I'm not so sure about the error amplifier part, but I see
why you are using an RMS converter, so you are controlling
the "effective" voltage to the lamp.  I take it you are
after constant brightness.  Of course, you could also
measure the lamp light output so you compensate for it aging
(and for the initial tolerance).

Are you very cramped for space, or is there some other good
reason you don't want to average the voltage of the pulses
with an inductor, so you can eliminate the RMS converter?
Having the inductor would lower the current ripple from the
power source and also the losses in the switch.

--
Regards,

John Popelish