PHILADELPHIA (Reuters) - The U.S. Federal Reserve should put a dollar cap and end date on its bond-buying program, a top central banker said on Friday, adding "the sooner we end (the program) the better." Philadelphia Fed President Charles Plosser, talking to reporters here, also suggested the central bank should convert its existing "thresholds" for raising interest rates to more predictable "triggers." The Fed has tied its first rate rise to future thresholds of 2.5 percent inflation and 6.5 percent unemployment. (Reporting by Jonathan Spicer; Editing by Chizu Nomiyama)