Be aware of exactly what you are trying to achieve. Do you want to track the maximum power point AND keep the output voltage constant? That means the load has to accept a current defined by the SOURCE not the LOAD. If the load is like a battery this is straightforward. If it isn't, and is time varying, then you have set up two inconsistent demands on your design. You have to resolve this at the design stage first.Then you can build a constant voltage output MPPT.