I see the beagleboard (C4) is basically powered on 4.2V, which I
assume is the voltage for a fully charged li-ion battery.
I was wondering, if there was a reason NOT to simply use 3.3V, which
would have saved a converter. The TPS65950 specs 3.2V as minimum
voltage to start.
A fully charged battery is 3.6V. The board was designed to run from a 5V supply. A voltage of 4.7V is the max the PMIC can handle so we dropped it down to the nominal voltage of 4.2V. You can try to run the board at 3.3V as long as you do not use the USB Host port at all as it requires 5V and 3.3V will not work. That is the reason we need the 5V supply. The PMIC will work down to 3.2V but the LDO is in the path so you will experience some voltage drop that could cause the PMIC not to get 3.3V and thereby fail.