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BQ2980: BQ2980's CHG & DSG can handle direct pull down

Christina Lim
Expert 5875 points
Part Number: BQ2980

Hi, 

Below is a diagram of the question: the circuit is exactly the same as what datasheet suggest, just there are other pull down circuit (red and orange N-MOSFETs) – when other protection in the system triggers, these MOSFETs will pull  both of either one of the CHG and DSG to ground.

The question  is: whether the CHG and DSG pin can handle this direct pull down to ground without being damaged? And do you have other comments about the idea and circuit?

bq2980_QUESTION.pdf

Thanks,

Christina 

  • 0
    TI__Expert 8526 points

    Hi Christina,

    I do not recommend using the configuration you showed in your document.  Pulling the CHG and DSG pins to ground when they are outputting a HIGH signal will stress the charge pump inside the BQ2980. Additionally, the charge pump capacitor will be continuously drained under this configuration.

    This charge pump outputs a voltage to the gate of the NFET that is higher than the voltage at the source of the NFET, providing the positive Vgs needed to turn the FET ON.

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    TI__Expert 5875 points

    Hi Andria, 

     Yes, we understand in normal operation CHG and DSG needs to turn on a high side N-FET, so it generates a voltage higher than system bus voltage Vdd from a charge pump. The question we were asking is however about abnormal condition where other protection circuit in the system needs to turn off the charge and/or discharge N-FETs.

     

    Since all of the charge pump output current will be pull to ground by the red/orange MOSFETs, but we would imagine the charge pump will keep trying to pump some current to the CHG and DSG pins, and all will be shorted to the ground via the red/orange MOSFETs, so our guess is the charge pump will therefore working at its max capacity to pump current at 10uA rate (according to datasheet Table 7.5 Iload).

    If that is right, do you mind elaborate a bit more on why and how the charge pump inside will be stressed?

    And if we really do so, what is the risk and consequence? (in another word, what will happen?)

     

      

    An additional question:

    If pulling down CHG or DSG to ground (like I drew in the initial email) is not a right way, is there a recommended way to turn off the CHG/DSG when other protection circuit wants to switch off the high side N-FETs (CHG FET, and DSG FET)?

    The protection design principle is to turn off the FETs directly – that is to do something about their gate voltage, instead of switch them off via a middle-man.

     

    Thanks,

    Christina 

  • 0 in reply to Christina Lim
    TI__Expert 8526 points

    Hi Christina,

    As I mentioned, the charge pump capacitor will be drained if it is pulled to ground.  This can cause an issue because both CHG and DSG FETs are relying on the charge pump for a voltage supply.  This could lead to excessive system current, as the charge pump capacitor is being simultaneously charged to a voltage high enough to turn on one FET while also being pulled to ground.

    For example, if you have the CHG FET ON and the DSG FET OFF, the DSG FET path is being pulled to ground.  This will be draining the charge pump capacitor that is also trying to supply voltage to the CHG FET to keep it ON.  Consequently, the system will be urgently trying to recharge the charge pump capacitor to provide high voltage to the CHG FET gate.  To charge the capacitor that is being pulled to ground, the device or system will be forced to source a high current.  This could lead to excessive heating, component burnout, or other complications.

    You could potentially place the MOSFETs that are controlled by the outside system between the charge pump and the CHG and DSG FETs.  This means that there will not be a direct path from charge pump to ground.  For this configuration, however, you will need to consider level shifting and achieving a gate voltage on the additional NMOSFETs that is higher than the charge pump output.

    Best,

    Andria