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BQ25798: V_SYS Regulation inconsistent with configuration

Ethan Armstrong
Prodigy 20 points
Part Number: BQ25798

Tool/software:

I am using a BQ25798 to charge a 3S li-ion pack. My input will be a USB PD source, but as of right now I am working with a standard 5V, 500mA adapter. I am trying to get the system to work as follows:
Battery Connected:
V_SYS ~= V_BAT

Battery Disconnected
V_SYS ~= VBUS

I am running into two problems getting this to work.
1. Setting the Minimal_System_Voltage register to 0x12 does not seem to set the minum V_SYS voltage to 4.5V (to my understanding 5V VBUS > 4.5V VSYSMIN ... no regulation occurs)
    this was tested with CE pulled high to avoid any issues with charging

2. IC is not able to properly detect the presence of a battery. Regardless of whether the battery is plugged into the BAT/BATP (via 100Ohm) pins the Charger_Status_2 Register declares VBAT as present.
    additionally, I tried with a dummy 10kOhm load, which should've set VBAT to 0V, and the Charge Status register says its in fast charge mode

In the case of #1, the V_SYS voltage is still regulated to the default setting for 3S (9V)

In the case of #2, BATP seems to be regulated to 12.6V for either a CC charge or a taper charge. Since the chip thinks there is a battery, BAT is connected to VSYS which also becomes 12.6V

  • 0
    TI__Guru**** 166106 points

    HI Ethan,

    Regarding 1, V(SYS) can never been less than V(BAT).  They are connected by a FET with a body diode. In other words, V(SYS) = max of MINSYS or V(BAT) as it charges up to VREG.

    Regarding 2, this charger does not have a robust battery detect circuit.  We recommend writing host software that monitors TS pin for COLD fault and then force IBAT discharge = 30 mA and use ADC to see if VBAT collapses.  If so then only the BAT pin capacitors are present.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points

    Thank you for the response Jeff.

    I understand the VSYS cannot be less than VBAT. However, with charging disabled (CE Pulled High), and no battery connected. The system regulates VBUS (5V) up to 9V instead of just passing it through. I assumed this could be changed via register 0x0, is this not the case?

    #2 is solved, I will implement this as part of the MCU control on startup.

  • +1 in reply to Ethan Armstrong
    TI__Guru**** 166106 points

    Hi Ethan,

    The VREG register is clamped to specific range around #cells x 4.2V, i.e. you can't have CELLS register = 2 and set VREG register to 4.2V.  I didn't think the MINSYS register had this limitation.  But the charger does not simply pass through VBUS to SYS with /CE =high.  The converter continues to switch and regulates V(SYS)=MINSYS.  

    You probably already know this but the MINSYS, VREG, CELLS and ICHG reset to defaults per PROG pin if WD timer expires.

    Regards,

    Jeff