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BQ24179: Can we connect the Vsys with batteries and not use the internal mos?

Nan Zhang
Intellectual 1895 points
Part Number: BQ24179

Hi team,

      Based on some special working conditions(they said the Vsys would be 19V in some condition and the BAT pin could stand 20V, they worry about the margin) , the customer has made the following design:
      A switch(mos) is added between the BAT pin and the battery. When not charging, disconnect the switch between BAT and the battery, and the battery is directly connected to SYS; when charging, close the switch, which means that SYS and BAT are connected in parallel to charge the battery.（btw, can customer use the SDRV pin to control the mos as they want to add）
Please help the customer to evaluate whether there is a problem with this use, and whether it will affect the charging control of the chip.

      

Yours

Nan

  • 0
    TI__Guru**** 165986 points

    Hi Nan,

    SYS is regulated to MINSYS, when battery voltage is low and then slightly higher than BATREG when battery is above MINSYS.  The BATFET measures the current into/out of the battery.  By shorting it, charge current is no longer regulated, so, I do not recommend this as there will be uncontrolled (potentially up to 5A) of current flowing from SYS into the battery.  

    Also, the BAT and BATP pins are 20V abs max and will be damaged if they see 21V. 

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 1895 points

    Hi Jeff,

          Thanks for your quick reply.

           BATFET is used to monitor the charger current. If we short the BATFET, the bq24179 will not know how much charger current is going to the battery. And  BATFET is the door to isolate the Vsys when batteries is low.

          And if the customer still want to put mos. Is that possible that placing a mos between the Vsys and batteries(marked in black). The mos would be open while charging the batteries; The mos would be closed while discharged the batteries. And the SDRV could control the mos marked in red to achieve the customer want.

    yours

    nan

  • 0 in reply to Nan Zhang
    TI__Guru**** 165986 points

    Hi Nan,

    If the FET is only on when the charger is in HiZ (either no VBUS or EN_HIZ=1 or ILIM_HIZ=0) then I see no issue.  SDRV cannot control that FET.  The user has no control over SDRV other than to enter ship and shutdown modes.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 1895 points

    Hi Jeff，

         Thanks for your reply. After talking with customer, the bigger voltage is due to the motor counter-electromotive force. Because of this, they have to place two mosfet to protect the BAT and BATP pin. So do you have other ways to protect the pins?

    YOURS

    NAN 

  • 0 in reply to Nan Zhang
    TI__Guru**** 165986 points

    Hi Nan,

    They will need to add a TVS diode to protect BAT and BATP.  During the pandemic, the only one I could find in stock with the closest spec was 

    20.6V (Typ) Clamp 36A (8/20µs) Ipp Tvs Diode Surface Mount US2H SOD323HE CUHZ20V,H3F

    Toshiba

    There are likely others available now.

    Regards,

    Jeff