I am working on an arduino-based custom board that has 2 1A LDO regulators. It’s meant to take power from a wall wart and provide 3.3V on one rail and 5V on the other.
Currently, I have my FTDI Vcc input tied to the VIN rails, which are pre-regulator. That should work for the 3.3v regulator, but I don’t think providing 5V to the input of a 5V regulator is going to work. What is the best way to tie the FTDI Vcc to the input of the 3.3v regulator and the output of the 5V regulator? Would a diode be enough? Is voltage drop going to be a problem?
Ok, you want to power the board in your schematic from the USB connection.
First be aware of the current limits of a USB port. Typical USB ports for PCs is 100 or 500mA MAX. If exceeded you may damage the PC motherboard (This happened to me once). A Powered USB Hub is safer and some have up to 1Amp available.
Ok now your circuit.
First run the 3.3V regulator from the output of the 5V regulator. This way the 3.3V regultor only needs to drop 5-3.3 = 1.7V and will then dissipate less power and heat.
To power the circuit from the USB or the external jack a pair of blocking diodes work but does have a Voltage drop.
I have seen and used circuits without the diodes and this also works if the regulators are in series.
You can have a look how it is done in the various Arduino schematics: using comparators and mosfets to cut-off USB power from the board 5 volt if Vin is high enough.
