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BQ25798: ACFET1 / RBFET1 - Detection - Dispense with RBFET1 with Backup Mode still available

Miguel Aguirre
Prodigy 70 points
Part Number: BQ25798

Tool/software:

Hello!

We are working on a device that requires very high reliability. It will source two loads, one from VSYS and the other from PMID, both with UPS requirements. The battery will be 1S Li-Ion.

We are planning to use Backup mode. There will be no OTG operation.

Our design will be powered through a 4 Schottky diodes rectifier bridge, so the customer wants to dispense with the RBFET1 to reduce components count, and because the circuit has inherent capability of blocking reverse power out to the VIN1 input.

1- How does the BQ25798 detects the presence of ACFET1 and RBFET1 at POR?

2- Is there a simple and reliable way to avoid using RBFET1 and still be able to set EN_BACKUP = 1 to enable Backup mode?

3- What happens if only ACFET1 is present at POR**? Will the BQ25798 still enable EN_BACKUP=1 (Backup Mode)?

** RBFET1 would be replaced with a short circuit between source and drain, leaving the gate connection open.

Thanks in advance!

Miguel

  • 0
    TI__Guru**** 166996 points

    Hi Miguel,

    First, backup mode is fast turn on OTG if VAC1 falls.  Also, while in backup mode, the load at SYS will tied directly to the battery.

    Regarding 1, impedance of ACDRVx to GND is tested at power up.  If ACDRVx is tied to ground, the IC thinks there are no FETs.

    Regarding 2, I assume that the source on the input side of the rectifier is AC so VAC1 cannot be tied there?  If so, curious how the charger know when to turn on backup mode. Regardless, the rectifier should provide blocking so no RBFET needed. 

    Regarding 3, backup mode does not activate unless VAC1 drops below a % of VINDPM.  If ACFET1 is present with the body diode connecting VBUS to VAC1, backup mode does not turn on.  So, not sure how this will work. Maybe connect to the middle point of the rectifier?

    Regards,

    Jeff