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BQ25601: How to power VSYS with VBUS alone without battery?

Sungwon Moh
Prodigy 70 points
Part Number: BQ25601
Other Parts Discussed in Thread: BQ24266,

We want the device (VSYS) to be powered-up whenever VBUS is available, regardless of the battery condition. With the battery physically disconnected, we notice that VSYS oscillates between 4.2V and 3.8V, so the device does not power-up. See the attached picture. How can we disable this VSYS oscillation under the SW control, so that the device (VSYS) can be powered on, and remain powered on whenever VBUS is available, even when the battery is completely depleted or removed?

   

  • 0
    TI__Genius 11850 points
    Hi,

    This VSYS oscillation is happening because with no battery connected, the charger enters and exits charging states continuously. One possible solution is to disable charging when there is no battery connected. With charge disabled, VSYS will be regulated above SYS_MIN and will not oscillate.

    Best regards,
    Angelo
  • 0 in reply to Angelo Zhang87
    Prodigy 70 points

    When the charging is disabled, VSYS stops oscillating, but now VBAT is oscillating by the charger. See the attached. It appears that the charger is trying to charge the battery. As a result, the system does not power on unless VBAT reaches certain threshold. With no battery connected, the VBAT cant never reach the threshold. How can I make the charger to bypass the battery during power-on, so that it will power up the system with VSYS alone?    

                 

  • 0 in reply to Sungwon Moh
    Prodigy 70 points
    By the way, I already tried to cut the charge current to 0mA, but the oscillograph still shows 100mA.

    Reg01 to 0x0A Charge Disable,
    Reg02 to 0x80 Charge Current 0 mA,
    Reg03 to 0x04 Precharge Current 0mA,

    Best,

    Sungwon
  • 0 in reply to Sungwon Moh
    TI__Genius 11850 points

    Hi Sungwon,

    The behavior you're describing is not the expected behavior. With charge disabled and no battery connected, VBAT should not oscillate. Please see the waveform below. I set VBUS to 5 V and had no system or battery connected to the EVM. Initially, VSYS (yellow) and VBAT (green) both oscillate during float charging, but after charging is disabled, VSYS and VBAT both stop oscillating. VSYS is regulated above the minimum system voltage, while VBAT decays to zero.

    Can you describe your test setup in more detail so we can determine what could be causing the behavior you're seeing?

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 70 points

    Hi Angelo,

    I agree with you when there is no battery connected - both VSYS  and VBAT does not oscillate with Charging disabled in register 1, although there is a repetitive VBAT pulses (see the attached) unless Termination is also Disabled in register 5.  I have not figure out yet how to make the VBAT stays low at 0V. It currently stays around at 2.8V (w/ no battery) after 'float charge' pattern disappears.     

    What I want is to be able to bypass the battery under the software control, whether the battery is depleted or removed, so the device (VSYS) can power-up from VBUS input, regardless of VBAT condition. I want no precharging and no CC/CV charging activities to occur - which only makes the device to wait indefinitely during power-up if the battery is dead.  

    Regards, 

    Sungwon      

  • 0 in reply to Sungwon Moh
    TI__Mastermind 25800 points

    Hey Sungwon,

    If all you want is to stop the behavior whether the battery is depleted or removed, I would disable charge. Just keep in mind that disabling charging will not charge your depleted battery.

    Also keep in mind that the decision to disable charge would come from your host controller; this charger will not detect the absence of a battery.


    Regards,
    Joel H

  • 0 in reply to Joel Hernandez II
    Prodigy 70 points

    Hi Joel,

    On the previous comment, I said I already attempted to Disable Charging by setting the REG01 to be 0x0a. However, since it requires the host controller to set the register after it is powered up, I was not able to stop the charging behavior (which was trying to revive the dead battery during power-up). There is no NVM for this controller to store the configuration setting for the next power-up? If I pull CE pin 9 high, will it disable Charging during power up? But, this would mean that there is no way to disable charging during power-up under the software control alone. Please advise if you have better idea to disable charging during power-up.  

    Thanks, 

    Sungwon                

  • 0 in reply to Sungwon Moh
    TI__Genius 11850 points

    Hi Sungwon,

    If you want charging to be disabled at POR without having to wait for the host to disable charging, then you can connect a pull-up resistor to CE and use an open-drain output from your MCU to pull CE low and enable charging, as shown below.

    Please take a look at this thread as well. Some of the suggestions may also be helpful to you, such as the TS pin solution and the BQ24266: https://e2e.ti.com/support/power-management/f/196/t/796672

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 70 points

    Hi Angelo, 

    Thank you for your suggestion to disable charging by pulling CE high. However, that would require hardware change. 

    I have been seeking if there is any way to disable charging in the charger chip under the host control, although it will not be possible unless the host does not powered on. 

    The problem is the host does not power up with VSYS alone. I suspect that it is because the charging is enabled in the charger chip with dead battery (VSYS is 3.5V)?  It keeps cycling the power-up routine, but does not complete it successfully. See the attached picture 1. But the host powers up normal if no battery is connected (VBAT is settled to ~2.9V).  See the attached graph 2 with no battery case. 

    What do I need to do in the charger chip to allow the load device to power-up with VSYS alone? Or, is it probably outside of the charger chip's control that prevents the load device to power-up?   

    Picture 1 Unsuccessful power-up w/ dead battery

    Picture 2   Successful Power-up w/ no battery

    Sungwon 

  • 0 in reply to Sungwon Moh
    Prodigy 70 points

    Hi Angelo,

    I already tried to disable charging by writing to Reg 1 bit 4, but the watchdog timer will reset it. I also tried to disable charging by disabling BATFET (Q4) by writing Reg 7 bit 5. Although it survived watchdog timer reset, it will be resetted by the next power-up reset, so the charging the dead battery continues - and the device load will not be powered up successfully.  

    What do I need to do in the charger chip to allow the load device to power-up with VSYS alone, and not affected by the battery condition? Does the charger chip measure VBAT and IBAT, and is there any threshold control that I can affect?    

    Sungwon

  • 0 in reply to Sungwon Moh
    TI__Genius 11850 points

    Hi Sungwon,

    There are a few ways to disable charging when no battery is connected.

    1) Pull the /CE pin high to disable charging, as I described in my last reply. However, this solution requires an additional pull-up resistor, as you pointed out.

    2) Disable charging via the CHG_CONFIG bit. You can also disable the watchdog timer so that the watchdog timer does not expire and reset the CHG_CONFIG bit. However, this solution requires that the host disable charging, so charging will not be disabled immediately at POR.

    3) Use the TS pin. The BQ25601 was designed assuming that, without a battery, the TS pin would fault and disable charge. Pulling the TS pin voltage high/low to force a battery COLD/HOT state disables charging. However, this solution requires that you use a thermistor so that the TS pin voltage is different depending on whether the battery is connected or disconnected.

    4) If the solutions above are unacceptable to you, then please look into a different charger IC such as the BQ24266. Some other customers with similar requirements as you have preferred this IC over the BQ2560x family. For example, see this related thread: https://e2e.ti.com/support/power-management/f/196/t/796672

    Best regards,

    Angelo