I'm trying to design a circuit that provides backup LiPo battery power to a +5v rail. The rail is sometimes powered by an external USB supply and sometimes not - I want my circuit to act as a power sink when the USB supply is present and act as a source when it is not. The main difficulty is that the USB supply and the load are joined together in an external system and cannot be separated - the only interface I have to this system is the +5v rail at point A in the schematic below:
This is the best solution, I can think of, but it is incomplete. My main questions are,
- Will the small resistance in series with the boost converter effectively prevent the battery discharging when Vusb is present?
- How can I obtain the active-low enable input (EN) for the battery charger, that is high when the boost converter is providing current and low otherwise?
- Can anyone suggest a better solution that charges/discharges a LiPo according to whether another power source is present or not, where the USB source and load are hardwired together and cannot be separated?