Tool/software:
Dear folk(s),
I am designing a new device and I want to include the BQ25303J in it.
My device will have a 3.7V 1-cell Li-Po battery, and will be charged from a USB. So, the BQ25303J comes handy for the battery charging purpose.
The thing is, I want the following behaviour :
- If no USB is attached, the system is powered by the battery
- If USB is attached, the USB must charge the battery AND power the circuit. The battery must only be charged in this case.
So, looking at the datasheet for the BQ25303J (https://www.ti.com/lit/ds/symlink/bq25303j.pdf?ts=1727334519693&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ25303J) I found on page 26, section 10.2.2 Typical Application with External Power Path. It looks just like what I want.
The problem is, when looking at this design, I am wondering, how does this work? Is the PFET Q4 properly connected?
I assume the voltage at PMID (VSYS) is 5V when USB is connected and is floating when USB is not connected.
Let's check Vgs for each case and see if the mosfet is ON or not
USB Connected
Vgs = Vg - Vs = (5V - 0.7) - (VPMID) = 4.3V - 5V = -0.7 V MOSFET NOT ON The voltage provided to the circuit is the voltage from the USB.
USB NOT Connected
Vgs = Vg - Vs = 0V - (floating) = ??? Then, here maybe the VSYS sees the voltage on the battery because of the diode on Q4. In that case, assuming the battery has its nominal voltage 3.7V:
Vgs = Vg - Vs = 0V - 3.7 = -3.7V --> THE MOSFET IS ON
But here now, what I don't understand is, the current on a PMOS flows from source to drain, right? In this case, this would mean from VSYS to Battery, but this is exactly the opposite of what we want.
If anyone could clarify this to me, I would be so grateful.
Cheers.