We are looking for a power switching device.
Power Train of module is powered through USB and coin battery.
1. Power Train
A. Battery - Use 3.0V coin battery
B. USB power - After the coin battery is consumed, supplying the coin battery, the main power is supplied via USB. (At this time, the coin battery is connected - there is no battery current)
==>We are looking for the parts that switch the above two power source automatically.
Please check LTC4419 datasheet. I think this is the part that you are looking for. Following is the datasheet link.