To your question:
What I need to do to allow the 5V from usb to charge my coin cell batteries and power the rest of the circuitry at the same time?
The example shown with LTC4054-4.2 uses a Lithium rechargeable battery and your mention to “coin cell” would mean a small Li-ion battery, then:
It is possible to specify proper parameters for charging the battery and monitoring its state of charge, while using it to power your device circuit at the same time, having some points of attention for proper sizing:
- Li-Ion battery will probably be 100% of the (charger-connected) time maintained at 4.2V as this is the “fully-charged” or termination voltage.
Some can argue about Li-Ion being maintained at 4.2V could shorten the long-term life, but cycling number, readiness and convenience of use may be the most important features, so for me 100% always charged = “ok”.
- Charging current should be higher than average consumption of device, but could be lower than maximum consumption; in this later case, battery would just supplement the worst case drain. As your are considering “coin” sized batteries, the charging current will ok for both USB and LTC sides.
- There might be cases where LTC4054 stops charging the Li-ion cell if the consumption of device happens to be less than 10% of the programmed current - PROG’s resistor sizing in the datasheet.
These features could be designed using the LTC4054 controller, but other ones may not have all these features. How important each will be in your device, would depend on the expected way of using the device, its expected lifetime, how much effort you expect to put to optimize the project and the parts to find, etc.