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BQ76952: Powering the BQ76952 from CD (middle of Protection fets)

Dominik Wenger
Prodigy 200 points
Part Number: BQ76952

Hello,

is it possible to connect the BAT pin of the BQ76952 to the CD point (middle of the protection fets) to allow the BQ76952 be powered from a charger if the battery is fully flat ? Or does the BAT pin absolutely nee to be connected to the top of the cells ?

kind regards

  • 0
    TI__Guru 54715 points

    Hi Dominik,

    The BAT pin is the BQ76952's reference to the charge FET gate.  It is not a perfect reference of course since there is the filter and diode in the line.  During a heavy load the FET source will drop below the BAT pin and CHG pin, but the load will turn on the FET due to both the voltage drop of the battery voltage and the body diode protection feature of the IC, so that is ok.  During charge with the CHG off  the BQ76952 will keep the CHG at the BAT pin voltage and the FET will be off.

    If BAT were connected to the CD point the reference to the charge FET would be ok during discharge since the source would only be a diode above the CD point until the FET turned on, then the same.  However during charging if the CHG were to be off it would drive to the BAT pin voltage, if that were connected to the CD node and the charger voltage was sufficiently high the charge FET would operate as a source follower and continue to charge the cells.

    Powering the regulator source transistor from CD is desirable since the regulator load could be heavy and it is desirable to have the that current come from the higher voltage charger rather than the low voltage nearly flat battery.  If the cells are too far discharged they probably should not be charged, of course that will depend on the cell.  Check with the manufacturer.  Many times if the cell stack is below the 4V VPORA of the BQ76952 and does not recovery by itself, the pack should not be charged.  The 4V level would allow a 3 cell stack to be at 1.33V/cell, a 16 cell stack could be at 0.25V/cell

    In summary, we do not recommend powering BAT from the CD net.

  • 0 in reply to WM5295
    Prodigy 200 points

    Thank you for this long explanation, i really appreciate this and i now understand the issue with the reference for the FETs.

    I am guessing the LDOs also do not work when the BAT pin is below the 4V VPORA of the BQ76952, correct ?

    While charging the Battery is not really safe after a deep discharge of the battery, it would still be very helpful to be able to re-nable the electronics after this, to readout a history of the events which lead to this deeep discharge from the uC. So i am thinking of a way to somehow at least power the uC or maybe switch the BAT pin to CD only in the deep discharge state on command.

  • +1 in reply to Dominik Wenger
    TI__Guru 54715 points

    Hi Dominik,

    "...LDOs also do not work when the BAT pin is below the 4V VPORA of the BQ76952, correct ?"

    You are correct.  Although REGIN may have sufficient voltage and current capability to source current for the LDOs, if the BAT pin voltage is below the VPORA the logic in the device will not be able to enable the regulators.