As you can see if correctly chosen the P-MOS switches between USB and Battery, indeed I actually have a problem choosing the right MOS. Let me explain: Suppose Usb is not attached then we only have the the Battery. Then at the DCDC input I must have at least 3.5V (200mV dropout). I've 2 question:
Since I cannot afford any voltage drop can I eliminate the Schottky diode? (or it's extremely needed for inverse current...).
I initially select the PMV27UPE since from the datasheet it has a ultra low active resistance but emulating its behaviour with spice it has a voltage drop of .7V... Where did I make a mistake? (is it a problem it's working in linear region even if for switching purposes?)
Additional info: I need a maximum of 500mA delivered to the Load.