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BQ25619: Vsys selection

Raphael Jambor
Prodigy 70 points
Part Number: BQ25619


How can I adjust the system voltage of the buck. I just found the upper and lower limits in the datasheet but not how to adjust the system voltage it self. I would like to have a system voltage not higher than 3.6 volts.

Regards Raphael

  • 0
    TI__Guru*** 148825 points

    Raphael,

    You can set the minimum system voltage when the battery is discharged using REG01b[0:3].  Default is 3.5V.  However, after the battery charges higher than 3.5V, V(SYS) follows V(BAT) up to the battery regulation voltage + ~150mV.  So, when the battery is fully charged, V(SYS)=4.2V+150mV = 4.35V.  This is how all of our power path chargers function.  If you need a lower system voltage, then I suggest adding a buck converter with 100% duty cycle following V(SYS).

    Regards,

    Jeff  

  • 0 in reply to Jeff F
    Prodigy 70 points
    Hey Jeff, thanks for the quick answer. I would like to use a LiFePo cell for the circuit. It has a final charge voltage of 3.6 volts. Hence the 3.6 Volt maximum voltage. If I set the end-of-charge voltage to this value, is the system voltage also at 3.6 volts? Regards Raphael
  • 0 in reply to Raphael Jambor
    TI__Guru*** 148825 points

    Hi Raphael,

    If using a LiFePO battery, I recommend that you start the charger with /CE pin = high, With /CE pin=high, the SYS output will initially regulate to MINSYS=3.6V. Then use I2C to change the VBATREG register = 3.6V and the MINSYS voltage register to something lower than 3.6V like 3.3V.  The I2C settings will stay assuming you periodically write to the watchdog timer bit or disable watchdog timer, unless the charger is completely powered down.   Then you can use your host GPIO to pull /CE pin low for LiFePo charging.

    Regards,

    Jeff