buck - ORing 2 power supplies for 3.3V output

Is it possible to add two independent power supplies together to achieve a steady 3.3V powers supply? One power supply will be running from a 9V battery and the other is a 12V source. They both don't have to necessarily run together i.e. the if the 9V battery is just connected, it should still supply 3.3V. I have a buck converter that can output a steady source of 3.3V (LM2576-3.3V) and can handle a wide range of voltages (up to 40V). Is there some kind of passive voltage adder circuit that I can use that has some power protection diodes?

Answer

If you don't care about efficiency, use two diodes, like below:

^{simulate this circuit – Schematic created using CircuitLab}

Otherwise use:

• Ideal/smart bypass diodes (such as SM74611) instead of D1 and D2.


• Some ORing FET controller such as LM5050 + nMOSFETs.



• Some some autoswiching power mux.

If, instead, you must have 2 separate 3.3V regulators, then use something like a TPS2113 (2.8 to 5V autoswitching power mux).