Hi,
I'm used to connecting the serial USB cable to my UNO even if it is being powered by an external power source, because of its built-in supply swtich.
Now I need to do the same with the Pro Mini, but it seems like there is not such supply switch in this board, according to the Datasheet (or maybe I missed it). What would happen if I connected my FTDI serial-to-usb board while the Pro Mini is powered through the RAW pin? I don't want to burn my chip 
Thanks in advance
It should be fine, I've done it plenty without burning anything up yet. I suppose there is a small chance of one of the voltage regulators being damaged, but I haven't had it happen yet (knocking on wood). Voltage into the RAW pin runs through a regulator before it gets to the chip. The FTDI adapter should be putting out "clean" voltage too. Just make sure the FTDI Vcc voltage is the same voltage as your Pro Mini (ie. don't mix 3V3 and 5V).
Ok, thanks again!
This is a matter of some considerable contention!
There are (not surprisingly) two considerations here.
- Does it matter if the USB feeds power back to the regulator? No, the regulator simply shuts down if its output voltage is greater than its set point. It does pass some current through the voltage reference, but that is not greatly dependent on the voltage (i.e., there is not an actual Zener equivalent across the output).
There is a consideration that the regulator may pass a heavy current from the output to the input in order to charge the input reservoir capacitor when that is not being powered; the on-board capacitor value is not a significant problem but another power supply including a capacitor connected to the input may be however this is only a particular concern when the power to the regulator output is applied suddenly and the USB source is probably unlikely to be applied that suddenly.
You probably should not "hot-plug" the USB with a un-powered power supply connected to VIN; if you intend to, you should put a diode feeding from +5V (anode) to VIN (cathode).
- Does it matter if the regulator feeds power back to the USB? Theoretically it might, but given that both are close to 5V, it is unlikely to cause a problem; in fact resistance in the USB cable will likely minimise and back-current.
Hm, I've emailed Sparkfun's tech support, here's what I've been answered:
There might be some sort of conflict with the power from the FTDI and your other power supply from the RAW pin. Usually, there is some sort of diode so that there is not any power conflicts to the board. I recommend removing the second power supply from the Pro Mini as you are using the FTDI, or not connecting the Vcc pin from the FTDI.
I'm going for a diode, just to be sure.
Thank you for your precise and clear answer!