[SOLVED] difference between vreg divider and resistor divider formulas

[d123]

Hi,

Silly question, really. Just curious. Maybe there is no answer.

Why are the formulas for a resistive divider and an adjustable voltage regulator divider so different?

e.g. The former divides lower resistor by sum of upper and lower, the latter divides upper resistor by lower resistor value.

The image is an example of both cases:



Thanks.

 

[betwixt]

They are both resistive dividers, the difference is that one provides the voltage directly (right) and the other provides feedback against a reference voltage built in the the IC. For the one shown on the left, even if the bottom resistor is shorted, it still produces 1.21V out.

Brian.