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BQ77905EVM-707: BQ77905 / BQ77905EVM-707 CHG PIN / OV Questions

Andrew Henry82
Prodigy 30 points
Part Number: BQ77905EVM-707
Other Parts Discussed in Thread: BQ77905

Hello,

I have a few questions regarding the CHG pin and OV conditions related to the BQ77905 and the EVM module.

I have used the EVM module to initially test my cell configuration and it works great. I have also tested my personal design and it also works exactly like the EVM module.

I noticed that the CHG pin will be "low" when I'm below the OV threshold of 4.25V (my cells were 4.15V/each). I am unsure as to whether the hysteresis range comes into effect in this scenario (200mV)?

Do my cells actually have to be less than 4.25V-0.2V = 4.05V before CHG pin will be "high"? What is the reasoning for such a large hysteresis range if this is the case?

Kind regards,

  • 0
    TI__Genius 14985 points
    Yes, once you exceed the OV threshold of 4.25V, the voltage must drop below 4.25V-0.2V=4.05V before the CHG pin will be enabled again. This hysteresis is to prevent "chattering" because your voltage will quickly drop once the charge current is removed. If the CHG FET was enabled very quickly after the voltage dropped a little bit then the charger might continue charging and the voltage would trip again. Then you might end up pulse charging your battery even though the voltage is no longer safe. Therefore the delay and hysteresis are both important components to ensure safety.
  • 0 in reply to dMax
    Prodigy 30 points

    Hello dMax,

    Thank you for your response. I just wanted to ask one last thing before my issue is resolved.

    When I first connected the cells to the BQ77905, they were at an individual cell voltage of 4.15V. Does this mean it still puts it into the OV hysteresis range even though I have not exceeded 4.25V? That's where I'm confused.

    Kind regards,

  • 0 in reply to Andrew Henry82
    TI__Guru 54715 points

    Hi Andrew,
    I believe your answer is in 8.3.2.1 of the data sheet. The part "... assumes OV fault after device reset." So if you assemble the pack with a cell in the OV hysteresis range CHG will remain off after cell attach. If you discharge at sufficient current CHG will come on from the body diode protection feature, see 8.3.8, but the OV will not clear until below the hysteresis threshold.
    We understand cells are normally stored, shipped and assembled at a low to mid state of charge, so this should not be a production issue. Of course in the lab with an EVM you will have a wider range of situations.

  • 0 in reply to WM5295
    Prodigy 30 points
    Hi WM5295, I must have missed that in the datasheet "assumes OV fault after reset"!! Thank you for clarifying that for me!

    Kind regards,