Backup Battery circuit with charging
I'm not very familiar with LiFe technology, but a simple resistor in series with the LiFe battery might not be ideal. While charging, depending on the current drawn by the LiFe battery, the series resistor will generate a voltage drop that may fall below the nominal voltage of your battery thus reducing its usable capacity and life span. Same matter when the main battery is removed, you should double check the low voltage supply limit of your load, for the voltage drop across the series resistance may be too high.

Perhaps you could use an integrated LiFe IC charger which will go through the constant current / constant voltage cycle. If you reckon the circuitry simple enough it will ensure proper charging of your battery.

As for the LDO, I am not sure what will be its behavior regarding the LiFe powering up the output pin. But I guess a diode wouldn't do the trick. You could put a N type MOSFET whose gate would be driven by the LDO. With a pull-down resistor to ground, when the LDO is not powered by the main battery, the MOSFET will turn OFF and prevent the LiFe from damaging the LDO.







simulate this circuit &ndash; Schematic created using CircuitLab

Side note about the buck converter : why choose a 7V output converter instead of a 5V one? Does your LDO need a voltage drop larger than 1.7 V? Again, just a minor question that does not help with your question.