Due to advances in manufacture technology, leakage current increases dramatically in modern ICs. Power gating technique is an efficient and effective method to resolve this problem. In order to turn off supply voltage in a low-power domain, it has to insert power switches into designs. However, chip area and IR-drop of circuits are impacted by the number and locations of inserted power switches. Unlike previous works using greedy algorithm to handle this problem, this paper proposes a simple model to approximate the equivalent resistance of power switches in a low-power domain, and uses the binary search method to get the precise value. Based on this value, power switches are allocated by a partition-based approach. Experimental results demonstrate that our approach can insert less number of power switches and still satisfy the IR-drop constraint than other approaches. Moreover, this method is very efficient.