I am trying to implement a secondary battery in one of my projects. When the main battery (24V) dies, the 3.3V battery takes over and the system operates in limp mode (some emergency functions). I am planning to connect the batteries like this, as my system runs on 3.3V and the secondary battery I’m going to use is a LiFePO4 cell (3.2V nominal). This way, my secondary also gets charged (to a safe 3.3V) when main battery is available. My load is hardly 500 mA with full functioning, and 200mA in limp mode. Is there anything I’m missing here? Will the LDO be fine when mains battery is not present? I’m attaching LDO’s functional diagram as well.

I checked the oring diode methods and LT’s powerpath ICs, but I want the circuit to be as simple as possible. LT has supply issues as well in my country, so they have to be imported.


My Circuit Diagram

My LDO Block Diagram