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BQ24780S: Charger not starting properly

Mark Cullen
Prodigy 180 points
Part Number: BQ24780S
Other Parts Discussed in Thread: BQ24780, BQ40Z50

Hi Everyone

I have a BQ24780 charging a single cell battery via a BQ40Z50 gas gauge.

During startup we set the charger to output 4.2 volts with a 512 mA limit, sometimes it works OK but sometimes the output voltage is only 1.6 volts.

The oscillator is running at 800 KHz and I don't think it's current limiting, if I change the charge current to 4096 mA the output voltage only goes up by about 10mV and the input current doesn't change.

The PROCHOT status register shows 0, ACOK pin is high, charger is not in sleep and is not disabled.

Has anyone seen anything like this before ?

Thanks

Mark

  • 0
    TI__Expert 5330 points

    Hi Mark,

    A few questions will help us track down the issue:

    1.  Are you testing with your own hardware or an evaluation module?  If your own hardware, do you have an EVM to use as comparison?

    2.  How are you representing the battery in your test?  Is it a real battery or do you have an eload or similar bench equipment?

    3.  What is the voltage at the battery or eload when running the test?  Can you measure the current?

    Also, one thing to look at that -- the device has a battery short feature that limits the current to 500 mA when the battery voltage is below 2.5V.  (Reference BAT_LOWV in the datasheet.)    This could be why increasing the charge current is not having an effect when the battery voltage is at 1.6V.

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    TI__Expert 5330 points

    Hi Mark,

    To clarify question #3 -- you already told me that the voltage is 1.6V when you are having an issue.  What I meant to ask is how the voltage is being set or determined?  I.e. if you have a real battery, this should be nominally higher than the actual battery voltage, but if you are using bench equipment, how are you configuring that equipment?

    Thanks,

    Steve

  • 0 in reply to Steve Preissig
    Prodigy 180 points

    Hi Steve

    Thanks for the reply sorry it took a while to get back to it.

    1) I'm testing on my own hardware.

    2) I'm using a real battery

    3) The battery voltage is around 3.6

    I must admit I hadn't seen the BAT_LOWV feature and it does fit my symptoms.

    When the input power first comes on the battery may be disconnected from the system by the gas gauge, in that case the voltage at SRN could be well below 2.5, do you think that may be triggering the BAT_LOWV feature ?

  • 0 in reply to Mark Cullen
    TI__Expert 5330 points

    Hi Mark,

    If the voltage as seen by the controller is at or below 2.5 V, then the BAT_LOWV will be engaged.  Once the battery voltage goes above 2.5V, however, the current clamp is removed without any further action required.  Note that there is a 200 mV hysteresis, but no more than 2.7V at SRN should be required to release the clamp.  So once the gas gauge connects the battery, if the voltage at SRN is above the BAT_LOWV threshold, then you should see full charging current restored.

    One way to test if the BAT_LOWV is the issue would be to set the charge current below the 500 mA clamp.  If you see that changing the current between 256 mA and 512 mA has the expected effect but that anything over 512 mA does not change the charge current, then this would support the BAT_LOWV hypothesis.

    There is also a possibility that the gauge/protection IC on the battery is engaging a protection of its own.  This is a separate team and I am not very well versed with the features of the gauge products, so you might want to reach out to that team as well.

    I am going to go ahead and mark this ticket as resolved since the BAT_LOWV seems the likely explanation, but if you continue investigating and find that this is not the case, please either press the "this did not resolve my issue" button on this thread or start a new thread and we will continue.

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    Prodigy 180 points

    Thanks Steve

    I will do some more testing to try and verify that this is the issue.

    lets mark this one as resolved.