I'm designing a circuit that has a built-in single-cell LiPo battery with its charger for a GSM circuit. The board feeds from a 12V power supply and uses the battery as a backup, and when the power supply goes out, the battery must power the load, and when the supply goes back battery should begin charging, and the load should be fed from the power supply. I used two Schottky diodes to select between the two. But I'm not sure what happens when the battery is being charged and powering the board at the same time. Another problem is that both regulator circuits supply 5 volts, but how can I make it use the step-down regulator (power supply) when both regulators are working? My load uses 5V and 1~2A, and the maximum dropout should not get over 600 millivolts.