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BQ25306: Clarification on dead battery condition

Monisha M
Prodigy 170 points

Part Number: BQ25306

Hi Team,

  1. In the application circuit of dead battery condition, an external MOSFET Q4 is added. Could please explain the purpose of the diode at gate of Q4 for? Is there any specification for the diode to be used there?
  2. Does BQ25306 support zero volt charging?

Best Regards,

Monisha 

  • +1
    TI__Mastermind 35950 points

    Hi Monisha, 

    Please see my comments below. 

    1)The application circuit shown in section 10.2.2 is for if your system requires an external power path, which will allow VBUS input to directly power system rail then when VBUS is removed the PFET will turn on to allow battery to discharge to system rail. The purpose of the diode is for reverse blocking back to VBUS input adapter/supply. 

    2)Yes the BQ25305 does support charging from zero volts. If VBAT is < Vbat_short (typ 2.2V) the charge current will be 35mA. Once battery voltage rises above this threshold it will transition to precharge phase where charge current is increased. 

    Best Regards,

    Garrett  

  • 0 in reply to Garrett Kreger
    Prodigy 170 points

    Hi Garrett,

    In this case if diode reverse block flow to VBUS Adapter, is the voltage source from the battery reason for the current flow? Is it applicable when the adapter is plugged in to the charger IC? If VBUS/Adapter is no present, then there is no voltage source at VBUS, so the reverse current has no effect.

    The Mosfet leaks from its  source (Vsys)  to gate and then to VBUS ?.Can you please mention is the voltage source that leaks current through gate of Mosfet  to VBUS/Adapter is the battery voltage?

    If adapter/VBUS is present, then there will no reverse current flow to gate. Right? Please clarify.

    Also, please comment whether we use an normal diode or an ideal diode is recommended at the gate of MOSFET?

    Regards,

    Monisha M

  • 0 in reply to Monisha M
    TI__Mastermind 35950 points

    Hi Monisha, 

    When adapter is plugged into charger IC the external PFET will be off. Yes the inclusion of the diode is so there is not leakage from battery output back to input rail VBUS when adapter is not plugged in.  

    Monisha M said:
    If adapter/VBUS is present, then there will no reverse current flow to gate. Right? Please clarify.

    Yes I am in agreement with this statement. 

    Monisha M said:
    Also, please comment whether we use an normal diode or an ideal diode is recommended at the gate of MOSFET?

    I recommend a Schottky diode. BAT54ws-7-f is an example part number of a widely available diode other customers have used for this purpose. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 170 points

    Hi Garrett,

    Okay. Also, Could you please confirm is the diode used at the gate of MOSFET is also to make sure that the Q4 does not turn on when VBUS is present? 

    Best Regards,

    Monisha 

  • 0 in reply to Monisha M
    TI__Mastermind 35950 points

    Hi Monisha, 

    The intent of the design is for Q4 to be off when VBUS is present. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 170 points

    Hi Garrett,

    Thank you for the clarification. Could you please review the external mosfet section added below and share your comments?

    Best Regards,

    Monisha

  • 0 in reply to Monisha M
    TI__Mastermind 35950 points

    Hi Monisha, 

    The circuit for the external mosfet looks good to me. Additionally, while figure 10-8 in the datasheet shows external power path use case, we are in the process of trying to modify this. Connecting a powerpath to PMID pin is not recommended on this device. Instead it is recommended to connect VSYS to VBUS and add an external reverse blocking FET. 

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 170 points

    Hi Garrett,

    We will consider this recommendation. Meanwhile, Could you please also clarify on our below query?

    For a dead battery condition, without implementing the external MOSFET, can't the battery charger IC(BQ25306) charge the battery and power the system load simultaneously? If so, then why is the external circuit implemented as shown in figure 10-8 in the datasheet?

    Best Regards,

    Monisha

  • 0 in reply to Monisha M
    TI__Mastermind 35950 points

    Hi Monisha, 

    Please see my comments below. 

    Monisha M said:
    For a dead battery condition, without implementing the external MOSFET, can't the battery charger IC(BQ25306) charge the battery and power the system load simultaneously?

    For the dead battery condition the output will be regulated to the deeply discharged battery voltage. Often the system output needs to be regulated to a higher voltage than this, hence the use of the external FET where SYS is source directly from the 5V input while the low voltage battery is charged through the converter. 

    Additionally, when battery is very low voltage device will be in trickle charge or precharge phase where the charge current is very low. Datasheet section 9.3.2.1 shows the charge profile and outlines the output current in each phase of charge. 

    For these two reasons you most likely cannot power SYS through the converter for dead battery condition until the battery has been charged some. 

    Best Regards,

    Garrett