The phone/tablet limits the amount of amperage needed.
So the charger's ability of amperage could be bigger than the phone's/tablet's needing, but should not less.
And how does the phone/tablet know that it can charge in "AC" mode?
It "looks" for jumpered data pins
HTC and most of phone producers follows the recommendation of USB standards for USB chargers ... they jump the data pins of their modified chargers for to bring the phone in 'AC charging' mode, means rapid charging with high amperage.
Wikipedia: Jumpered data pins on USB chargers
With generic chargers this modificaton with jumpered pins is rare feature to find.
So you'd look for a HTC charger