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BQ77905: 3s charger & protector => BQ24618 & BQ77905: VTB of bq77905 always remains 0V

Prathamesh Tadaka
Prodigy 185 points
Part Number: BQ77905
Other Parts Discussed in Thread: BQ24618

Hello,
           
         I testing BQ24618 as an 3's Li-ion charger with BQ77905 as an protector. BQ24618 charger works fine but in bq77905 i observe that VTB pin always stay on 0V.  I think it should be arround 3V.
Because of VTB Pin remains 0v TS Pin is also getting 0V. so bq77905 always remains in fault  & DSG AND CHG turn off. Can anyone tell me what is the issue?

  • 0
    TI__Expert 5090 points

    Hi Prathamesh,

    Our experts on this device are out this weekend, someone should be able to look into it when they return in the next few days.

    Thanks,

    Terry

  • 0 in reply to -Terry
    TI__Guru 54715 points

    Hi Prathamesh,

    VTB is pulsed, it is only turned on for the time to make the measurement, then off again.  When checked casually it will look low.  Inspect VTB with a scope, it should approach the AVDD level during the samples. TS will be lower amplitude depending on the temperature of the thermistor.  Be sure the RTS_PU and RTS (thermistor) are not shorted or open, any will cause a permanent fault. Also note in table 8-3 that open wire and current faults can turn off both FET outputs.