Alan, All,
Note the LMV431 here in your circuit, that device is a zener 3 pin not a five pin op-amp as shown here correct, I'm confused?. I have implimented a similair circuit using the LM3481 as suggested and my own op-amp to the FB pin, but the drop across the 100K to the FB pin is so large that the input inpedence of the FB seems to be about 50K is that what you would expect? The way the circuit is drawn it seems the input impedence would be much greater not to lose a large voltage drop before getting to the FB pin. Can you advise?