Hello,
I'm developing an application where I intend to use a BQ25895 with a Li-Ion battery as a power bank: I need to be able to charge the battery from VBUS, and output 5V on PMID to charge external devices. I'm using the BQ25895 in host mode with a microcontroller that talks to it over I2C. I've read the datasheet a couple of times now, but I still have some questions:
a) From what I understand, if only VBUS is present, the internal converter will be in buck mode charging the battery, and if VBUS is not present, the converter will be in boost mode charging an external device through PMID. This behavior is fine. However, I'm not sure what happens on PMID when VBUS is present. According to the functional diagram, it will have the same voltage as VBUS, since they are connected through RBFET (Q1). But what if I have a charger with MaxCharge, and the VBUS voltage is raised to, say, 12V? I would also have 12V on PMID, which is supposed to be a 5V output.
b) Section 8.2.6.1 (Narrow VDC Architecture) of the datasheet claims that VSYS is always kept over the minimum system voltage, "even with a fully depleted battery". From my understading of the functional diagram, this is only possible if an external charger is connected to VBUS, since VBAT and VSYS are connected via the BATFET. What happens on VSYS once the battery voltage becomes lower than the minimum system voltage? Is the BATFET simply turned off, shutting down VSYS?
Thanks!