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BQ24040: Charge Complete When Under Load

George Ioakimedes
Genius 4950 points
Part Number: BQ24040
Other Parts Discussed in Thread: BQ24075

I have a question that perhaps someone has solved before. My system is using a 1000mAh Li Ion and the BQ24040 is configured

  • for 0.2C charge (200mA) by using 2.7k pull-down on ISET.
  • I have left PRE-TERM pin floating so termination charge is when current drops to 20mA (10% of ISET)
  • ISET2 is pulled low so charge current = ISET

I have CHG and PG pulled high and connected to my processor and am able to detect the power good and charge done signals. My board has a string of LEDs that can be turned on in 3 different brightness levels which consume ~30mA, ~70mA, and ~100mA.

Is there some way that I can determine when the battery will be charged if the user turns on the LEDs either before or during the charge cycle? I'm thinking that right now if this happens the charge cycle will timeout and stop but I will never receive a charge done signal because the load will be higher than the termination

  • 0
    TI__Prodigy 640 points

    George,

    If system is operating, this charger cannot determine if battery current has reached a termination level. You are correct that if system load current remains higher than the termination current, charging will eventually time out.

    I recommend looking at a power-path device to resolve this issue, such as BQ24075. Power path allows the current to battery and the current to system to be monitored separately.

  • 0 in reply to Diana Bodenmiller
    Genius 4950 points

    Hi Diana,

    Thanks for taking a look. I had hoped that someone had come up with a trick to determine this. I was even looking into measuring ISET so I could try to provide some sort of time left to charge indicator but I think that voltage will reflect the load current too. I might be able to still use that since I'll know what mode the board is in and what current is flowing to the LEDs. I could use that to subtract from what I measure at ISET. I haven't thought this through but it was 1 idea I had.

    The problem with the BQ2407x is that it looks to be 2X the cost of the BQ24040.

    Regards,

    George

  • 0 in reply to George Ioakimedes
    TI__Prodigy 640 points
    Hello George,

    If you know the current Iout and the current going to the system, you could determine the current to the battery, as you said. However, we do not have any official instruction for determining battery charge level in this way, since power path devices are designed specifically to handle this issue.

    I understand cost is an important factor. I will notify you if I encounter a solution using bq24040 or other non-power path device.