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BQ24610: What is the purpose of Q3 and SYSTEM path?

Michael Mast
Prodigy 60 points
Part Number: BQ24610

I have a few general questions about Figure 19 in the Datasheet, Typical Application:

Given that Q4 and Q5 are the low and high drivers, respectively, what is the purpose of Q3?  

What is SYSTEM and how is it intended to be used?

My application is a battery charger for a removable battery pack.  The battery pack will be inserted, charged and then removed.  The thing that the battery powers will not be connected to the circuit containing the BQ24610.  Is is NOT a design objective to be able to power some device with the charging circuit, even if the battery has been removed.  Given that, do I need R15, R19, C9, C15 and Q3?

Thanks.

  • 0
    TI__Expert 8690 points
    Hi Michael,

    Q3 allows for the system to be powered by the battery when there is no adapter.

    You don't need those components if there will be no system connected. BATDRV can also be left floating.

    Regards,
    Steven
  • 0 in reply to Steven Bangdiwala
    Prodigy 60 points
    @Steven Thanks. The application is inside a vehicle, so the BQ24610 based charger will be connected to the vehicle's 12V power supply. Because of the running engine and alternator, voltages can fluctuate from around 11.5VDC to 14.5VDC. As there will be no SYSTEM path (as discussed above), do I need Q1, Q2, RAC, R14, C14, C2 and C3? Also I'm thinking I would not need R3 and R4. In such a case, I think Q4's drain can be directly connected to vehicle's 12VDC supply, is this correct? And what would I do with the ACN and ACP pins? I assume ACDRV can just float.
  • 0 in reply to Michael Mast
    TI__Intellectual 2320 points
    Hi Michael,

    You don't need RAC RAC, C2,C3 if there will be no input current limit request. and you can short ACN and ACP and connect to VIN after Q1 and Q2. you don't need R3, R4 connect ACSET to VREF.

    I think Q1, Q2 is necessary, because they can provide reverse current protection, OVP, Reverse input protection ....

    Alen Chen