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BQ24171: BQ24171 disable Q3

Star Xu
Genius 9645 points
Part Number: BQ24171

Hi 

I have  a query need your help.

If I do not need the power path, the SYSTEM voltage is equal to the battery voltage, does the battery voltage pass through BQ24171?

Waiting for your reply.

Thanks

Star

  • 0
    TI__Guru*** 146695 points

    Hi Star,

    If the system is connected to the battery, Q3 is not needed.  Keep in mind that the charger can't distinguish between system current and charge current in this configuration so the charger may never report termination if ISYS>ITERM.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Genius 9645 points

    Hi Jeff

    Thanks for reply.

    The customer add an under voltage protection circuit is placed at the position of ×. When the battery power supply voltage is lower than 9V, the under voltage protection disconnects the system power supply.

    The actual test is that Q3 and Voltage protection circuit does not work. The system and the battery are connected together under the battery power supply.

    If I want to add an under voltage protection  at the system, how to do it?

    Waiting for your reply.

    Thanks

    Star

  • 0 in reply to Star Xu
    TI__Guru*** 146695 points

    Hi Star,

    Q3 body diode connects SYS to battery all the time.  To disconnect, a second, in series FET with diode facing the other direction is required.  The charger turns on Q3 after the input voltage drops below the UVLO point per OVPSET resistors.  A supervisor and reset circuit that monitors V(BAT) and controls the back to back FETs instead of or in addition to /BATDRV pin could provide  undervoltage protection.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Genius 9645 points

    Hi Jeff

    Thanks for your patience.

    The actual test is that after Q3 is removed, other parts of the circuit remain unchanged. Connect the battery and input 15V for charging. After 15V input voltage is removed, the system still has power, which is equal to the battery voltage. Q3 has been removed, how does the system power come from?

    Waiting for your reply.

    Thanks

    Star

  • 0 in reply to Star Xu
    TI__Guru*** 146695 points

    Hi Star,

    There is a path through the inductor, to SW pin through through the buck high side FET body diode to PVCC to give V(BAT)-0.6V.  If Q3 is installed, V(SYS)=V(BAT).

    Regards,
    Jeff