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BQ25700A: VBUS_UVLO

Jelly li
Intellectual 290 points
Part Number: BQ25700A
Other Parts Discussed in Thread: BQ25700

The OTG operation can be enabled, the conditions include:

 VBUS is below VVBS_UVLO;

The questions are: 

1. VVBS_UVLO can be set by register or not? if can, which register?

2. Normal input voltage is 12V, if input voltage drops to 9V, higher than VVBS_UVLO falling threshold 2.4V, EN_OTG is invalid？？？？？  How can I enable OTG when VBUS drops to 9V?

wish your reply ASAP, the project is urgent, thanks

BR

Jelly

  • 0
    TI__Guru**** 166626 points
    Jelly,

    The BQ25700A support team is out until Monday for the US Holiday. I apologize for the inconvenience.
  • 0 in reply to Jeff F
    TI__Expert 5330 points
    Hi Jelly,

    Qn 1:
    VBUS_UVLO cannot be set by register and is hard-set to 2.4V, which is the lowest voltage that BQ25700A can reliably operate at.

    Qn 2:
    OTG mode should only be enabled when no adapter is present on VBUS. If you have a 9V adapter present on VBUS and were to enter OTG with VBUS voltage of 12V, you would get excessive current into the adapter as the 9V adapter and 12V source are shorted together. The VBUS_UVLO voltage is basically used as a condition to determine when the adapter has been removed from the system.

    Is your intention to enter OTG while a 9V source is still attached on VBUS, or are you trying to sense the loss of the adapter more quickly (i.e. at 9V instead of 2.4V) so that you can switch into OTG mode more quickly? If the latter is your intention, please provide more details on the timings you are attempting to meet, although I am afraid that you will find that, in general, the time required to switch from charging to OTG modes is large compared to the time difference between VBUS decaying to 2.4V versus 9V when adapter is removed, and that, even if this change were possible (which it is not), it would have little effect on the overall time required for the transition.

    Regards,
    Steve
  • 0 in reply to Steve Preissig
    Intellectual 290 points
    Hi Steve,
    The typical application is: the device A has 12V output support the BQ25700 as charger. When the device power-fail, BQ25700 enter OTG mode, output need >4.5V to support the device. Charger switch the OTG mode has 10ms gap, we add a LDO to
    support the 10ms and enable the OTG at the same time, then diable the LDO after 10ms, expect the OTG start already. the questions is: once the LDO enable, the voltage on the VBUS still >2.4V, EN-OTG is invalid！！！
    how to avoid the 10ms OTG converter time?
    thanks
    Jelly
  • 0 in reply to Jelly li
    TI__Expert 5330 points
    Hi Jelly,

    Unfortunately, the BQ25700 was not designed to hold up the VBUS rail in the manner you are describing. I understand the issue you are describing and why you want to change the VBUS_UVLO threshold, but I am afraid it is hard set.

    It might be possible to "fool" BQ into thinking VBUS dropped below 2.4V by adding a MOSFET switch at BQ VBUS pin, but nothing like this has ever been validated, and we can't support using the part in this way.

    Steve