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BQ24179: questions about the SDRV

Nan Zhang
Intellectual 1895 points
Part Number: BQ24179

Hi  team,

      According to datasheet, the SDRV pin should be connected to ground via 1nF cap,when the ship MOS is not assembled. But in customer's design, they use the ship mos and connect the SDRV to ground via cap. Is there any potential risk in their system? Their pack is 2s-batteries.

    In addition, the the customers measure waveform which is show below. The vbus is removed. The Vsys is only supported by the 2s-batteries.The internal resistor of the oscilloscope probe is 1M ohm.

    

     I have search the on the E2E. The SDRV is powered by charge pump which has weak ability(nA) to load the caps or Cgs of MOS.  Using such oscilloscope probe with 1M ohm could trigger the oscillation. Here are my questions:

     Q1.I also find that the SDRV would work on 50% duty. So could you help me to confirm which reason causes oscillation tested by customer ?

     Q2.why the SDRV output runs at 50% duty cycle rather than make ShipFET fully on when only battery is connected. I find that this could reduce the Iq. But it not makes sense. Because continuous charging and discharging the ship MOS(named Q3)  would waste the energy and increase the power consumption. And for SDRV working on 50% duty could lead to the ship mos close and open over and over again causing the Vbat shaking. This confuses me a lot.

     

YOURS

NAN

  • 0
    TI__Guru**** 166996 points

    Hi Nan,

    In battery only mode, SDRV functions at 50% duty in order to minimize Iq.  Any resistive load, even 10Meg scope probe, can cause SDRV output to droop.  If driving only a capacitor, there is no droop.  You can measure using a muiltimeter set to highest output impedance (usually 1 Gigaohm).  I have not tested with both a cap to ground and attached to the shipFET gate so I cannot recommend doing that.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 1895 points

    Hi  jeff,

    Jeff F said:
    SDRV functions at 50% duty in order to minimize Iq.

          I am really confused about this.  Is this kind of wasting energy to charge and discharge the ship MOSFET over and over again?

         And in addition, i have idea about  the potential risks with both caps and ship mosfet on SDRV. In battery only mode, the SDRV would work 50% duty. The more caps would slow down speed of charging and discharging.

    YOURS

    NAN

  • 0 in reply to Nan Zhang
    TI__Guru**** 166996 points

    Hi Nan,

    The only reason you see the SDRV periodically dropping is due to the 10Mohm scope probe on SDRV.  If you were to use a high impedance JFET probe or a multimeter set to 1Gohm impedance, SDRV does not drop.  Additional caps on SDRV will slow down ship FET turn on and off.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 1895 points

    Hi  Jeff,

          SDRV functions at 50% duty in order to minimize Iq.  Do you mean that the SDRV internal charge pump work at 50% duty？

    YOURS

    Nan

  • 0 in reply to Nan Zhang
    TI__Guru**** 166996 points

    Hi NAN,

    Yes.

    Regards,

    Jeff