Thanks for your response! Yes, I totally forgot about the current... but I thought the voltage would start dropping when it can't produce enough current. So if I feed the pre-DC/DC voltage to the OpAmp the current will be taken care of?
Also, for the DC/DC converter, its output voltage is adjustable. I can configure it to produce any voltage I want, as long as it is lower than the input voltage. I am not sure what is going to happen if the output voltage is higher than the input voltage, so I decided to set the output voltage as 5.7V and a forward diode to subtract 0.7V. But again, I do not know if it is necessary. Do you think I can just set the DC/DC converter's output voltage as 5V and get rid of the diode?
This is the converter I am using:
http://www.dhgate.com/lm2596s-dc-dc-step-down-module-35-v-turn/p-ff80808133cfcdc10133fdc66906743f.html