I am building a lipo charging circuit based on Microchip's AN1149: http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdf. It seems to have all the functionality that I need to build a circuit that can charge a lipo while also powering the main circuit.
The part that I am trying to understand is the p-FET that switches between battery source and USB source for the system load. This corresponds to figures 6 and 7:
The following are my questions:
Is this rationale for figure 6 correct: Vd = 4.2V (lipo), Vg = 5V, Vs = 5V -> Vgs = 0V so FET is off, body diode is reverse biased so no current flows through the FET.
Is this rationale for figure 7 correct: Vd = 4.2V (lipo), Vg = 0V, since anode of D1 = 0V, body diode of FET conducts, so Vs = 3.2V (assume 1V forward voltage). Once body diode starts conducting, Vgs = 0-3.2V = -3.2V -> Q1 turns on (logic level FET), and eventually settles at Vgs = -4.2V since the body diode will be bypassed by the conducting FET.
I would like to use this circuit to drive some tiny DC motors, probably pulling a maximum of 2A (only off the lipo, external power unplugged). If my rationale for #2 is correct, will the system load see a voltage of 4.2V (minus losses in the drain-source resistance), or does the forward voltage drop of the body diode come into play? Minus the drain-source resistance, is there any difference between the state in figure 7 (external power unplugged, running off battery) and directly powering the load off the lipo battery (no charging circuit)?
I found part Si4497DY (sorry, not enough reputation to post another datasheet link). It is logic level, has very low Rds, and can easily handle the current I'm using. It seems like it'd be reasonable for Q1, but I don't really have any experience hooking up a p-FET in this manner. Is there anything that I'm overlooking here?