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BQ29209: EN signal and OV protection over single wire

Stefano Sivera
Prodigy 221 points
Part Number: BQ29209

Hi guys, 

in my application i need reduce the number of wire that connect the balanced circuit with charger circuit composed by step up and linear battery charger. The overvoltage must disable the step up and the balancing must be  active only in charging, this  is for avoid extra discharge when the device is not connected to the power supply.

Schematic enclose.

Can you share you opinion on this connection ?

Thank you 

Best RegardsTI_BQ.pdf

  • TI__Mastermind 42060 points

    Hello Stefano,

    It does not seem to me like this would work. Q3 would essentially always remain off (cell balancing would always be disabled), there would be nothing pulling CB_EN to Vss. If OUT goes high, Q1 would turn-on, but because Q3 is a NPN, it would remain off as the current pulled down would be coming out of the base. To turn-on Q3 there must be current going into its base. 

    Also, using a FETs may be better than a BJT as these are voltage controlled, rather than current controlled. So consumption would be less and they would be easier to control. The drive current is in the uA range. So I am not sure if it'd be strong enough to drive the BJTs.

    Can you explain exactly how is it that you want the cell balancing control circuit to work? You want CB to be enabled solely during charging or during OV and charging?

    Best Regards,

    Luis Hernandez Salomon

  • in reply to Luis H. S.
    Prodigy 221 points

    Hello Luis,

    I explain what is in my mind and I hope even Grinin the schematics.

    The CB_EN enable is active when tigh low, when the power suppy is removed (J2 connector) the balancing is disabled by R15 pull up. When I plug the power supply  the SHDN signal go high through R8 and Q3 pull down the CB_EN.
    The balancing is activated only when is present the bower supply and disabled when the PS is disconnected.
    In all normal condition the OUT is low and Q1 is disabled.
    When overvoltage condiction occours OUT go high and Q1 pull down SHDN dsabling the step up and removing the power for the battery charger.
    Only point wher I have adoubt is : when the CB_EN is disabled by Q1 switched ON, the OUT singnal continue to remain high until the battery overvoltage decrease.

    About the use of a FET: Q3 is not a problem because the base current is present only with PS connected, effectively you have right for Q1 is better use a MOS because the out current is around 80uA .

    With PS connected the CB mus be active of shure,
    when the PS is disconnected I think that be better disable the CB for avoiding useless current consumption
    when PS present and OV occours, it would be better continue to have CB active but with only one wire control it cannot be possible.

    Best Regards

    Stefano 

  • in reply to Stefano Sivera
    TI__Mastermind 42060 points

    Hello Stefano,

    Thank you for the explanation! That makes a lot more sense to me now. Yes, I see how it works now!

    When the CB_EN is disabled by Q1 switched ON, the OUT singnal continue to remain high until the battery overvoltage decrease.
    You would be correct. You would not be able to cell balance if at OV with this configuration.

    About the use of a FET: Q3 is not a problem because the base current is present only with PS connected, effectively you have right for Q1 is better use a MOS because the out current is around 80uA .
    Yeah, with Q1 a FET is best. But now that I understand how Q1 works, a BJT should work here.

    With PS connected the CB mus be active of shure,
    when the PS is disconnected I think that be better disable the CB for avoiding useless current consumption
    when PS present and OV occurs, it would be better continue to have CB active but with only one wire control it cannot be possible.
    Yes, you are correct. Not possible in this configuration unfortunately.

    Couldn't you just connect an additional resistor from VUSB to Q3 directly and avoid using Q1?

    Like so:

    Best Regards,

    Luis Hernandez Salomon

  • in reply to Luis H. S.
    Prodigy 221 points

    Hello Luis,

    Luis H. S. said:
    Yes, you are correct. Not possible in this configuration unfortunately.

    You see some problem on this configuration ?

    Luis H. S. said:
    Couldn't you just connect an additional resistor from VUSB to Q3 directly and avoid using Q1?

     Is connected to the base of Q3 the added resistor ? Q1 is used for shut down the step up and remove the power supply to the battery charger.

    Stefano

  • in reply to Stefano Sivera
    TI__Mastermind 42060 points

    Hello Stefano,

    You see some problem on this configuration ?
    No more than what you've already pointed out. I was just agreeing with you.

    Is connected to the base of Q3 the added resistor ? Q1 is used for shut down the step up and remove the power supply to the battery charger
    Yes exactly. The additional resistor could connect to the base of Q3. This way you can still use Q1 to pull-down SHDN and power Q3 separately. I think that could work, do you have any concerns with that?

    Best Regards,

    Luis Hernandez Salomon

  • in reply to Luis H. S.
    Prodigy 221 points

    Hello Luis,

    Luis H. S. said:
    Yes exactly. The additional resistor could connect to the base of Q3. This way you can still use Q1 to pull-down SHDN and power Q3 separately. I think that could work, do you have any concerns with that?

    Understand, but in this way we have 2 wire of control line and this discussion is born for reduce the control line. I will mantain this schematic.

    Thnak you for your support.

    Best Regards

    Setefano Sivera