# 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?

• If both power supplies are connected, do you care which one supplies the current? – Dan Laks Sep 24 '17 at 6:43
• I would prefer if the 12V power supply was the one supplying the current. – Guy Lee Sep 24 '17 at 7:35

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).

• if I'm considering using a FET controller or more specifically the LM5050, what are some nMOSFETs I can consider using? – Guy Lee Oct 24 '17 at 5:27
• What is your maximum load current? In section 8.1 of the datasheet they suggest how to choose the MOSFET. In any case, be sure to use logic level MOSFETs! – next-hack Oct 24 '17 at 6:56
• My maximum load current will reach up to 1A. It's for a radio application whereby if the battery for a 9V battery runs out, they can supply power to it via a 12V power supply. – Guy Lee Oct 29 '17 at 1:18
• Well, the constraints are set. Absolute minimum 1A, minimum 12V Vds and logic level MOSFET. I would then choose any MOSFET with a $V_{DS,max}$ larger than 20V, logic level, and with $R_{DS,on} < 0.2\ \Omega$, and a drain current of at least 2A. Also you have to make sure that the maximum $V_{GS}$ is within the IC specifications. The cheapest I can find on digikey with such requirements is PMV130ENEAR. – next-hack Oct 29 '17 at 11:30

Use an electromechanical relay whose coil is powered from the 12 volt supply.

When the 12 volts is not present, the relay isn't powered and the 9 volts routes through the normally closed contact to the relay's contact switch pole. When 12 volts is there, the relay switches and disconnects the NC contact from the pole and connects the normally open (NO) contact to the pole. You have 12 volts also connected to the NO contact.

Or you use diodes as mentioned in another answer but diodes drop a small voltage that you may (or may not) be able to live with.