When I searched for "power path and battery charger chip" a few options come up. At the following website I selected LiFePO4 and selected power path and battery charger. Boy did I open a can of worms!
Mixed-signal and digital signal processing ICs | Analog Devices(PowerPath,Battery_Charger)#!1806_LiFePO4!1069_Linear|Switching|Shunt!2170_Linear%20PowerPath|Switching%20PowerPath!chem_LiFePO4!vmax!icharge_!bat_bat!path_path
The first of the two options was the LTC4098-3.6 chip (link below). It says "700mA Maximum Load Current from USB Port, 2A Maximum Input Current from Internal Switching Regulator and 1.5A Maximum Charge Current with Thermal Limiting". What would be the maximum output current to the load? When charging, would it be the same as the 700mA input current via the USB. My load needs to get just under 1A. I'm not sure if the "easy" option in the diagram on page 1 is sufficient. Maybe I have to go with one the two diagrams on page 15. Page 22 has additional circuitry for Dual Input Overvoltage Protection. Page 23 has additional circuitry for Dual Polarity Voltage Protection. Not sure if I need these extra components or not.
The second option was the LTC4156 (link below). It says it may draw up to 3A from a high power wall adapter. It seems current-wise this one is good enough as is. My head is starting to hurt looking at all the numbers.
Any thoughts on the use of one of these chips? Remembering that I will draw just under 1A at peak load. It is like that for a second or so when the servo starts moving. The diagrams for these chips have 3.3V battery supply labeled. Am I limited to 1 lonesome LiFePO4 battery or can I put a bunch in parallel?
How about using a LiPo? I saw a single cell 5000 mAh for $10. I read that they can be safely trickled charged but I think they would require regular discharge which would no doubt complicate things if this was to be automatic.
@Robin. As I replied to Chagrin I don't know of a way to do the load sharing with the LM317. As for the LiPo, if I was to use one then I would be happy to put it in one of those flame proof bags from hobbyking or something similar. But alas I think they are only fire resistant so I would need to check that out.