UC3854 is a PFC controller IC employing Average current mode control. Output of the voltage error amplifier is divided by square of the average input voltage & the resultant gets multiplied by rectified input voltage signal. Squarer, multiplier & divider circuit plays a pivotal role . This extra circuitry keeps the gain of the voltage loop constant. If this circuit is not present, the loop gain would vary as square of the input voltage.
How????...
Let's assume that output of voltage error amplifier remains constant(No variation in Load) & input voltage is increased by 2 times .So EA output multiplied by average input voltage(I.e. A.B) doubles up but at the same time gets divided by square of the input voltage(C) so effectively resultant (Current programming signal)of A.B/C is reduced by half. So the effective input power remains constant.
Due to this, output of the error amplifier acts as a power control i.e. it controls the output power delivered to load.
Note:- if you want, You can refer to some of the old but well written documents by TI/Unitrode. They nicely explain about the complete operation. kindly, go through those before you start designing a PFC circuit.