Help with designing negative part of Dual-Sided SMPS
In principle, you can make inverted buck converter; however, this will be difficult in real life since suitable parts are rare. You may want to try a different approach; generate negative output from the same positive input using inverting or Cuk converter topology. Read Jim Willams' "Switching Regulators for Poets" -> http://cds.linear.com/docs/en/application-note/an25fa.pdf if you desire more elegant solution.