What I suggest you is to check for the various solutions adopted by the producers of this kind of DC/DC converter.
You can take a look at the block diagram (if the internal schematics are not available) of the IC and you can read the description of the inner functioning of the device: I think you will learn a lot about problems you can occur in the developement of such converter.
Personally I have never had such requirements, so I'm not sure if I can help you.
BTW, when you make such requests, it would be better if you will give other parameters like maximum input/output current, output ripple, ...
It is a bit difficult to help without knowing them!
BTW, what is the reason you do not want to use a ready made IC: price/dimensions/...?