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I'm designing a switching circuit between USB (5V) and Battery (li-ion 3.7V) for a portable device, below is shown the schematic: Schematic

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.

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You connected Q1 drain to the wrong place.

With no USB power, R2 pulls the gate low, turning Q1 on. Its drain then pulls the gate high, turning it (partially) off again; it settles at 0.7V drop.

I think you want the drain connected to TEST-POINT1, then D1 prevents the gate voltage rising. (That way the Schottky diode drop only happens under USB power, where it doesn't matter).

Note also. Q1 body diode will charge the battery from USB power, which may be very bad. You may need the usual back-to-back pair of MOSFETs to provide isolation in both directions..

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