I have an arduino and a telit GM862 module which requires 3.4V-4.2V power supply. So far I have constructed a 12V limiter to plug it into a car power supply and a step-down converter which converts 12V to 3.8V so the custom board which consists of arduino and telit is plugged into a car power supply and arduino is powered with 12V and telit with 3.8V.
Now I would like to modify the project such that the board could be powered by 3.7V Li-ion or Li-Po battery or 12V. To do that I would still need a step down converter which converts 12V to 3.7 (or little higher) when plugged into 12V charger and 3.7V to the voltage required to power arduino as the battery so the battery would power both, telit module and arduino.
Has anyone constructed a similar project that could serve as a reference for the project I would like to build? I would be very thankful if anyone could suggest any helpful resource or name any ICs which are useful to construct such step-up and/or step-down voltage converters.