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BQ25606: VBAT behavior when USB is connected and no battery is present

user4725373
Expert 1645 points
Part Number: BQ25606
Other Parts Discussed in Thread: BQ34110

Hi,

Our design uses a BQ25606 for charging a Li-ion single cell battery, and a BQ34110 as a battery fuel gauge. The battery connections are +VBAT and -VBAT, and there is a current sense resistor between -VBAT and GND for the fuel gauge.

The BQ34110 requires a calibration during production (voltage offset and current offset/gain). The currently calibration requires a fixed current going through RSENSE, and the voltage calibration requires a fixed voltage at the VBAT pin of the BQ34110, which is shared with the VBAT pin of the BQ25606.

Currently, our plan is to have the USB powering our PCB, but not have a battery connected. This way, the PCB is powered via the BQ25606. We can then use a current sink circuit between +RSENSE and -VBAT for the current calibration. However, the voltage calibration requires a fixed voltage at +VBAT. We observe that +VBAT is unstable when USB is connected while there is no battery due to the BQ25606. Our question is: can we connect a 4.2V LDO to +VBAT to fix the voltage? Would the BQ25606 detect that the voltage is high enough that the charging can stop?

Thank you,

Fred

  • 0
    TI__Mastermind 25800 points
    Hey Fred,

    The "instability" you are seeing is due to the battery not being present and exiting/entering charging states continuously.

    Directly connecting the LDO at 4.2V may not be ideal for your LDO that may not operate correctly when trying to sink current. However, you can place a diode in series with your LDO and make sure the voltage is high enough to force the charger to terminate.


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Expert 1645 points

    Hi Joel, thanks for answering.

    What value of voltage would be high enough? It wasn't very clear for me from the datasheet. Would 4.2V be enough?

    Also, once the charger detects that the "battery" is full and stops trying to charge it, but the charger is still connected to USB, does V_SYS still come from USB or it now comes from VBAT?

    Thank you,

    Fred

  • 0 in reply to user4725373
    TI__Mastermind 25800 points
    Hey Fred,

    From the datasheet, the VBAT voltage will depend on the state of the VSET pin. From section 6 Pin Configuration and Functions, if the VSET pin is floating, the VBATREG voltage will be 4.208 +/- 5%. If you had an adjustable output LDO, that might be best.

    Is this only for testing purposes? If so, just use a power supply with the same diode.

    In that scenario, VSYS still comes from VBUS.


    Regards,
    Joel H