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BQ24618: Consistent Weird Bug

Kim Drange Hansen
Prodigy 20 points
Part Number: BQ24618

Tool/software:

When I try to test my circuit with this IC using an electronic load functioning in CV mode at 14V to emulate a 4S battery, it initially does not behave as expected. At 16.5V in, the load that's supposed to be at 14V is forced to sway somewhere between 7-8V and it does not pull nearly as much current as programmed (around 0.2A as opposed to the intended 3A). What is odd is that this problem is IMMEDIATELY remedied when I try to measure the voltage from the output to the Vfb pin. This is consistent and the IC will not behave as expected before I put the two terminals of a digital multimeter in voltage mode on a testpoint on the output and what would be the lowside of R2 on the datasheet.

Here are some things which may help diagnose the issue:

- While working as intended, supplying 3A at 14V, the STAT1 output is OFF while the STAT2 output is ON, indicating that it is finished charging even though it's in the constant current phase of charging

- While not working as intended, the PG pin sways from 1-2V, as opposed to 0 or the 5V reference

- While not working as intended, the Vcc pin goes from low to high at a rapid rate

  • 0
    TI__Mastermind 31976 points

    Hello Kim,

    Kim Drange Hansen said:
    When I try to test my circuit with this IC using an electronic load functioning in CV mode at 14V to emulate a 4S battery, i

    I recommend setting the electronic load in CC mode with 1000uF -3000uF cap in parallel with the load.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    I tried initially without a capacitor and it only pulled 0.5A when the load is set to 3A, indicating that the IC went into precharge mode because the voltage on the outside was so low. I just added a 3300uF capacitor in parallel and the result is exactly the same. The voltage at the load stays at 0.02V and I can't test the IC when it should be operating with CC charging.

  • 0 in reply to Kim Drange Hansen
    TI__Mastermind 31976 points

    Hello Kim,

    Did you set the electronic load in CC mode as I mentioned before? for battery simulation we recommend setting electronic load in CC mode.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    Yes, this is what I described in my last reply. I've found that the IC only works when the electronic load is set to CV and after I've measured the voltage across R2.

  • 0 in reply to Kim Drange Hansen
    TI__Mastermind 31976 points

    Hello Kim,

    What is the current load programmed to when electronic load is set to CC mode?

    I recommend starting at 1A and slowly ramp the current load up.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    I set it to 3A, I will try ramping it up the next time I am in the lab.

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    Okay, so I think we might have solved it. This problem is far less persistent when the power supply is providing a higher voltage. Originally, the load was being charged at 14.4V with a 16.5V supply. When turning the supply up to 18V, the problem no longer persists. Is the "battery voltage" the regulation voltage programmed at the Vfb pin or the voltage of the battery being charged? Because the regulation voltage is set to around 16.4V, and it would make sense with the 1.5-2V recommendation if the "battery voltage" was this value, since this is when the issue is no longer encountered.

  • 0 in reply to Kim Drange Hansen
    TI__Mastermind 31976 points

    Hello Kim,

    Kim Drange Hansen said:
    Okay, so I think we might have solved it. This problem is far less persistent when the power supply is providing a higher voltage. Originally, the load was being charged at 14.4V with a 16.5V supply. When turning the supply up to 18V, the problem no longer persists. Is the "battery voltage" the regulation voltage programmed at the Vfb pin or the voltage of the battery being charged? Because the regulation voltage is set to around 16.4V, and it would make sense with the 1.5-2V recommendation if the "battery voltage" was this value, since this is when the issue is no longer encountered.

    Yes, We recommend the input supply to be 1.5V-2V greater than the maximum battery voltage. If you have the battery regulation voltage programmend for 16.4V, You will need an input supply greater than 18V.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    I see. But if I plan on keeping the battery in the flat region and only us CC charging, why does it matter? Is the voltage drop across the IC really that great? I was charging at 14.4V, but I needed 18V in for it to be consistent.

  • 0 in reply to Kim Drange Hansen
    TI__Mastermind 31976 points

    Hello Kim,

    Kim Drange Hansen said:
    I see. But if I plan on keeping the battery in the flat region and only us CC charging, why does it matter? Is the voltage drop across the IC really that great? I was charging at 14.4V, but I needed 18V in for it to be consistent.
    • I don't think this is an issue with the charger, this is most likely an issue with your test setup.
    • Are you still using the Cap in parallel with the Eload?
    • is the Eload still in CV mode?
    • I only see this behavior if I  use 14.5V eload in CV mode, and I start with a 16V input.
    • If I lower input from 24V-16V, this issue does not occur.
    • if I add 27,000uF( the only cap I have available) this issue does not occur.
    • I think issue is with the battery simulator.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    I did try lowering the supply from 20-16V while running, but started encountering this issue around 17V (with the load at 14.4 CV). I have tried to use the electronic load to pull a constant current of 3A, but the voltage on the feedback pin of the regulator drops too low and the load ends up pulling the precharge/termination current of 0.5A, even with a capacitor on the outside.

  • 0 in reply to Kim Drange Hansen
    TI__Mastermind 31976 points

    Hello Kim,

    Do you have the capacitor in parallel when you are operating in CV mode? Are you able to increase this capacitor value to 6000uF?

    Best Regards,

    Christian.