It should add 3 V to 12 V by means of 5 PWM V.
... can boost 12V up to about 24V very easily.
Well, the charge pump voltage, should it be called unsteady? To decrease, yes, but I thought voltage regulator wasn't necessary in terms of steadness here, the load is very small, 0,001 mA, and it won't change.3,3 volts would sink below 3 on 2 diodes. To increase 12 to 24, will it need a transistor again, because PWM is 5 V? So better, to compactize, maybe 5 V PWM should be added to 12, and those 16 reduced to 15 by regulator?