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bq769x0 CHG FET drive

Fred P
Expert 2015 points

Looking at Figure 7-2 in bq769x0 datasheet, I can't find a charge path for Q1 gate when Q2 is open.
That would mean you can't charge without allowing discharge.

I must miss something...

Can someone show me the path?

Regards,

Fred

  • 0
    Intellectual 370 points

    Hello,

    when Q2 is open the body diode is active in charge, when the charge sensing is done by current measurement you can close the MOSFET Q2 to avoid loss in the body diode of Q2.

    Otherwise you can use a dedicated channel for charge.

    Regards.

  • 0 in reply to LILITH
    Expert 2015 points

    Hi,

    You may have misunderstood my question since you mention "charge sensing".  

    I am interested by the Q1 gate-source charging current path (a few nC only), not by the cells charge path which, I agree, is through Q2 body diode when its channel is open.

    CHG pin is referred to GND.  Current must flow from CHG to GND.

    Q1 body diode must be part of the gate-source charge path, but not Q2 body diode. When Q2 is open, its diode is blocking whatever current would want to flow from Q1 to the GND.  That's its purpose....

    Thanks for trying

  • 0 in reply to Fred P
    TI__Guru 54715 points

    If there is no load or charger, both CHG and DSG are off and you measure the voltage of the pack, your meter will provide a load.  You will measure a voltage dependent on the meter load, the power FET leakage and the CHG pull down.  This will likely be a few volts below the battery voltage depending on your meter's resistance.

    If you turn on CHG, you will see the pack voltage drop because CHG has gone to ~12V and Q3 is on.  Your meter is providing a negative bias on the gate-source voltage of the charge FET Q2.  If you remove the meter, Q2 is still off because R2 holds the Vgs ~0 depending on Q1 leakage.  Q2 will not be on until DSG turns on Q1 or a charger voltage is applied which brings the source of Q2 low relative to its gate. With sufficient charger voltage present the charger can provide current to the battery through Q1 if on or its body diode if off.

  • 0 in reply to WM5295
    Expert 2015 points

    Biasing through the meter, charger or whatever external load is connected is the key!

    I get it.

    Thanks a lot!

    Fred