Idles too low with A/C on.

[ugabd57]

Can anyone tell me which component makes an engine (1.6L) idle up when the A/C compressor kicks on? My 91 Celica ST Idles about 1000 RPM without the air on and goes down to about 550 when the air is turned on instead of the previous 1200. A couple of people have told me that it is in the computer but if it could be something more simple and less expensive I sure would like to know. The car runs great otherwise. Thanks