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Original question:

BQ76930: BQ76930

BQ76930: External circuit questions

James Zhan
Expert 4365 points
Part Number: BQ76930
Other Parts Discussed in Thread: TIDA-00449

Hi team,

My customer is using BQ76930 for 6S li-on battery in  their design and have some questions as below:

1.In the EVM board, there is a MOS between C10 and REFSRC, why we need the MOS here? Could they connect the REFSRC to the third or fourth cell instead of C10? By the way, why put the C4(1uF) cap in this place? Would all the voltage from C10 add to the C4 and cause some issue when the MOS turn on?

2. What's the use of D4 and D6?

3. Why VC5X need C12  to VC5B, and C9 to VC6? Is it because VSS and VC5X is two reference "GND" here? How does it work internally?

Let me know if you have any suggestions, thanks.

James

  • 0
    TI__Guru 54715 points

    Hi James,

    See the application note http://www.ti.com/lit/pdf/slua749, particularly sections as identified below:

    1. The source follower allows supplying current for the REGSRC supply pin from the top of the stack while preventing high voltage on the pin. See section 9 of the application note.  The designer of TIDA-00449 did not explain C4, perhaps it is to filter transients from C10 to help maintain the peak voltage below the 60V limit of Q3.  For a 6S design REGSRC may be in range of the battery+.

    2. D4 and D6 avoid potentially damaging voltages during assembly or dynamic pack operation.  See figure 20 and related discussion in section 10 of the application note.  Use a conventional diode, not Schottky.

    3. Yes, VC5X is a power reference pin.  See section 3 of the application note for additional description of the device architecture.

  • 0 in reply to WM5295
    TI__Expert 4365 points

    Hi WM5295,

    Thanks for the detailed analysis here, but I still have some questions as below:

    Actually the battery is 7 series here.

    1. What's the meaning for C4, if we have C4 here, would the Battery voltage all charge on C4. And if we turn on the MOS, would this cause overshoot and damage?

    2. Why do we need the Q3 MOSFET here?

    3. Could they connect the REFSRC to the third or fourth cell instead of C10?

    3. What's D4 use here? D6 is connected to BAT, which may have some transient, but D4 connect to VC5X, which is just a internal reference. Could I remove D4?

    4. CAP1 and CAP2 each have a 1uF cap, what's the use of this? what's the internal circuit use?

    5. How to detect if we have adapter pluged in?

    Thanks,

    James

  • 0 in reply to James Zhan
    TI__Guru 54715 points

    Hi James,

    1. Yes the capacitor will charge to the level of the battery if there is no current into REGSRC.  The MOSFET is normally always on in this circuit and current will flow as needed if the REGSRC load changes or VC5 rises.  No overshoot or damage is expected.

    2. Q3 drops the voltage from the battery voltage to the REGSRC pin to keep it in the limitations of the REGSRC pin range.  See the apnote section referenced above. With 7 cells you may not need Q3, but the part would likely be operating above the recommended input range for REGSRC.

    3. D4 keeps VC5 from dramatically exceeding VC5X.  D4 is recommended.

    4. The CAP1 and CAP2 capacitors stabilize internal supplies used for some of the internal circuitry.

    5. The BQ76930 will not know if an adapter is plugged in.  The MCU will need some signaling or alorithm to detect if an adapter is connected to the system.