• TI Thinks Resolved

BQ25890: Why VSYS is only 1.87V with a 4.7Om load?

Part Number: BQ25890

I use the schematic just as the BQ25890evm.

The system works well with battery. The Ilimit is 1.6A. And I did not program the chip through I2C.

When I remove the battery and only connect the VBUS:

1. When the VSYS is connect to a 10Om load, the VSYS is 3.65V.

2. When the VSYS is connect to a 4.7Om load, the VSYS is only 1.87V.

Question: Is that normal? Why the VSYS drops below 3.5V (VSYS MIN)?

Question: What is the maximum current that VSYS can provide when battery is removed? I didn't find the value in the datasheet.

The Figure 5 and Figure 6 in the datasheet only show the SYS Voltage Regulation vs System Load Current when the battery is on.

Thank you.

Frank

  • Guru 60655 points

    In reply to FrankXu:

    Frank,

    Can you remove the USB meter and short D+=D- and retry?

     

  • In reply to Jeff F:

    Hi Jeff,

         Thank you.

          I ordered a BQ25890EVM-664 from TI and today I got it.

          TEST1. I use a 4.7Om load for test.

          1. When no load is put at the VSYS, the VSYS is 4.2V. (D+ D- not shorted)

           2. Then, I put the load onto the SYS while keeping the board powered by the VSOURCE. The VSYS drops to 1.82V which is similar to the phenonmen that happened on my board (1.75V). Here now, the VSOURCE keeps on 5.08V and the current is 0.18A.

     

       3. I also repeat the above experiment with D+ D- shorted. The exactly same phenonmen happened, the VSYS droops to 1.82V.

      4. You can see that the Vsource did not drop. And the Vsource is a 2A charger and as I said, when the battery is put on, the meter shows the current is 1.08A.

          And I also tried the experiment without the meter. The exact phenonmen happened.

    5. If a 10Om load is used. The VSYS is 4.19V and everything is OK.

        So, I think the bq25890 VSYS can not provide > 4.2/4.7=893mA current when battery is off.

        Question1. What is the maximum current that the VSYS can provide when the battery is off?

        TEST2.  Compare my board with BQ25890EVM-644.

        Both the boards can not drive a 4.7Om load.

        When the load is 10Om, the VSYS on my board is 3.64V while the VSYS on EVM is 4.19V.

        Question2 : What is the difference between my board and EVM?

        Question3 : The STATE LED on my board blink once at a time, while the STATE LED on the EVM blinks much quickly. Why?

     

         B.R.

    Frank

  • Guru 60655 points

    In reply to FrankXu:

    Frank,

    I have used the bq25890 to provide well above 2A from 5V/3A input so I know it is capable. To summarize:
    1. At startup with SYS < ~2.0V, input current is limited to 200mA so the charger will not startup into 4.7ohm.
    2. After startup, the input current limit is clamped to the lower of the setting per the ILIM resistor or setting in the IINDPM register. With the 890, the IINDPM register is configured by D+/D- pins. With D+/D- shorted, IINDPM is 3A but, unless you disable ILIM resistor via I2C or also change resistor to set it to 3A, the ILIM resistor clamps the input current below 3A. If I recall correctly, the EVM ILIM resistor has a potentiometer in parallel with fixed resistor. If you increase the potentiometer to the maximum, so that help?
    3. If the load transient caused by quickly added such a large load causes the SYS voltage to drop <~ 2.0V, the charger could get confused and think it is in startup, which returns you to condition one above. In that case, you will need to add additional capacitor from SYS to GND that can absorb more of the transient and prevent such a droop.

    With the EVM, you can use the bqStudio I2C software to read the charger's status register, which might tell you the problem. Do you have an EV2300 or EV2400 I2C box to use with the software?

     

  • In reply to Jeff F:

    Hi Jeff,

        Thank you very much.

        I find that the JP10 is shorted with a jumper on the EVM, and the VSYS is 4.2V. Remove the jumper in JP10, the VSYS become 3.65V.

        In my test, the RLIM is 130Om on the EVM (the R19 potentiometer is set to 0Om). So the ILIM is 355/130=2.73A.

        In my test, the 4.7Om load is in parrel with a 1uF cap. With the 4.7Om load, the VSYS is 3.13V.

        And with the 10Om load, the VSYS is 3.65V, when I open the power first, and then connect the load to VSYS manually.

        

    With the 4.7 Om load:

    The adapter is recognized as a SDP, and the current limit is 500mA. 3.65/4.7*3.65/0.9/5=0.63A.0.5A. 

    That's why the VSYS drops. And if I set the current limit to 3.25A, the VSYS can be 3.65V even with the 4.7Om load.

    Thank you for your advice.

    But I still have some problem with the EVM:

    Problem1: With a 2A adapter and D+/D- shorted on the EVM board, the adapter is still recognized as a SDP. (current meter is not used). But on my board, I did the experiment many days ago and it's recognized as DCP.

    Problem2: With the battery on, the adapter never charges the battery. But with my board, the adapter charges the battery with 1.08A.

    For the above 2 problems, I've also tried other adapters, but always SDP and never charge.

    I have not found the reason from the registers, could you help. Thank you so much.



    B.R.


    Frank

  • Guru 60655 points

    In reply to FrankXu:

    Frank,

    First, please check the EVM to ensure that it has a bq25890 installed and not a bq25892.  Then, confirm that JP1 shunt is not installed and JP2 shunt is connected between pins 1 and 2.  If you are using a power supply, you can install the shunt on JP5 to short D+=D- and simulate a DCP.  If you are plugging in a DCP adapter, use J5.  If that doesn't work, try flipping the S2 switch.