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BQ24780SEVM-583: Current on bench supply slowly drops, then charging stops abruptly (using BQ40Z50 EVM as pack manager with the charge controller EVM)

Jordan Restifo
Prodigy 110 points
Part Number: BQ24780SEVM-583
Other Parts Discussed in Thread: BQ40Z50, BQ24780S, BQSTUDIO

I have a BQ24780S EVM wired up with a BQ40Z50 EVM. The pack output from the BQ40Z50 is wired to BAT on the BQ24780S. I have a bench supply attached to VIN, feeding 19.5V. I have 2S batteries on the pack manager, total charge voltage of 8.4V.

I set the charge controller to 8.4V, and turn on current. The batteries begin to charge, but on my bench supply, the current is slowly dropping for awhile, until it abruptly goes to 35 mA (the sudden drop occurs somewhere in the 700 mA range) and charging stops on its own. The charge current register does not reset to 0 but in this condition of course the batteries have stopped charging. I have tried various input current settings, thinking it was too low, then maybe too high, but I always have the same behavior. I also tried with very low charge current, only 512 mA but the same thing happens.

Any idea on what could be the issue? So far I have been unable to complete a charge cycle due to this. It seems to me to be an issue with the charge controller, not the pack manager, since the pack output stays on, and in the log I don't see any register changes except voltage and current dropping when the event occurs.

  • 0
    TI__Genius 11850 points

    Hi Jordan,

     

    This behavior is unusual, and I have not noticed it when testing a BQ24780S EVM.

     

    What is the current capability of your 19.5 V power supply, and do you have any load connected to SYS on the BQ24780S EVM? If so, please remove the SYS load for now for debugging purposes.

     

    Can you please capture a waveform showing VBUS, VBAT, IBUS, and IBAT on the same graph? It would be helpful to see the big picture of how IBAT gradually decreases and then abruptly drops down to 35 mA. Please also share your full BQ24780S register settings initially (when the charging current is normal), as well as another register dump later (after the charging current drops to 35 mA). A BQStudio screenshot showing all the register settings would be fine.

     

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 110 points

    Unfortunately I don't have good enough equipment to log the current at the moment, but will work on that can get back to you.

    In the meantime, I removed any loads from SYS. I also tried with a different bench supply and putting voltage slightly above the nominal voltage. I am not seeing the slow drop in current now, but rather an abrupt drop to 0. Again, the actual charge current register isn't affected, it stays at the value I set it.

    I also turned off ICRIT thinking that somehow could be affecting it.

    I also then tried by taking the BQ40Z50 out of the equation, and simply hooking up a single cell to the charge controller BAT, and the same issue occurred, so it definitely is the charge controller.

    Is there any other condition where the charge controller "gives up" or reaches some termination threshold? It seems to me if that were the case I should see a register change, but I am not. Does it seem likely that the EVM is somehow damaged?

    This is the register dump. No registers change even a single bit when the issue occurs, so I am just posting the one screenshot. I should note I have tried it with 0x38 set to 0x384 as well, same result.

    Also here is a video showing the bench supply, note the current just drops out. I have the same issue with another bench supply from a different manufacturer, so I don't think the supply is defective.

  • +1 in reply to Jordan Restifo
    TI__Genius 11850 points

    Hi Jordan,

     

    Thanks for the detailed information.

     

    The BQ24780S has a watchdog timer. When the watchdog timer expires, the converter is suspended, so charging stops. The register settings are preserved, which is consistent with your description. Please try disabling the watchdog timer to see if that resolves the issue.

     

     

    Best regards,

    Angelo