or a FET driver IC can be added to DSG.  CHG is more challenging since the driver must float with the PACK- pin.

    When CHG is off a charger can push the PACK- voltage below BAT-,. when CHG turns on the gate can have a high Vgs, the zener diode shown in figure 8 of the data sheet can be important to protect the gate if the FET does not turn on fast enough.