I have a board that I am working on that works like this. I have a 48v battery that is going though a buck converter to drop the voltage to 5v. This 5v (when active) is then going to power the V_MODEM pins and the LDO that outputs 3v3 for the VIO power.
When the 5v is not present a 3.7v lipo will be enabled and now providing direct battery to V_MODEM pins as well as powering the 3v3 vregulator ldo for the VIO power. The schematic looks like this.
My question is is this ok? The MOSFET’s are going to switch power rails based on whether the 5v input is present or not. However, as you can see the 5V_VDD and the VBAT_OUT share the same pin. But only one is active at a time. Wanted to know if you thought this was ok or would go a different route to achieve this dual power switching mode type of circuit.
Why not always have the 5V exclusively running the Charger, and always have the LiPo powering VMODEM and the VIO?
Why not use an efficient 3v3 buck-boost regulator rather than an LDO, such as ISL9120 or TPS63001? That would allow the Notecard to work below 3.3v, which is important IF you want to get the most out of a lipo.