Must it be a single chip? The circuit on the link below from RMCybernetics.com uses a 555 and a 393 (both 8-pin DIPs) that I have always had luck with. Because they each have only 8 pins, they take up the same amount of space as a single 16-pin DIP. If you're not worried about space, but complexity instead, the PWM circuit on this site is actually quite simple:
RMCybernetics - DIY Homemade Power Pulse Controller
I'm not sure this is what you wanted (I haven't done much with buck converters), so please let me know. Thanks.
-Der Strom