I believe the current of the battery causes the juice to taste burnt, not so much the voltage.
I have a 5volt USB PT and CE2 carto's work great on that. No burnt taste at all (no problem vaping for 3-4 seconds).
But at the same time I have a 3.7v KR808D-1 Battery and the juice burns very easily (anything over 1 second burns).
Most 5 volt PT's vape at a lower voltage. My "store bought" PT measures 3.5 volts while in actual use.