Can I split my positive supply voltage into a positive and negative voltage for better energy-efficiency?

Question

I have a question about an idea of mine, concerning the split up of a 5 V supply voltage into a 3.3 V and a -1.7 V voltage. My circuit has to be as efficient as possible and therefore I don't want to use a combination of a simple 3.3 V voltage regulator and a voltage conversion IC (like the ICL7660). The negative voltage is only used as negative supply voltage for a precision operational amplifier whose output voltage will never be under -100 mV, but for sure will be negative too.

My idea was to use something like this, to stabilize the important 3.3 V voltage for the microcontroller on my board and leave whatever difference between the supply voltage (normally 5 V, but can vary because of battery usage) and the 3.3 V remains as negative reference voltage:

(The used parts are only symbolic, I didn't take the time to find exact components for my circuit and needs, before I know the concept even works)

I don't need high currents, like I said it is a fairly low power circuit which this supply-circuit should power. Would this be a feasible option or did I miss/overengineer it too much? I tried simulating it and the results looked promising in the sense, that I could vary the supply voltage and load resistance (not in the picture) to a certain degree and everything looked stable.

So my questions now are:

1. Can this concept work or what flaws can you think about?
2. Will this be more efficient as the voltage regulator/voltage inversion-approach? Like I said I don't really care how big my negative voltage reference voltage is, because the op-amp will never go rail-to-rail on the output.
3. Building on the last question, is it even a good idea to supply an op-amp with an unsymmetrical supply voltage, like 3.3 V and -1.7 V in my case?
4. Any tips and tricks you could help me out with or do you have a better or easier approach in mind? Please let me know if that is the case.

Answer 1

Can this concept work or what flaws can you think about?

Yes, it's fine.

Note that if you connect 3.3V stuff between the +5V and +1.7V rails, current goes into the 1.7V rail towards ground. You need to be aware of this because most voltage regulators are only designed for current to go out of them. Op-amps work both ways so you don't have this problem.

Note that all of that current goes through the op-amp. Make sure the op-amp is rated to handle all of the current that goes through your 3.3V stuff. This is a linear type of regulator - excess voltage is dissipated by creating heat. So make sure the power dissipation of the op-amp is okay too.

Will this be more efficient as the voltage regulator/voltage inversion-approach? Like I said I don't really care how big my negative voltage reference voltage is, because the op-amp will never go rail-to-rail on the output.

A minimum of 1.7 5.0ths, which is 34%, of the power gets wasted as heat. Whether that's okay for you, I can't say. It's up to you. You might be able to do better with some type of switching regulator.

If the amount of power isn't very much, it might not be a problem.

Building on the last question, is it even a good idea to supply an op-amp with an unsymmetrical supply voltage, like 3.3 V and -1.7 V in my case?

Op-amps don't actually know where 0V is. No circuit knows where 0V is. Op-amps, though, don't even have a 0V pin. They need a voltage that's higher than all your inputs and outputs, and a voltage that's lower than all your inputs and outputs. As long as all your input and output voltages are between the + voltage and the - voltage, with some margin (check the datasheet), and the + voltage and - voltage aren't too far apart (check the datasheet) this isn't a problem.

Any tips and tricks you could help me out with or do you have a better or easier approach in mind? Please let me know if that is the case.

The op-amp to create the voltage is fine, however, the way that you get the 1.7V reference voltage for the op-amp input is unnecessarily complicated. You use a 3.3V regulator to get 3.3V above the ground rail, and then you use op-amp math to "flip it upside down" to get 3.3V below the +5V rail.

For one thing, these regulators are designed to handle reasonable amounts of current and you are only demanding a tiny trickle. Some regulators require a minimum amount of load or else they won't regulate properly.

For another thing, it's just overly complicated.

Make a voltage divider between +5V and ground. But instead of a resistor, in the top half put a 3.3V shunt voltage reference (like this one but not this exact one).

An alternative is a 3.3V Zener diode. However low-voltage Zener diodes actually aren't that accurate as the voltage changes with the current. If you use a Zener diode you can put a trimpot in the bottom half and adjust the current while checking the voltage with your multimeter.

Of course you can also just make a plain old resistor divider. 1.7V is 1/3 of 5.0V so the top resistor should be twice the bottom one. I just assumed you wanted to regulate the 3.3V in which case you can use one of the other two methods.

Anyway, now that you have a voltage divider with 3.3V in the middle, just connect the middle of the voltage divider to the input of your op-amp voltage follower.