Hello,
I have a question regarding the use of MOSFETs.
I have a circuit with two 3V3 power supplies : 3V3AUX which is always ON, and 3V3 which is not always ON.
I want the output of my circuit to be connected to the not-always-ON 3V3 power supply when it is available, if not, I want it to be connected to 3V3AUX. I hope that's make sense, English is not my primary language...
So I came up with the circuit attached. Its an Operational Amplifier configured as a voltage comparator (I'm using the MCP6001 rail-to-rail OpAmp). Its output then drives two MOSFETs, one is N channel and the other one is P channel.
The idea is that when the not-always-ON 3V3 power supply is available the comparator outputs a '1' which turn off the PMOS and activate the NMOS. And when it's not available then the comparator outputs a '0' which turn off the NMOS and activate the PMOS.
The two MOSFETs that I'm using are assembled in a single package : the AO4604, I had it on hands.
As you can see in the attached shematic, I have connected the two Drains together, same for the two Gates. And each Sources is connected to one of the two power supplies.
The problem that I have is that when the not-always-ON 3V3 power supply is OFF (so NMOS OFF and PMOS ON) I have 3.21V at the ouput (which is fine I guess for a 3.3V power supply minus the Rdson drop), but when it is ON (so NMOS ON and PMOS OFF) I only have 2.7V. Why this difference ?
Also, do you think that my schematic is the best solution to the problem ?
Thank you