gmc envoy 2005 ac problems

at first when i turned on my ac or heater on my 2005 envoy my rpm would lower and the car would stall now when i turn on my ac it blows hot air and i get a burning smell the fan still works but i get this really bad burning smell so i have to shut off the ac .please help

First thing I would check is if the A/C compressor pully is locked up, or possibly the compresor itself is siezed...