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BQ25628EVM: The question about NVDC waveform

Ryker She
Prodigy 680 points
Part Number: BQ25628EVM
Other Parts Discussed in Thread: BQ25619, BQ25628

Hi

The VSYS would fall about 50mv when charge disabled and VBAT above the minimum system voltage initially. But there isn't fall in datasheet. Is this phenomenon normal? And what causes this fall? Thanks.

Best regards,

Ryker

  • 0
    TI__Prodigy 595 points

    Hello Ryker,

    Reference section 9.3.4.1 "Narrow VDC Architecture" of the datasheet. When charging is disabled and VBAT is above the minimum system voltage, VSYS is regulated 50mV above VBAT. When BATFET is turned on from the battery voltage rising above the minimum system voltage, the difference between VBAT and VSYS is the VDS(ON) of BATFET.

    Regards,
    Alec

  • 0 in reply to Alec Lehman
    TI__Prodigy 680 points

    Hi Alec,

    My confusion is the VSYS would fall 50mV when charging is disabled and VBAT is above the minimum system voltage initially. Is the BATFET incompletely turn on switch to fully turn on cause this? The below picture in BQ25619's datasheet has not that fall. Thanks.

    Best regards,

    Ryker

  • +1 in reply to Ryker She
    TI__Prodigy 595 points

    Hello Ryker,

    That picture can be ignored as it does not apply to BQ25628.

    This behavior is normal. In the scope picture, while VBAT < VSYSMIN, there is an offset between VSYS and VBAT while charging is disabled. The spec for this offset, typically 230mV, is found on page 9 of the datasheet and is called VSYS_REG_ACC (for VBAT < VSYSMIN).

    As VBAT rises above VSYSMIN while charging is disabled, VSYS is to be regulated 50mV above VBAT, following the same spec but under a different condition VSYS_REG_ACC (for VBAT > VSYSMIN). Thus, the reason VSYS drops about 50mV at this instance is so that it can be regulated at 50mV above VBAT instead of 230mV above VSYSMIN.

    Regards,
    Alec