> So a linear regulator, like LM1084 which i had in mind, won't work?

Ignoring the power drawn by the regulator itself (which is usually very
little), the current going into a linear regulator is equal to the
current going out of it. So if you draw 5A from the 12V side of the
regulator, it will draw 5A from the 18V side and dissipate 6*5=30W. I
doubt you want 30W ending up on the regulator heatsink. There is no way
around this fact with linear regulators - if you want to minimize the
power dissipation of the regulator (6W per amp in your case) and have an
input current smaller than the output current, you'll need a switching
regulator.

-- 
Hector Martin (hector@marcansoft.com)
Public Key: http://www.marcansoft.com/marcan.asc

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