I'm working on a circuit design with a microcontroller that runs on 3.3V. I'd like to be able to power it either through a 3.3v supply or a USB port. I'm using an spx3819 regulator.
What would be the best way to achieve this? What I have in mind so far is something like this:
Basically, the 3.3V signal is fed into an inverter, which disables the regulator. This 3.3v signal is also directly fed into the microcontroller (not shown in the schematic).
Is there any better way?
EDIT: The Vout is directly fed into the microcontroller. Vout is 3.3v.
When high, the 3.3v source will disable the chip, regardless of the voltage at Vin (the enable pin disables the chip at < 0.4v).
Best Answer
I know the answer is subjective.
I use this whenever there is a need to switch between two supplies with at least 1 V difference automatically. Here, assuming VIN_EXT is less than 5 V, the circuit will draw power from PC whenever USB power is available, else from VIN_EXT automatically.
simulate this circuit – Schematic created using CircuitLab
Major advantage here is that, drop across the FET will be just a few 10s of mV at max compared to diode drop of about 300 or 400 mV