http://www.onsemi.com/pub_link/Collateral/AN920-D.PDF
 
 http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF

MC34063 does it for me. For what you want just about any cheap
buck switcher would do. But you don't specify other conditions.
If you're not fussy about efficiency it's possible to drop (eg pre-
regulate with a zener or just brute force with a resistor) the i/p
voltage and use any oscillator, even a 555 or a PIC for example,
as a switcher

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