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BQ25756EVM: Issues with EVM

Boris Yanchev
Prodigy 120 points
Part Number: BQ25756EVM
Other Parts Discussed in Thread: EV2400, BQSTUDIO

Tool/software:

Hello,

I am facing some issues with this EVM.

Test setup:

- bench PSU, set to 23V, 5A

- DC load, set to 23.5V (no PSU for option 2.4.1 in guide available)

- EVM board, standard configuration as per user's guide

- bqStudio (v1.3.127, Charger_1_00-bq25756.bqz) and EV2400 set up correctly, verified communication with IC

With the above setup, charging did not go into CC mode, only pre-charge. Furthermore, there was a discrepancy in the current values reported by the IC and the DC load (setting 1A, ADC 868mA, load 0.97A).

Between different tests, the IC would go into battery overvoltage protection every second test or so.

Testing further, I disabled precharge. The IC then goes into CC mode, however still with a discrepancy in the current values: ADC 1586mA, load 1.7A, setting 2A.

One thing I noticed at a later point (cannot verify if this was the case all along) was that VFB_ADC always reported 0.

After some more tests without a different outcome, the IC is cycling between CV and "charge termination done" without the DC load connected.

I have ordered spare ICs as I believe this one is damaged, however any information as to how this could have happened would be helpful. I have also ordered the correct resistors to test with the actual battery (4S).

Kind regards,

Boris

  • 0
    TI__Mastermind 23160 points

    Hello Boris,

    Thanks for working on this.

    I have a few questions to help troubleshoot the charging going into battery over-voltage protection:

    • Was the DC load in constant current mode or constant voltage mode?
    • How much capacitance was in parallel with the DC load? We recommend at least 1000µF or 2000µF in parallel with the DC load when testing.
    Boris Yanchev said:
    Furthermore, there was a discrepancy in the current values reported by the IC and the DC load (setting 1A, ADC 868mA, load 0.97A).

    This discrepancy is expected. The ADC is intended to provide a general indicator for the voltage and current level.

    Boris Yanchev said:
    One thing I noticed at a later point (cannot verify if this was the case all along) was that VFB_ADC always reported 0.

    The VFB ADC is disabled by default. We recommend turning OFF the VFB ADC while the charger is charging. Turning ON the VFB ADC while the charger is not charging should cause no problems.

    Boris Yanchev said:
    After some more tests without a different outcome, the IC is cycling between CV and "charge termination done" without the DC load connected.

    I think the charger is charging up the capacitors and then reaching the termination current threshold. Once charging terminates, the capacitor voltage falls to the recharge threshold and the IC starts charging again. This is expected behavior.

    Let me know if you have any questions about this. Also, from what you've told me, it sounds like the IC is still functioning. I don't think the IC needs to be replaced yet.

    Also just for reference, here's the link to the BQ2575X FAQ page.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 120 points

    Hi Ethan!

    Thank you for your prompt and comprehensive reply!

    Adding the parallel capacitance fixed the strange behaviours (pre-charge, overvoltage protection).

    Thank you for sending the FAQ, it will come in handy later on!

    Kind regards,

    Boris