dc to dc ocnverter with uc 3842
I think that if you have no experience about Dc/Dc converter, the best way is to start with a complete (monolithic) DC/DC converter as suggested by mr_ghz.
The parameter to know are:
-the range of input voltage (normally thi ranges are standard; for example 18-36V)
-the voltages and the relative output currents
-if it is necessary the isolation between input and output
An important parameter to consider is efficiency. Normally for the integrated Dc/Dc Coverter it is around 80%.
There are many manufacturers (Powerbox,Gaia,etc.) that propose their monolithic Dc/Dc Converter and some of them are cheaper especially if they come from oriental market like YDS.
Normally the integrated solution is much more expensive than a discrete solution, but for testing your project is better to have a reliable power supply.
In a second momet you can try to reduce the cost of your application either designing the Dc/Dc Converter by yourself (if you have time!) using the tools that many manufacturers make available on the net, like WEBENCH Tool by National Semiconductor, or you can ask third party to design and to realize the sample with your specification.
If you need the Dc/Dc Converter only for a project the best solution is the monolithic one.
If you need the Dc/Dc Converter for a market product the cheaper solution is the secod one.
Best Regards