[ubuntu] maximum CPU frequency when on AC

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http://ubuntuforums.org – hello, i am using ubuntu 12.10 32-bit on my laptop DELL inspiron N4030 with ATI Mobility Radeon HD 5430 Series. the problem is: when Laptop on AC adapter, CPU frequency always be the maximum (2.39 GHz), that's make out loud fan, even i try to change that frequency, it returns automatically (to the maximum) as long as the laptop plugged in. on the other hand ubuntu seems to (automatically) scaling the processors correctly in battery-mode. nevertheless i cannot change the CPU frequency In all cases. what to do? i can't work comfortably when my laptop plugged in AC adapter! (Hardware)