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BQ25306: BQ25306 cann't charge

Part Number: BQ25306

The schematic diagram of our design is shown in the figure. The charging voltage is 12V, and the battery voltage is 8.4V (two batteries in series). Our problem is as follows: 1. When the battery is discharged to a voltage below 6V, BQ25306 will not charge,the charging current is always 0mA

  • Hi,

       What is the status of the STAT pin? Is it indicating charge in progress, off, or toggling at 1Hz frequency?

    What is the range of battery voltage that you see charging occur like normal?

  • Hi Kedar,

    I describe our situation in detail,

    1. We charge at 12V, and the battery is fully charged at 8.4V;

    2. When the battery is discharged below 6.0V, it will enter over-discharge protection, the voltage output is 0V ;

    3. Then plugged in the charger, BQ25306 will not charge, and there will not start 35mA trickle charge;

    4. At this time, the status of STAT is low, the waveform of SW is as shown in the figure below,it's DC 1.0V, and the voltage of REGN is 4.96V

  • Hi Kedar,

    If the battery voltage is between 6.5V-8.2V, BQ25306 will charge normally

  • Hi,

    Through further testing, I found that only when the battery voltage is less than VBat_Short, the BQ25306 will not charge in this case, and there is no 35mA charging current. Other Pre-charge and Fast Charge are normal.

    Please help,thanks!

  • Hi,

      VBAT_SHORT = 2V for falling threshold. This feature has been thoroughly verified. Do you have an external gauge or protector in your system? If that external part has an undervoltage lockout voltage through which the path disconnects, then there could be an issue. have you tested your configuration on the evaluation board?

  • Hi,

    We did not add external gauge or protector

    1. We actually use a 1.27V battery to charge, the charging current is aways 0mA, STAT is low, and after 2 hours, STAT starts toggling at 1Hz frequency 

    2. We switch to using a 2.5V battery to charge, the charging is normal, and the charging current is 75mA;

  • Disconnect the PMID power supply to the downstream system. At this time, when the battery voltage is 1.27V, there will be a charging current of 30mA.

    But our design must be compatible with the adapter to start the machine directly, is there any solution?

  • Hi,

       If the system load is too much for the adapter to provide with the short current of 30mA will not be enough. This is intended as using your system load when battery is discharged so low is dangerous. If system load is active, and adapter is removed, then battery will supply the system load. When battery is discharged so low, providing discharge current to supply external system load on pmid is dangerous. Shouldn't system architecture be such that battery cannot be discharged so low? It would be better to include a protector with an undervoltage lockout

  • Hi
    This is the problem. The battery has over discharge protection. When the battery discharge is lower than 5V, it will enter the protection mode, and then the output voltage is 0V. At this time, bq25306 does not have 30mA current to activate the battery because PMID supplies power to System, so the battery can never be recovered
  • Hi,

       Let me test this configuration in lab on the EVM. What is the load current from PMID in your application?

  • Hi,

    I have tested this,when the PMID current exceeds 30mA, BQ25306 will not charge, it should be current limited by the first MOSFET at the VBUS input. Our system load is about 500mA. 

  • Hi,

       I have verified the architecture of the device, and the trickle charge current of 35mA is sensed across Q1. If you have load at PMID, then the behavior you see is expected, as the current sensing across the RBFET will try to regulate the current to 35mA when VBAT<VBAT_SHORT. To get around this and have system wakeup, you will need to implement external powerpath with the addition of a PFET. You can refer to section 10.2.2 Typical Application with External Power Path

  • Hi,

    Our current design is same as Typical Application of  section 10.2.2,Please refer to the schamatic attachment befor.

    The Typical Application will not hlep for this issue.

    Our current design is the same as the typical application, please refer to the previous schematic attachment. If the power supply of system is not through PMID, but directly through VBUS, is this feasible?

    For typical applications, just the PMID is disconnected, and then directly connected to VBUS, and the others remain unchanged.

  • Hi,

       Yes, it should be directly through VBUS. This way when adapter is connected, it can directly power SYS and provide trickle current to the battery through the IC (Q1 FET will only sense the trickle charge current that way). The risk is if adapter is not connected, as then the battery will try to provide SYS load, which is dangerous for deeply discharged battery. However this risk is same as if it is off of PMID as well. 

    Note that as the path is not through RBFET anymore, the body diode of the RBFET cannot act as reverse blocking diode. You will need to consider using an external diode in the VBUS path to SYS.