Maybe you should explain what you mean, the
first page of Google hits is all about pneumatic
/ hydraulic pressure regulators.
I have seen fixed duty cycle PWM "regulators"
used as intermediate bus converters and as
VTT regulators for RAM but no way to tell if this
is in fact the topic.
I mean a proportional regulator. Despite all the junk that you find in google, true engineering is missing there. In real life there are 3 types of very important regulators. This is P, PI and PID. In the first article I have shown the P(proportional regulator). This regulator has a transfer function which works on proportions, its very useful, because if your input changes by 1 V, the output can change by 2V, until the input reaches the desired value. On some systems this is not only necessary, but leads to faster regulation of the controlled value in the desired borders.