I'm trying to design a simple circuit that has the ability to charge a single cell Li-Po battery and can be used as +5v power source. I chose the circuit from here.
It seems that when
VUSB is active, the
+5V pin would select the
VUSB instead of the step-up regulator because of two Schottky diodes, and the charger chip would charge the battery at the same time. When the
VUSB pin disconnected, the charger would go off obviously and it seems that the
EN pin of step-up would go high (Why?) and enables the regulator, then the
+5v pin selects the output of the step-up regulator. My first question is what is happening to FET switch?
My second question is, does this circuit protect the LiPo battery from under-voltage discharge? For example, when
VBAT goes below 3.2V. I'm using a different step-up regulator (AP3031) and I think I should calculate a different value for