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BQ25619: SYS, BATFET and operation without battery connected

Mickael Churet
Intellectual 260 points

Part Number: BQ25619

Hi there, 

As  we are working on the design for the BQ25619, a few questions came up: 

  • If we are not using the the BOOST function, what is the best termination for the PMID pin?
  • If we understand correctly:
    • as explained in the Narrow VDC Architecture paragraph, the battery charge voltage is fully controlled by the Buck operating voltage point SYS (at least when VBAT is > Minimum System voltage + 180mV). In this case BATFET is fully ON and the buck is operating in current regulation as long as it is in the CC part of the charge - is that correct? 
    • But during DPM, when charge current needs to be reduced to supply SYS Loads in priority, what is the mechanism used to control ICHG - is it BATFET operating in LDO mode with current regulation mode?
    • And, in this case, what voltage point is set for VSYS? Does it increase to make headroom for the increased VDS of BATFET?  
  • What is the behaviour of the chip if no battery is connected? What is for example the voltage point for VSYS ? 

Best Regards,

Mickael 

  • 0
    TI__Guru**** 181076 points

    Hi Michael,

    Regarding PMID pin, PMID still needs its minimum capacitance = 10uF even if not using boost mode.  It is the input to the buck converter.

    Regarding operation, when V(BAT) <SYS_MIN, V(SYS)=SYS_MIN+0-150mV if charge is disabled or no battery.  So if SYS_MIN=3.5V, V(SYS)=3.5V-3.65V. If this mode, if the [I(SYS)+I(BAT)]*SYSMIN < efficiency*VBUS*IINDPM (meaning there is enough input power to provide system load and battery charge current), when charge is enabled (CC mode), the BATFET is in linear regulation mode where the BATFET itself regulates the charge current (assuming not clamped as explained below). 

     When V(BAT) >SYS_MIN, V(SYS)=V(BAT)+0-50mV if charge is disabled or no battery.  So if V(BAT)=3.7V, V(SYS)=3.7V-3.75V. If this mode, if the [I(SYS)+I(BAT)]*SYSMIN < efficiency*VBUS*IINDPM (meaning there is enough input power to provide system load and battery charge current), when charge is enabled (CC mode), the BATFET is fully on and charge current is regulated by modulating (clamping) the buck converter duty cycle. 

    In both cases above, if one of the other control loops (VINDPM, IINDPM, TREG, CV) turns on, the buck converter duty cycle gets clamped which clamps input current and therefore total output current.  Charge current naturally reduces to provide as much power to SYS in order to prevent SYS collapse.  If there is not enough input power to provide I(SYS), the IC turns on the BATFET fully in supplement mode and V(SYS)=V(BAT)-RDSONQ4*(IBAT).

    When no battery is connected and charge disabled, the charge operates the same as explained for V(BAT)<SYSMIN.  If charge is enabled with no battery, you will see the V(BAT) and V(SYS) oscillate between approximately SYS_MIN and BATREG as the output capacitor on BAT charges to BATREG and discharges.   

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 260 points

    Super clear explanation - Thanks a lot Jeff!