The PCB circuit I'm using expects 5V input voltage which it then converts to 3.3V using an AMS1117-3.3 voltage regulator. The power supply I'm using is 3.3V. What happens if I feed the 3.3V to the input of the 5 to 3.3 voltage regulator?
The input of the voltage regulator has an easily accessible jumper header. Whereas bypassing the voltage regulator would require the use of bodge wires. Can I get by with just feeding the 3.3V into the regulator? What are possible drawbacks if it even works? What would happen when the PSU voltage drops below 3.3V (as I doubt it will provide a precise and constant 3.3V)?
Best Answer
The critical parameter is drop-out voltage, which I'll call \$V_{do}\$, listed in your datasheet on page 3.
The output voltage will generally not be more than \$V_{in}-V_{do}\$. (Obviously this breaks down when \$V_{in}\$ is very low. If \$V_{in}<V_{do}\$, for example, all bets are off)
For the AMS1117-3.3, the drop-out voltage is listed as 1.1 (typ) to 1.3 (max) volts. That means to be sure of getting 3.3 V out, you need to provide at least 4.6 V in.
And if you provide only 3.3 V in, you shouldn't expect more than 2 V out. In fact, that far below the nominal input voltage you shouldn't count on the listed drop-out voltage applying, so you can't even be sure of getting 2 V out.