In this battery and USB load sharing circuit, the diode to prevent backflow is replaced with another PMOS. When turned on fully, the PMOS will will less loss than a schottky diode.
Care must be taken to ensure the PMOS will turn on fully when the battery is full at 4.2V, where there is only a 0.8V difference in voltage, if not the power loss from RDSon might be more than a conventional schottky diode.
The top circuit is the original one using a diode and the bottom one uses a PMOS. You can flip to switch to toggle between 3.6V Li-Ion battery or 5V USB.
