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Original question:

BQ25505: Doesn't start up after a short voltage drop/undervoltage lock

BQ25505: Solutions to discharge VSTOR on power on - Section 7.4.3 of BQ25505 Datasheet

Tom Stein
Prodigy 20 points
Part Number: BQ25505
Other Parts Discussed in Thread: TPS22919, TPS22917, TPS22992

Tool/software:

Hello there, 

Problems during startup when attaching a secondary battery to the BQ25505 have been reported several times on the forum. Basically the chip does not continue to provide power to the system if there is a short power outage during startup which can happen due to connection bounce when inserting a battery into a battery holder, placing a jumper connector or flipping a power switch. The image below illustrates the problem.

I want to propose and discuss possible solutions to this problem.


Image taken from Daniel E. in his post.

Related topics in the forum

Datasheet

The Datasheet describes the problem precisely but does not provide a solution. Datasheet section 7.4.3 "When no input source is attached, the VSTOR node should be discharged to ground before attaching a storage element. Hot-plugging a storage element that is charged (for example, the battery protector PFET is closed) and with the VSTOR node more than 100 mV above ground results in the PFET between VSTOR and VBAT_SEC remaining off until an input source is attached."

Solutions

  1. Waiting or manually discharging the capacitors on VSTOR. Not really a solutions for a non-prototype PCB. 
  2. Delay power connection from battery using a delay like 500 ms + a load switch (delay could be realized using RC circuit, or startup sequencer)
  3. Bonus: utilize external QOD functionality of load switch and connect it to VSTOR (not VBAT_SEC as usual). 
  4. other ideas?


For the QOD load switch I could identify two parts from TI: TPS22992, TPS22917, TPS22919. From a first look theTPS22917 looks most suitable because of its low Iq of 0.5uA, low Ron of 80-120 mOhm. TPS22992 looks good as well but has high Iq with 10 uA (note that this will be always active which is not desired for a low power device). 

Please check the draft schematic below.

Checklist

We need to ensure a few things about the solution to keep BQ25505 functioning.

  1. Allow bi-directional current flow (discharge and charge the secondary battery)
  2. Low Quiescent current Iq (uA)
  3. Low Ron (mOhm)
  4. Slew rate compatible with BQ25505



Would this work or is there anything I am overlooking? Looking forward to other ideas.

  • +1
    TI__Mastermind 40845 points

    Hello,

    My schematic review is as follows:

    1. I see you choose MPPT at 80% of VOC by tying VOC_SAMP to VSTOR

    2. VBAT_OV is actually calculated at 3.86V. Please check the calculation.

    3. VBAT_OK_PROG is calculated at 1.52V. Please check the calculation.

    3. VBAT_OK_HYST is confirmed at 3.04V.

    This seems like a plausible solution. You will need to make sure the delay to turn on the load switch has some margin for when VSTOR is completely discharged. Please test according to your application for how the load switch behaves to hot plugs (as if it makes the ON pin turn on accidentally). I would open a separate thread for that part so an expert on that team could confirm the application.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 20 points

    Hello, 
    thank you very much for pointing out this other issue. It was a copy error from another design. 2 pairs of resistors were swapped (R15 with R16, R12 with R13).




    I will ask on another forum topic about the load switches. Thank you very much. 

  • 0 in reply to Tom Stein
    TI__Mastermind 40845 points

    Hello,

    Happy to help!

    The VBAT_OV seems correct at 3.42V.

    The VBAT_OK_PROG seems correct at 2.73V.

    Regards,

    Mike Emanuel