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BQ25883: Undershoot In The Output Supply At Regular Intervals

Chirag thakar
Prodigy 50 points
Part Number: BQ25883

Hi,

We have used BQ25883RGET IC in our design for Battery Charging. We observed undershoot in the VSYS output at regular intervals. These undershoots are also causing its input VBATT to have undershoots at regular intervals. Please refer attached waveforms for the same.

We are interested in knowing a possible cause of this undershoots. Can this Undershoots be removed?

Please help us with the same.

Note: Waveforms are taken with AC coupling so that the deeps in the supply can be observed clearly.

Thank you!

Regards,
Chirag Thakar

  • 0
    TI__Guru**** 165886 points

    Hi Chirag,

    I do not see any attachments.  What event (load transient on SYS?) precedes the undershoot?  If a battery is attached, the undershoot on SYS should only be the as low as the battery voltage.  What are the charger's input current limit and VINDPM settings?  Are they limiting the input power and therefore the charger's output power?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 50 points

    Hi Jeff, 

    Thank you for your response.

    We have attached the waveform of the SYS(In both DC as well as AC mode of Oscilloscope) and the schematic of BQ IC here.

    Charger's input current limit is set around 3A, which is not limiting the input current limit.

    Thank You!

    Chirag Thakar

    Regards,

  • 0 in reply to Chirag thakar
    TI__Guru**** 165886 points

    Hi Chirag,

    How much capacitance on is on the SYS output?  To confirm, the D+/D- lines set input current limit initially.  In order to get 3A current limit from the D+D- detection, the charger must be connected to a DCP adapter.  See table 3 on page 18.  You can overwrite the IINDPM register after startup detection with 3.0A.  Keep in mind that 5V input at 3A is 15W so at approx 7V on SYS, the max output to SYS and BAT is 15W*0.95efficiency/7V = 2A.  If the SYS load is 

      Can you show VBUS, SYS. IBUS and ISYS zoomed in on one SYS dip?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 50 points

    Hi Jeff,

    Thank you for your response.

    Please note that only battery is connected as a power source. USB as well as Adapter are not connected.

    For capacitance on VSYS, Please find below the attached schematic of BQ IC:

  • 0 in reply to Chirag thakar
    TI__Guru**** 165886 points

    Hi Chirag,

    If only the battery is connected, then your system load at SYS pin (or load at VBUS if OTG is enabled)  is pulling too much current (>9A) from the battery.  Or the battery's own internal protection, with overcurrent limit, is turning on.

    Regards,

    Jeff  

  • 0 in reply to Jeff F
    Prodigy 50 points

    Hi Jeff,

    Thank you for your response.

    Are there any flags that sets and indicates this kind of condition(internal protection, overcurrent limit, etc.). Can this be checked by reading any register setting? Please help.

    Regards,

    Chirag Thakar

  • 0 in reply to Chirag thakar
    TI__Guru**** 165886 points

    Hi Chirag,

    You can check IBAT OCP in REG26 and/or VSYS Short in REG27.

    Regards,

    Jeff