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BQ24125: Open Instance in FB

Jejomar Ildefonso
Genius 9010 points
Part Number: BQ24125
Other Parts Discussed in Thread: BQ24123, BQ25125

Hi Team,

Our customer has a question about the feedback resistors on BQ24125.The resistor network connected between the battery and FB pin determines the the charger output voltage.

They're wondering if one of the feedback resistors were to break open, how would the part respond? We would like to understand in this fault condition, does the part protect the battery pack by disabling charging or would the control loop no longer regulate and cause the output voltage to rail? 

Thanks in advance!


Kind Regards,

Jejomar

  • +1
    TI__Mastermind 24075 points

    Hi,

       FB is just a sense line, to sense battery voltage. If it senses over 2.1V due to bottom resistor open, then it will suspend charge due to a battery overvoltage condition. If top resistor is missing then bottom resistor pulls FB to GND signaling a 0V battery, and charger would only charge at Ishort at ~35 to 65mA. This is not enough to significantly raise the battery voltage in a short amount of time, and after 30 minutes the inbuilt precharge timer will kick in and suspend charging.

  • 0 in reply to Kedar Manishankar
    TI__Genius 9010 points

    Hi Kadar,

    Many thanks for your response. It was really helpful. Our customer has follow-up questions that I was hoping you could help us with:

    1. We are designing a PCB that is compatible with both the BQ24123 and BQ24125. There would be resistors that would be populated for each config. In the event that there is an error with the way that the boards are assembled do you see any issues if the BQ24123 had a resistor divider from the battery voltage to the CELLS pin? See attached schematic picture.

    2. What can we recommend for the tolerance of the resistors used in the resistor divider for BQ24125? It seems that if I used 0.1% tolerant resistors combined with the 1% voltage regulation accuracy of the part, then the BQ25125 cannot be as accurate as the BQ24123 across the resistor variation.


    Thank you for your continuous support!


    Kind Regards,

    Jejomar

  • +1 in reply to Jejomar Ildefonso
    TI__Mastermind 24075 points

    Hi,

       The VIH and VIL levels for CELL are defined in the EC table: 

    It is possible the voltage division due to the resistor network will trick the charger into thinking BQ24123 is configured for 2S application. You will need to check with battery vendor how dangerous that would be. If you define variants in schematics and send the right variant for fabrication. then you wouldn't ever really run into this issue. It wouldn't be a vendor issue but an engineer fault. I define variants for different schematics quite frequently and have never seen issues.

    Yes, this is the reason we have different variations for the IC family. Using the CELL configuration part will always have higher vreg accuracy as we can use internal divider corresponding to voltage on CELL pin. The downside is the limited output regulation offered, as you can only do 2 different configuration. With resistor divider you have a larger range of vreg values for non standard battery voltages as well. Tradeoff is the accuracy can be slightly lower and is affected by resistor tolerance.