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Voltage dip on VSYS when connecting VBUS

Rashid Talib
Prodigy 20 points
Part Number: BQ25629
Other Parts Discussed in Thread: BQ25628, BQ25619, BQ25895M

Tool/software:

Hi

We use VSYS to power the main processor (using a 3.3V LDO). VSYS is stable as long as only the battery is connected, but as soon as power is supplied to VBUS I measure a voltage dip on VSYS which resets the main processor (see image below). No voltage dip can be observed when power is removed from VBUS.

Steps to reproduce:

  1. Remove VBUS voltage, attach battery, VSYS is stable at 4V
  2. Main processor boots up and runs stable
  3. Supply power to VBUS
  4. VSYS drops to 2V for 12ms then returns to 4V
  5. Main processor reboots
  6. Remove VBUS voltage
  7. VSYS stays stable

It seems like the BQ25629 switches off Q4 after VBUS is supplied. I suspect that the IC needs some time to discover the VBUS type so I tried to set EN_AUTO_INDET to 0 to disable automatic D+/D- detection when VBUS is plugged-in but it does not solve the problem.

Are there any other settings I need to change?

Thanks for your support & best regards

Rashid

  • 0
    Prodigy 20 points

    I need to mention, that this problem only occurs, when boost operation is active. When I disable the booster, no VSYS voltage dip can be measured. Is this problem related with the fact, that the boost operation is disabled as long as VBUS is supplied? I also see a long voltage dip on VMID whenever power to VBUS is supplied and a short voltage dip on VMID when power on VBUS is removed:

    VMID with booster enabled and applying power to VBUS:

    VMID with booster enabled and removing power from VBUS:

  • 0 in reply to Rashid Talib
    TI__Mastermind 31910 points

    Hi Rashid, 

    Thank you for reaching out via E2E.

    I have confirmed the BQ25629 is at risk of brief (~10msec) BATFET turn off resulting in loss of SYS power when transitioning directly from boost mode operation to charge mode due to adapter plug in. 

    I can also confirm the behavior will not occur when the device is initially in battery only mode rather than boost mode then adapter is plugged in at VBUS. 

    To ensure SYS is not at risk of brief loss of power you will need to disable OTG boost mode via I2C write (EN_OTG=0, REG0x18[6]). For your reference another customer was able to realize this fix by using a power mux between input port and VBUS pin. 

    Please let me know if you have any follow-up questions.

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 20 points

    Hi Garrett

    Thanks for your reply. Unfortunately disabling OTG boost mode is not possible because the system needs to provide 5V to an accessory at any time and it does not know in advance when USB power is applied. Adding a mux as you described is not really an option because that's one of the features why we choose the BQ26529.

    Do you know if the BQ25628 has the same problem? We don't actually need the USB adapter detection algorithm.

    Thanks for your reply & best regards

    Rashid

  • 0 in reply to Rashid Talib
    TI__Mastermind 31910 points

    Hi Rashid, 

    Unfortunately, both the BQ25628 and BQ25629 are susceptible to this behavior. 

    We do have a couple alternate IC options with OTG boost mode to PMID output which do not have the same risk of brief BATFET turn off due to adapter plug in when in boost mode. These options are BQ25895M and BQ25619.

    Additionally, please be aware that a short PMID voltage drop when transitioning from charge mode to boost mode (due to VBUS removal) is expected. All of our buck charger ICs with OTG mode to PMID require a short delay to transition from charge to boost operation and are not able to support an always on 5V at PMID. 

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 20 points

    Hi Garrett

    Thanks for your reply. I managed to buffer the voltage dip with a 220uF capacitor, enough to keep the MCU running and not enter brown out reset. This is not a nice solution but for now it seems reasonable.

    The voltage dip on PMID isn't nice either, but I think we can live with it.

    If we do a redesign, I might consider looking at the suggested alternate parts.

    Best regards
    Rashid

  • 0 in reply to Rashid Talib
    TI__Mastermind 31910 points

    Hi Rashid, 

    Thank you for your reply. 

    Please let us know if you have any further questions. 

    Best Regards,

    Garrett