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BQ25798: VSYS not present if VBUS under ~6.55V

Eric LOUVEL
Prodigy 60 points
Part Number: BQ25798

Tool/software:

Hi,

I use BQ25798 chip with external charger and no battery.

When the VBUS is 6.6V (or more), VSYS is equals to ~3.88V, BUT when VBUS is 6.55V (or less), VSYS is NULL.

Why VSYS is not present with a VBUS ~6.55V ?

(note: VPMID is equals to VBUS).

My feeling is that depends of VAC1 and VAC2 voltage with this feature

But I don't understand the relationship. Can you help me ?

Best regards

Eric

  • 0
    TI__Guru**** 152756 points

    Hi Eric,

    VACx are only high impedance sense lines that sense if a valid input voltage is available. What is the load on SYS?  The input power, VBUS*IINDPM, must be higher than MINSYS*ISYSload.  If not, SYS collapses.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 60 points

    Hi Jeff,
    Thank for your response.

    The load on SYS is about ~50mA (buck-boost + µP + ...). I known MINSYS (3.7V for 1S) but I don't know the value for IINDPM ? Can you give me please ?

  • 0 in reply to Eric LOUVEL
    TI__Guru** 103925 points

    Hello,

    Due to the U.S. holiday our responses will be delayed, sorry for the inconvenience.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Eric LOUVEL
    TI__Guru**** 152756 points

    Hi Eric,

    IINDPM autosets to the LOWER of ILIM_HIZ resistor clamp or the D+/D- detection.  The lowest recommended value is 500mA.  The charger also has VINDPM, which is autoset at power up to 0.7V below the unloaded voltage at power up. VINDPM also limits input current to prevent the charger from crashes the input source. If you manually lower the input voltage below the VINDPM value, the converter stops.  Can you measure the voltage at the IC VBUS pin voltage to see if it is lower than what the power supply reports?  If so, then your cables are highly resistive, causing a voltage drop and activating VINDPM.

    Regards,

    Jeff