This will possibly  fail on price, and availability looks not
marvellous * but otherwise looks superb.
(*Not listed by Digikey except evaluation board. Mouser, Arrow, ...
show $ but 0 stock)
IR3897 - lowest capability member of a family.
4A.
300 kHz - 1500 kHz.
1 <=3D Vin <- 21V.
Should be comfortably above 90% efficiency in your application.

       http://www.irf.com/product-info/datasheets/data/ir3897m.pdf

Family

         http://www.irf.com/whats-new/nr120628.html


         Russell
--=20
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