Optimum CMOS Stack Generation with Analog Constraints

An algorithm for the automatic generation of full-stacked layouts in CMOS analog circuits is described in this paper. The set of stacks obtained is optimum with respect to a cost function which accounts for critical parasitics and device area minimization. Device interleaving and common-centroid patterns are automatically introduced when possible, and all symmetry and matching constraints are enforced. The algorithm is based on operations performed on a graph representation of circuit connectivity, exploiting the equivalence between stack generation and path partitioning in the circuit graph. Path partitioning is carried out in two phases: In the first phase, all paths are generated by a dynamic programming procedure. In the second phase, the optimum partition is selected by solving a clique problem. Original heuristics have been introduced, which preserve the opthnality of the solution, while effectively improving the computational efficiency of the algorithm. The algorithm has been implemented in the ??C?? programming language. Many test cases have been run, and the quality of results is comparable to that of hand-made circuits. Results also demonstrate the effectiveness of the heuristics employed, even for relatively complex circuits.