Re: Charge pump DC to DC
The three design variables that you mentioned, frequency, capacitor value, and switch resistance will all have primary impact on the output resistance of yor converter.
If you are running open-loop, none of these parameters will vary with output voltage, so they will directly impact output voltage. If you know your input voltage and output voltage specifications, you can determine an output resistance specification. From this, you will be able to determine how the design variables constrain each other. From other design considerations (available clocks, available capacitor sizes, etc.), you may be able to further constrain the design variables in order to determine your solution.
If you are running closed-loop, one or more of the parameters will vary with output voltage, usually either switch resistance (in one or both phases) or switching frequency. This will effectively modulate the output resistance to give you a more consistent output voltage. The solution for this case would be similar to the first case, only one of the design variables would be variable when in use.