Voltage drop
Well if drop has to be introduced, it could be done in 2 way :
- about 2 Watts has to be disspated somehow (zener ldo resistors ...)
- or voltage must be transformed DC DC convertor with minimum loss .
If you can't change your input and output constraints, there is nothing more than mentioned above 2 methods .
Added after 11 minutes:
Make a switching capasitor DC-DC converter :
- design comparator with hysteresis
Vh = 5,5 Vl= 5 , put its output to FET, so when Voltage is below 5 - FET must be opened and if above 5,5 - closed.
- Connect one side of FET serially with resistor (0,5 Ohm) to 8 V and another to capasitor about 1000 uF.
- Comparators input should control voltage on capasitor above .
Well , i dont know whether this switching capasitor DC-DC converter will satisfy objectives, but at least you can try to test it .
OR
redesign your DC - DC converter to provide 2 voltages