> I wish to buy (or will need to design) the smallest and most efficient
> constant current ballast that directly takes 230vAC UK mains (6% deltaV,
> 50Hz), to supply 40mA to a power LED string with a voltage drop of 216v.

There may be a few more issues. Depending on volume and application,
powrr factor may be an issue. If you are working from steady DC
without any preprocessing from AC to DC then you'll end up with around
1.4 x 230 =3D 300+ VDC and you'll have to buck from there. You MAY wish
to phase controlled rectify, when power factor may become an issue.

Gargoyling eg offline LED driver produces many ideas.
eg

Phase controlled Power Factor correcting TRIAC systems
A large number of app notes - some highl;y relevant

                 http://www.powerint.com/applications/led-lighting

eg Far far far too complex - but gives some ideas:

           http://www.powerint.com/sites/default/files/PDFFiles/di130.pdf

Equally nasty

         http://www.powerint.com/sites/default/files/PDFFiles/der167.pdf

This is more like it.
Most unusual.

       http://www.powerint.com/sites/default/files/PDFFiles/di92.pdf

Another

               http://www.powerint.com/sites/default/files/PDFFiles/di92.pd=
f



LEDs magazine - specific comments here and also much else relevcant in
ongoingf dfree magazines

          http://www.ledsmagazine.com/features/5/4/9


http://www.maxim-ic.com/app-notes/index.mvp/id/4387

This may be useful
http://www.national.com/cat/index.cgi?i=3Di//221
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