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BQ24081: Problem with termination detection current

Jordi Cabrera
Prodigy 20 points
Part Number: BQ24081
Other Parts Discussed in Thread: BQ25188, BQ25185

Tool/software:

Hi,

I have designed a battery charger using the BQ24081DRC. 

I have the Fast Charge Current set at 970 mA, and according to the equations in the data sheet, the Charge Termination Detection Current is 97.5 mA. So the Rset is 820

When I monitor the Vset, it starts at 2.5V as the datasheet says, but when the voltage decreases, it doesn't stop at Vterm value and decreases below 90mV. So the termination detection current doesn't stop at 97.5mA .

Theoretically, the Vterm value doesn't have to stop at 250mV?

I don't know if I have done something wrong. Can anyone help me?

  • 0
    TI__Expert 8315 points

    Hello Jordi,

    The BQ24081 terminates charging when VSET is below 250mV and VOUT is at the battery regulation voltage for longer than the 375ms deglitch time.

    Do you have anything other than the battery connected to the OUT pin? If so, I recommend disconnecting it and testing termination with only the battery connected to OUT.

    What is your typical load profile? Is it a pulsed load?

    What do STAT1 and STAT2 indicate? Are they stable or are they toggling between states?

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 20 points

    Hello Alec,

    The OUT pin is connected the battery and the circuit power input.

    It's not possible to disconnect the circuit, because it includes a microcontroller that reads the STAT1 and STAT2 inputs to indicate the battery's state.

    The load profile is a continuous power without any pulses.

    The STAT1 and STAT2 are stable and indicate fast power.

  • 0 in reply to Jordi Cabrera
    TI__Expert 8315 points

    Hello Jordi,

    If the load is drawing more current than the termination current, the charger will not terminate. How much current is your load drawing?

    If you need to power the system load while charging the battery, I recommend switching to a power-path charger, such as the BQ25185 or BQ25188.

  • 0 in reply to Alec Lehman
    Prodigy 20 points

    Hello Alec,

    The load is drawing around 30mA, and the charge termination detection current is around 97.5mA. (Rterm = 820 ohms)

    In any case, I understand that if the output current is higher than the termination current, it keeps the Vterm above 250 mV, and I can read values of 90 mV without switching the state to end of charge.

    I have done the test with different circuits and different xips, and all of them do the same.

  • +1 in reply to Jordi Cabrera
    TI__Expert 8315 points

    Hello Jordi,

    I did some testing on a BQ24081 EVM to try and recreate the behavior you're describing.

    Without any external load connected in parallel with the battery, I confirmed that termination does occur when the output current falls below the termination current threshold.

    With a constant current load, I confirmed that termination does not occur if the load current is higher than the termination current, as expected.

    With a pulsed load, it's possible for termination not to occur. This is because the pulsed load causes VSET to pulse as well, so it doesn't stay below the 250mV termination threshold for longer than 375ms

    How are you measuring the 90mV on VSET? I recommend capturing waveforms of VSET, VOUT, and IOUT to confirm whether VSET is stable when this issue is occurring.

  • 0 in reply to Alec Lehman
    Prodigy 20 points

    Hello Alec,

    When I captured the waveforms of Vset, I could see fast micropulses from power supply noise.

    As soon as I filtered these micropulses, the module started to work correctly.

    Thanks!