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Original question:

BQ25895: what's Vsys when battery is absent

BQ25895: Vsys will be 3.3V for 9 seconds at start-up

Howard Zou
Genius 12365 points
Part Number: BQ25895

Hi,

my customer tested BQ25895 in their final board with battery unattached.

When adaptor plugged in, Vsys will be varying from 3.8V to 4.2V which is normal since there is no battery and Q4 is on.

But Vsys will fall to 3.3V for 9 seconds during the period Vsys varying from 3.8V to 4.2V.

Why would that happen?

schematic shown below, OTG connected to GND.

  • 0
    TI__Genius 12365 points
    And we're not able to do the test by pull CE high at the beginning. Because it's QFN package device and CE is connected to GND on the board.
  • 0 in reply to Howard Zou
    TI__Guru 57331 points
    Hi,

    What does Isys look like during the start-up?

    Thanks,

    Ning
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    TI__Genius 12365 points

    The waveform is as below:

    VBUS- Blue, VSYS- RED, SW- YELLOW, inductor current- Green.

    When adaptor connected, VSYS start up and will be varying between 3.8V and 4.2V for 2seconds (which is normal). And then VSYS will fall to 3.3V and last for 9 seconds, and then return to varying between 3.8V and 4.2V for about 500ms, then the MCU start working and write I2C to disable charge and set Vsysmin=3.7V. Then VSYS is stable to about 3.8V.

    The main problem is why VSYS will fall to 3.3V with default registers.

    By zoom in:

    zoom in again at the period VSYS varying from 3.8V to 4.2V

    zoom in at the period Vsys=3.3V

    zoom in at the period Vsys is stable at 3.8V

  • 0 in reply to Howard Zou
    TI__Guru 57331 points
    Hi,

    The BTST cap should be 47nF. Please provide all BQ25895 register values.

    Thanks,

    Ning
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    TI__Genius 12365 points
    It's default value because the MCU hasn't started working at the moment Vsys falls to 3.3V.
  • 0 in reply to Howard Zou
    TI__Guru 57331 points
    Hi,

    Please try the followings:

    1. Change the BTST cap to 47nF.
    2. Disconnect SYS load from customer board. Try again and see if VSYS drops.
    3. Use separate power supply for MCU and try to read the register values. It is possible for VSYS drops to 3.3V if in DPM.
    4. Use BQ25895 EVM instead of the customer board and see if the start-up behavior is similar.

    Thanks,

    Ning
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    TI__Guru 57331 points
    Hi,

    The thread has been quite for a while. It is considered solved and closed.

    Thanks,

    Ning.