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BQ24753A: What is the reason for using bidirectional current flow switch?

Ras Sharif50
Genius 3455 points
Part Number: BQ24753A

Dear All,

Please consider the schematics provided in the datasheet of BQ24753A:

Why should to place back to back mosfets to flow current in two directions to/from adapter?

Best Regards, Rasool

  • 0
    Genius 3455 points

    Also I don't understand the direction of the Q3.

    As I think, it is for powering the system when the adapter there is not connected.

    So, Should not the Darin- and Source be replaced to work correctly?

    Regards, Rasool

  • 0
    TI__Expert 5330 points

    Hi Rasool,

    The reason that we have both Q1 and Q2 FETs back-to-back is so that current can be blocked in both directions.  The Q2 FET blocks current from flowing from adapter to system when it is disabled.  This is used to protect the system if there is an over-voltage condition at Adapter+ or if there is an overcurrent condition.  The Q1 FET blocks current flow from the system back out to the adapter.  This is used when the adapter is removed to keep from having a "hot" voltage on the Adapter input and to protect in case there were a short at the adapter input.  The device will wait until it sees a valid adapter voltage (as programmed at ACDET) before enabling that FET.

    Regards,

    Steve

  • 0 in reply to Ras Sharif50
    TI__Expert 5330 points

    Hi Rasool,

    The Q3 (BATFET) is oriented so that the backgate diode will conduct from the battery to system but not the other direction.  This architecture supports only buck-mode charging, so the adapter must always be higher voltage than the battery.  When an adapter is plugged in, SYS will be at the adapter voltage, so if the BATFET backgate diode allowed current flow from SYS to BAT, there would be no way to stop this flow when we wanted to terminate charge.

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    Genius 3455 points

    Hi Steve,

    Thank you for the reply.

    Why mosfets should be in the position of Q1 and Q3?

    Does not the diodes instead of Q1  and Q3 do the task?

    Regards, Ras

  • 0 in reply to Ras Sharif50
    TI__Expert 5330 points

    Hi Ras,

    Because there is no OTG mode on this device (where power is supplied out of the adapter), Q1 could be replaced by a diode.  And yes, the BATFET could be replaced by a diode as well.  (Note that this is not true for NVDC architecture, but on this architecture the Q3 BATFET could be replaced by diode.)

    However this is not typically done because there is much larger power loss across a diode than a turned-on FET.  Usually the FET has an R_DSON of about 10 mOhm.  A Schottky diode will probably have about 300 mV drop.  So even at 6A across one of these FETs, you only have 60 mV drop across the turned-on FET, so about 1/5 the power loss, which helps with thermal as well as efficiency.

    Regards,

    Steve

     

  • 0 in reply to Steve Preissig
    Genius 3455 points

    hi Steve,

    Please consider delivering power from the adapter to to the system/battery. So the Q1 current passes through the body diode of Q1 and no from the Rds of the mosfet.

    Regards, Ras

  • 0 in reply to Ras Sharif50
    TI__Expert 5330 points

    Hi Ras,

    Whenever an adapter is detected (ACDET > 2.4V), the ACDRV will drive ACFET/RBFET to enable both FETs.  As a result, there should never be backgate diode conduction across Q1.  Please refer to section 8.3.4 Adapter Detection and Powerup for more detail on this process.

    Regards,

    Steve