• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 3 answers
  • Subscribers 63 subscribers
  • Views 181 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25798: VAC MosFets and Ship Mosfet question.

Dicoel DICOEL
Prodigy 20 points
Part Number: BQ25798

Tool/software:

Hi,
I intend to use the BQ25798 in a 12VDC backup system.
I need to use the backup capabilities of this IC.
I will enter the power on VAC1 and I will supply the system on the PMID.

So, on that scenario, two questions:


(1) Is it possible to avoid using the back-to-back input mosfets? My power supply support applied voltage when off. For example if I leave de ACDRV1 floating... or connected... but not shorted. The IC will enter on backup mode?

(2)On the other hand, Without using the ship mosfet... It is possible to power the system from battery... then shutdown the system... and later re-start when power is applied again? If yes, what will be the total Iq current drained from battery during off without ship mosfet? (it is for storage purposes, system can be absolutely off until power will be connected again).

Thanks in advance for your answers...
REGARDS.

  • 0
    TI__Guru**** 166916 points

    Hi Dicoel,

    Regarding 1, backup mode requires at least the reverse blocking FET (diode pointing to VBUS pin). Otherwise the backup voltage on PMID (and VBUS), presumably 12V, appears at VAC1 and host cannot read VAC1 ADC to know when the 12V power source has been reattached.

    Regarding 2, in battery only (HiZ) mode, the battery is connected SYS. If the load is on SYS, the battery supplies the load.  But is the load is PMID, the charger must enter back up mode to supply the load on PMID.  In battery only (HiZ) mode, the Iq is shown below:

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points

    Hi Jeff,

    First of all, Thanks for your fast reply!

    About 1... Ok I understand.

    About 2, Could you explain a little bit more? I have loaded only on PMID. I need to supply PMID always!  (from VAC1 or from Battery).

    Except when the battery falls UVLO or when the system decides to power off. (In that case, "power off" we can only get up powering VAC1).

    So, I understand that it is possible to enter in backup mode without the ship FET, and assume around 24uA at power off?  In this scenario, the board is off, and only the battery is connected.  And additionally, if I need to reduce the 24uA to a lower value, then I need the ship FET...

    Are both ideas correct? 

    Thanks again to clarify.

    REGARDS.

  • 0 in reply to Dicoel DICOEL
    TI__Guru**** 166916 points

    Hi Dicoel,

    Regarding 2, in battery only mode (i.e. backup mode not enabled), the battery voltage less 2 diode drops is at PMID.  

    Other ideas are correct.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points
    Hi Jeff,
    A little bit more concrete....
    Without ship FET, Can I enable backup mode, and it will work correctly? And PMID will function non-interrupted?
    Regards.
  • 0 in reply to Dicoel DICOEL
    TI__Guru**** 166916 points

    Hi Dicoel,

    ShipFET is not required for backup mode.  PMID sits at V(BAT)-2*Vdiod-IPMID*(Rdson-batFET+DCR) or VBUS - IPMID*Rdson-inputFET.

    Regards,
    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points

    OK Jeff,

    The second   PMID=VBUS-IPMID*Rdson-inputFET is my intention.

    Thanks a lot!  It was a pleasure!

    REGARDS.