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BQ25672: BQ25672 Reregulation VSYS

Serhii Yantsevych
Prodigy 40 points
Part Number: BQ25672

Tool/software:

Dear TI support,

I have a circuit with the BQ25672 and use an Application Note for the auto-restart for a poor source qualification.
The configuration of the input circuit looks like the circuit desctibed in the 8.3.5.2 One ACFET-RBFET part of the datasheet.
The IC has default configurations. +VEXT = 12V.



The issue that I see is re-regulation of the VSYS rail during the dropout of the +VEXT rail, which affects the regulator and periphery in the VSYS downstream.
This reregulation looks like :


2ch - ACDRV1, 3ch - VSYS, 4ch - VAC1

Could someone help me define the reason and potentially help with fixing that behaviour?

Best Regards,
Serhii

  • 0
    TI__Guru**** 166576 points

    Hi Serhii,

    I'm not following what is happening.  I assume you have no battery attached but battery charging is enabled?  Then the load on system is making SYS voltage collapse?  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Hi Jeff,

    Thanks for quick comunication.
    There is no battery connected. The charging is dissabled by CE pin, and the IC is not configured by I2C.

    There is no big load on the SYS, about 100mA.

    This case hapends, when AC/DC external power supply is disconecting. So, voltage on SYS start rising, and the prifery that depends on SYS go to reset and booting again fo some period.

    Best Regards,
    Serhii

  • 0 in reply to Serhii Yantsevych
    TI__Guru**** 166576 points

    Hi Serhii,

    When no battery is attached, the charger needs a voltage greater than 3.6V to provide a regulated voltage at SYS.  From your scope plot, VAC1 is falling below 3.6V and not recovering so there is no SYS voltage.  The app note circuit is intended for solar panel sources that start at a higher voltage and then dip low but not below 3.6V.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Regarding dropping VSYS when VAC1 is falling below 3.6V, that is expected behaviour. But this area of the SYS is not clear to me:

    I expect that the SYS voltage should drop linearly, like this:


    Regards,

    Serhii

     

  • 0 in reply to Serhii Yantsevych
    TI__Guru**** 166576 points

    HI Serhii,

    The charger sets the VINDPM threshold at startup either 0.7V or 1.4V less than input voltage depending if the voltage is > or < 10V. After the input voltage drops below the VINDPM, the converter essentially stops switching so SYS falls as ISYS discharges the SYS capacitors until it hits the battery voltage.  The host can overwrite the VINDPM threshold to a lower value to prevent this.

    Regards,

    Jeff