May be Ip(avg) is the average of the peak two levels of the current wave. For example for a square wave(0A then 2A then 0A, then 2A like this), we get 2A just after rising edge and also 2A just before falling edge. Now what if, after rising edge it is 2.3A and before falling edge it is 1.9A. Then we have to make an average of 2.3A and 1.9A like (2.3A+1.9A)/2=21A to express the peak current level of that squarewave. Then we will treat it as it is a squarewave like 0A then 21A then 0A then 21A then 0A etc.