Well we are talking about voltage enduced in the tag. Not sure what the t5577 needs but the issue is more to do with antenna coupling.
For example in prototyping the HF LED Power Bracelet I can easily light the HF led on the diagnostic card at 5v (the lowest I have tried) I pass up to 30v to the same circuit to get a decently bright light out of the xLED unless I get the coil design just right.