Electronic – Running Regulator Under Voltage

They are regulators as you thought.

To properly control the MOSFET on each regulator, the gate needs to rise several volts above the source, probably over 6 volts greater than the source. If the source voltage is 3.3 volts the gate might need to rise as high as 9 or 10 volts when the MOSFET is driving a heavy load current. This means the op-amp MUST be powered from a supply greater than 10 volts.

The non-inverting input is the demand voltage for the regulator - for the 3V3 supply the non-inverting input will be 3.3 volts and ditto the 2V5 supply.

The MOSFET is rated at 20 amps and this means the supply current to the circuit might be in the realm of 5 to 8 amps - try and find a linear voltage regulator that would fit the bill an be a low drop-out type - realistically with a drain voltage of fractionally above 3.3 volts the output would still be in regulation. This is of course due to the op-amp being powered from 12 volts.

As to whether it is a bad design, I'd say not BUT I don't know the full spec of the circuit.

To add to the other answers, here is some general information about getting a low noise power supply from a car:

• The voltage from your car will normally be between 12V (engine off) and 14.5V (engine on) but may have drop-outs, glitches and spikes up to about 30V. So you are correct to think about smoothing it for precision use.
• The simplest starting point is a DC-DC converter, such as one described in the other answers. On its own, it'll give you the set voltage with probably 100mV or ripple, depending on how much you paid for it.
• If you need a cleaner power supply, you can pay for a better DC-DC converter, or you can add a filtering stage. A low-ESR ceramic capacitor connected across the output is a good start, and a LC pi filter will do better, but be careful to select low resistance parts to make the filter.
• If that's still not good enough, you can set your DC-DC converter to 14V and use a linear regulator to drop the voltage to 12V. That will usually produce much lower noise on the supply, but make sure the linear regulator has a good PSRR at the switching frequency of the regulator.
• For absolute best results, use a DC-DC converter, then an LC filter, then a linear regulator, and finally a large bypass capacitor near each IC you are powering. Put the DC-DC converter in a metal box, the regulator in another box and your actual circuit in a third box, so they are all shielded from each other, and use shielded cable to connect the 2nd and 3rd box.

There are very few applications where you'll need to go beyond a decent DC-DC converter and maybe a capacitor. Taking all of the steps above would only be necessary if you were doing something like powering an ultra-low-noise instrumentation amplifier to make sub-nV measurements.