Schematic was inspired by various schemes from internet forums (TODO links).
load sharing - when the Vcc is present the battery is charged and it is cut off from the load (by the Q1) while the load is powered from Vcc (via D1). When there is no Vcc the load is connected through the Q1 with the battery. Contacts LOAD are for "raw output" without the voltage regulator.
The advantage of MCP1252 is automatic buck/boost feature, it will maintain the regulated output voltage whether the input voltage is above or below the output voltage (2.1 to 5.0V input range) so it is ideal for the lithium battery voltage.
If you read the datasheet for the MCP1252-33X50I/MS there is clearly specified what type of MLCC capacitor should be used.
The PCB layout was heavily inspired by MCP1252 datasheet.
Output parameters are:
- max. output current 120 mA
- output voltage 3.3 V
There is also a MCP1253 which is identical to MCP1252 but has higher switching frequency (1MHz).