You could use a low dropout regulator in series with the solar panel output such as **broken link removed** from National Semi. You set the voltage to the maximum you want. An any voltage below this, its output will follow the solar panel voltage with less than a 0.5V drop, depending upon current.
Are you configuring the DS2715 for switch-mode operation to maximize its efficiency?
Why do you have a linear regulator following the switching regulator? That will reduce your efficiency. Are you concerned about its noise getting into your electronics?