An LM317 is just a 1.2V regulator. The output is always 1.2V above the
adjust pin (as long as we're in regulation). So, you can drive the adjust
pin with a voltage source that is 1.2V below the output voltage you want.

Another interesting adjustable regulator is the LT3080 (
http://www.linear.com/product/LT3080 ). It uses a reference current source
instead of a reference voltage source. The voltage that is compared with
the output to set the regulated voltage is developed across a "set"
resistor with the internally generated reference current. This is
typically used as a fixed regulator with a set resistor. It can be an
adjustable regulator (down to 0V) with a pot for the set resistor. It can
be a programmable voltage regulator by driving the SET pin with the
desired voltage. The output voltage will be the same voltage, but with
higher current capability.

Harold



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