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Original question:

BQ25798: BQ25798 register settings change automatically

BQ25798: VSYS fluctuating when powered up without battery

Ahmed Ali
Prodigy 80 points
Part Number: BQ25798
Other Parts Discussed in Thread: USB2ANY

Tool/software:

Dear TI team,

My charger is in a 4s configuration. I have double-checked the 17.4k/0.1% ohm resistor on the PROG pin. I have also connected the 4.7uF capacitor to the PMID pin. My issue is that when powering up the board without a battery, the VSYS(min) register sets itself to 3.25V, and voltages at the VSYS pin start fluctuating from 9-13V. The voltages at the VBAT pin also fluctuate with random voltage values. The stat pin starts blinking, and the inductor creates weird noises. The CHG_Flag_Int and VBAT_OVP_Int starts blinking. As shown in this figure, these registers are causing the issue. Even if I correct these, these are set automatically.

I am unsure why this issue is present. When I connect the battery, the charger works fine when setting up the registers.

I am looking forward to hearing back.

Regards,

Ahmed Ali

  • 0
    TI__Guru**** 166876 points

    Hi Ahmed,

    I do not understand what you are seeing.  The host appears to be losing I2C comms with the charger.  When VBUS has a voltage > BATUVLO applied, is REGN crashing to ground?  Can you provide an oscilloscope plot of VBUS, REGN, SYS and BAT?

      Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 80 points

    Hi Jeff,

    I am only using USB2ANY at this point. I have put the 100uF capacitor on the PMID pin also, due to which VSYS fluctuation has been stopped, and the following are the observed oscilloscope requested results on load and no-load conditions:

    ON LOAD:

    1. VBUS on power up and load

                      

    2. REGN on power up and load

    3. VSYS on power up and load

    4. BAT on power up and load

    ON NO-LOAD:

    1. VBUS on load turn-off

    2. REGN on load turn-off

    3. VSYS on load turn-off

    4. BAT on load turn-off

    These are the requested oscilloscope results. I apologize for the camera results, as the oscilloscope behaved weirdly when saving the plots onto the USB.

    I am looking forward to hearing back.

    Regards,

    Ahmed Ali

  • 0 in reply to Ahmed Ali
    TI__Guru**** 166876 points

    HI Ahmed,

    Is a battery attached?  It appears that your load on SYS is too much for the input power at VBUS.  What is the load power VSYS*ISYS?  What is the input power VBUS*InputCurrentLimit were InputCurrentLimit = ILIM_HIZ resistor clamp if EN_ILIM bit =1 or IINDPM register setting?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 80 points

    Hi Jeff,

    There is no battery attached. The load on VSYS is only ~0.6A, and the IINDPM register setting is 3A. 

    As you can see, the voltage divider on the ILIM_HIZ pin is set to 3A.

    I have connected the ON/OFF controller to the VSYS pin, as shown in the above image. When I turn on this controller, the load is connected. When there is no load, according to the PROG pin setting, there should only be 12V at the VSYS pin, but it rises to 16.8V, as shown in the oscilloscope result. When the load is connected, the VSYS drops to 3.25V.

    What is the reason behind this behavior on the VSYS pin?

    I am looking forward to hearing back.

    Regards,

    Ahmed Ali

  • +1 in reply to Ahmed Ali
    TI__Guru**** 166876 points

    Hi Ahmed,

    With charge enabled and no battery and no fault due to missing thermistor, V(SYS) = higher of MINSYS voltage or 16.8V.  Then charge terminates, V(BAT) drops below recharge voltage and the process repeats.  Can you try disabling charge?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 80 points

    Hi Jeff,

    Tomorrow, let me share the results of turning off the charge pin CE through the MCU and in the chargers' register.