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BQ24088: Force Charge Disable to Check Battery Voltage

George Beckstein
Prodigy 30 points
Part Number: BQ24088
Other Parts Discussed in Thread: BQ25100

Hi everyone!

I'm working on a design using the BQ24088 to charge an internal LiPo battery. We want to implement a simple battery charge percentage tracking algorithm based on the battery's voltage. To do this, we need to be able to manually disable the charger, check the battery voltage "at rest" and then re-enable the charger.

We also wanted to have a PMIC that monitored battery pack temperature for improved safety, thus why we chose the BQ24088 (that and we were able to buy it at the time).

My plan is to connect the TS (temperature sense/NTC thermistor input) pin of the BQ24088 to a 5V-tolerant GPIO on my microcontroller (STM32). Then, when we want to check the "at rest" battery voltage, we can pull the TS pin low, putting the BQ24088 into a temporary, fake "thermal fault", which disables the charger output.

We are using the BQ25100 in another design and the datasheet for this part mentions using the TS pin in this way. I could not find similar recommendations in the BQ24088 datasheet.

I want to confirm with TI that using the TS pin in this way on the BQ24088 will not cause any undesirable/unpredictable behavior. It seems to work fine on the breadboard -- shorting TS to GND disables charger output and removing the short returns the charger to "fast charge" mode.

  • 0
    TI__Expert 8245 points

    Hi George,

    The only thing to keep in mind is that while the BQ25100 has set thresholds at 0C, 10C, 45C, and 60C, the BQ24088 has user defined temp thresholds as determined by resistors RT1 and RT2. This makes your intended application a bit less straightforward. Since you are not actually using a thermistor here and are instead forcing the device in and out of a cold threshold, I am not sure of where the thresholds will be and the behavior you will observe. The RT1 and RT2 values are usually calculated knowing the spec of the thermistor you intend to use but I suspect if you short TS to GND as you said in your post, it should drop below the cold threshold and suspend charging. I would just make sure that when you remove the short, you are exceeding the cold fault threshold by a fair margin.

    Please take a look at the attached images from the data sheet for more info on RT1, and RT2.

    Thanks,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 30 points

    Hi Jackson,

    Thanks for the response. We are actually using an NTC thermistor. We would simply leave the GPIO in high-impedance mode during normal operation, then suspend charging manually by pulling the NTC net (TS pin) low for a brief period while we read the battery voltage.

    So during normal operation, the BQ24088 will actually see a valid NTC thermistor reading.

    I also plan to monitor the voltage on the ISET pin which should give us an idea of what stage the battery charge is in.

  • 0 in reply to George Beckstein
    TI__Expert 8245 points

    George,

    Thanks for clearing that up. In that case, the only thing to ensure is that you give the battery long enough to have it's voltage settle after charging is suspended. If you pull TS low then immediately read the voltage, your voltage measurement used to estimate SOC may not be entirely accurate.

    Best Regards,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 30 points

    Hi Jackson,

    One more question:

    What is the maximum voltage the ISET pin will be during normal operation? From the datasheet it seems this pin is typically regulated to 2.5V.

    My MCU is running at 2.8V, I don't want to apply a voltage to the ADC higher than this. If the ISET pin voltage does go higher, I will just not monitor the output current.

  • 0 in reply to George Beckstein
    TI__Expert 8245 points

    Hi George,

    according to the EC table in the datasheet the max value you should observe on this pin is 2.55V

    Best Regards,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 30 points

    Thanks for clarifying that!