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BQ25619: bq25619 - issue with SYS not powering with just external power source

Keith Jasinski
Prodigy 10 points
Part Number: BQ25619
Other Parts Discussed in Thread: BQSTUDIO

I've got an issue with the bq25619 lithium-ion charger and I've exhausted all self-help options.  The main issue is that from a cold-start, Vsys doesn't come up when just external power is applied.  It boots just fine with the battery installed but not from just external power.

Design information:  Schematic attached.  Design uses a removeable single-cell lithium-ion battery and has an external 5V power source.  Vsys is used to power the system so I need it to come up regardless of the input source (ie: battery or external power).  When just external power is applied, there is a distintive ringing on the SW output (scope plots attached showing waveform with and without external battery).  I've tried doing some loading of the BAT input but I couldn't change the results.  I didn't see any guidance in the datasheet regarding the need to load BAT if not battery present.

Are you able to lend any guidance before I explore other solutions?

Thanks,

Keith

BQ25619_schematic_partial.pdf

  • 0
    TI__Guru** 105760 points

    Hello Keith,

    The bq25619 should supply power to VSYS even when there is not battery present, from your waveforms it looks like the charger is in PFM. What is the VSYS pin when you only have voltage on VBUS without the battery connected? Can you export the registers in the this state for us to review?

    Sincerely,

    Wyatt Keller

  • 0
    Prodigy 10 points

    2V... registers are default at this point as there is not yet any interaction since the system hasn't booted...

  • 0 in reply to Keith Jasinski
    TI__Guru** 105760 points

    Hello Keith,

    The charger should regulate to VSYS_MIN which is defaulted to 3.5V, as long as CE is not enabled, if CE is enabled it will oscillate the STAT LED and regulate VSYS to the charge voltage (cycles between charge and charge complete)

    The issue may be hardware related, I can refer you to the schematic checklist:

    BQ25611D_BQ25616J_BQ25618_BQ25619_SchematicChecklist (1).pdf

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 10 points

    Wyatt - I did have the schematic checklist previously and am following it as far as I can tell.  But proofreading your own work is challenging at times.  I did post my schematic as part of my original submittal.  Do you see issues with it somewhere?

    So, follow up question... while trying different things to see if I could figure out what was going on, I stumbled across a different behavior.  If I remove my system load from the output, Vsys comes up normally.  If I then connect the system load, it seems to behave normally.  I eluded to possible instability of the output with regards to the switching output in the scope plot above.

    So, I don't see anything in the datasheet regarding max load or inrush during powerup with just the USB power.  The is a fair amount of bulk capacitance (hundreds of uF) in the system that needs to get charged up at power on.  However, while that might slow power up, I can't see why it would cause Vsys to sit at 2V.

    Keith

  • 0 in reply to Keith Jasinski
    TI__Guru** 105760 points

    Hello Keith,

    I think you are hitting the default IINDPM limit on power up, you should be drawing about 200mA from the adaptor in all your tests, if you check the registers with bqStudio you should see you're in IINDPM.

    You can refer to section 8.3.3.5 Power Up Converter in Buck Mode in the datasheet for more information about the power up default settings and conditions.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 10 points

    I can look into that but I'm not sure how that could be unless there is some sort of shutdown (if there is what resets it).  Is there some sort of foldback on the current limit?  In this mode, I'm drawing about 40mA on Vsys and slightly less than that from my 5V input source while in this mode.  In other words, if there is indeed some startup inrush that exceeds 200mA (have not yet verified that is happening yet or not), does it make sense that it would sit at a couple volts and 40mA?

  • 0 in reply to Keith Jasinski
    TI__Guru** 105760 points

    Hello Keith,

    During my verification I could see the IINDPM bit being set when the current was too high on VSYS. Can you export the registers before and after the test to verify if there are any faults? Is your STAT pin blinking at any point? First check the IINDPM status to make sure that is not the case for the reduced current.

    Sincerely,

    Wyatt Keller