Patellar tendonitis is characterized by pain below the patella (the origin of the patellar tendon) and is usually found in adults; the pain can sometimes be reproduced by pressing upon the patellar tendon itself. Similar pain symptoms in teenagers is typically due to an entirely different entity known as Osgood Schlatter disease
Also known as "jumper's knee" (because of its predilection for athletes), patellar tendonitis is typically brought about by repetitive contraction of the quadriceps and related muscles combined with repeated impact on the knee joint. Cold packs acutely followed by topical heat chronically, and NSAIDS (ie ibuprofen, Aleve) may provide symptomatic relief, but long term rest is necessary for healing and resolution of symptoms.
There is no clinical evidence of which I am aware that bracing of any kind is beneficial in treating patellar tendonitis, though theoretically the proprioceptive effect of a brace (iow, the sensation of a brace as a subconscious "reminder" to protect the knee) might offer some protection. In that case, a good neoprene brace would suffice.
The above is not medical advice, a diagnosis, or a recommendation for treatment, nor is it a substitute for medical care. If symptoms persist, consult your health care provider.