Champ with an ultimate/move that has NO cooldown.

The ultimate/move would be very strong and have no cooldown. How to balance this? As soon as the champion casts the ultimate, it deprives them of all of their resources (mana or energy or something) until they recall. It's an idea that I had a while ago, for a champ to have something (ult or move) with 0 cooldown whatsoever. I can see how this would be very OP, although it's** just an idea**. I just think it would be interesting for someone to have a move with no cooldown. :)