Linear Technology and Maxim make a number of ICs that will work in your
application. Search for "power path controller" and "battery charger".
Some of the newer ICs will integrate both functions. Some ICs also integrate
switching regulator and LDO functions too.
For a very simple switchover two diodes will work. The issue is the forward
voltage drop. The powerpath controller uses a FET which has a much
lower drop. TI makes an "Ideal Diode" IC. The higher the voltage drop
the less margin you have for the linear regulator.
I have used the LT4412 for powerpath control and the MAX1555 for Li-ion
charging.
(* jcl *)
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