This a theoretical question, I am happy with my current design, no intentions to try this yet.
Some PCB that has requirements of 5V, 3.3V, and 1.8V. The 5V is the supply from some standard linear dropout regulator, and the 3.3V and 1.8V current requirements are low and also fed from some linear regulator powered from the 5V supply.
Let's also pretend maybe our only goal is to reduce the number of components on a board, and that the components aren't particularly sensitive to noise or the exact voltages required.
Theoritically, would it be possible to only populate the boards with the 5V regulator to act as the supply and the 3.3V to act as a supplementary source for the required components. Then for the specific component that needed 1.8V, connect the VCC to 5V and the VDD to the 3.3V plane/source.
Would this not create a sufficient ~1.7V voltage potential across the chip? We could even design say to have a 5.05V source and a 3.25V secondary to get a closer 1.8V approximate assuming the other chips have good tolerance as well.
What would the greatest penalties be and considerations? Obviously a dirtier 1.8V source with power fluctuations and spikes. Also introducing the tolerances in output voltage for 2 regulators instead of one. But if our acceptable input range was 1.65-1.95V, could this work?