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BQ76920: Can I leave cells connected while BAT/REGSRC is disconnected

Scott Nelson
Intellectual 340 points
Part Number: BQ76920

If I leave the cells connected (VC0-VC5) but disconnect the BAT/REGSRC pins, could this cause damage to the IC?  I ask because I'd like to insert a breaker in the high side of the high current path.  If that breaker trips will the voltages seen at VC0-VC5 (but no longer at BAT/REGSRC) cause too much current to enter the VC0-VC5 pins?

Thanks!

  • 0
    TI__Expert 6085 points
    Hi Scott,
    First, please be reminded, the FET drivers are powered from a 12V-supply regulated from the REGSRC. It also powers the REGOUT. The REGOUT is used to power internal logic, communications & the CC. Even if the REGOUT is not used for external loads, the REGSRC needs to be hooked to the BAT source.
    If you disconnect BAT, the device can rapid decay to Vshutdown level very rapidly and enter shutdown(SHIPMODE). At which stage, the device will need a BOOT signal on the TS1 to get it back into normal.
  • 0 in reply to Vish Nadarajah
    Prodigy 80 points

    I too am interested in this question as it relates to my question as well: e2e.ti.com/.../2265375

    Vish, can you provide any more diagrams of the internals of the chip so Scott and I can see if there are conduction paths in the chip that would impact our respective applications? Or go into more detail on this matter in general?

  • 0 in reply to Nino Abundes7
    TI__Expert 6085 points

    Please refer to application note -SLUA749- bq769x0 family Top 10 design consideration". refer to the REGSRC Power Option section from pp12-17.

  • 0 in reply to Vish Nadarajah
    Prodigy 80 points

    I have read that entire application note and the only part related to this question is the following:

    "While there is not a value in the recommended operating conditions table, it is expected that the BAT pin voltage is normally approximately the same value as the top cell."

    I understand this but why? What conduction paths are between BAT or REGSRC to VC0/1/2/3/4/5? If we go outside of the recommended operating conditions, but stay within the absolute maximums, what happens internally to the chip? 

  • 0 in reply to Nino Abundes7
    Prodigy 80 points

    In my personal case, if I place a diode in series with the BAT pin to force that current only flow into the pin I see the following conditions with an attempt to pull the BAT/REGSRC voltage low:

    • VC0/1/2/3/4 currents are very close to zero.
    • VC5 current ~46uA <-- This is very high
    • Current coming OUT of BAT/REGSRC ~10uA

    That last point means that the Cell5 power draw is enough to not only float the voltage on the BAT REGSRC pins, but also drive a diode in to reverse bias such that it leaks 10uA.

  • 0 in reply to Nino Abundes7
    TI__Expert 6085 points
    when you make this measurement, what is the status of the ALERT pin?. 15-25uA can get added to the BAT current when ALERT is triggered.
    refer to dI(alert) spec. where is the 46uA is measured?@ the PIN?
  • 0 in reply to Vish Nadarajah
    Prodigy 80 points

    When I make this measurement, the alert pin is the same voltage as VSS. It is pulled down by a 560kOhm resistor that goes into a high impedance, so if that much current was exiting the alert pin, the voltage would be well above VSS.

    The 46uA is measured going into pin VC5. The 10uA is measured as coming out of REGSRC/BAT (I can't tell which because I would have to ruin the board by cutting the trace to tell).

    This is all done after power was cut. Nothing on this board is on at all. The only current in the entire system draws is exclusively from the VC5 pin.