A new block-structured Lyapunov sum for partitioned matrices is defined and basic properties are established. It has similar properties to a Lyapunov matrix, but preserves the block structure, and has similar properties to the block Kronecker sum, but with about half the dimension. It is used to solve a maximal stability problem of integral controllability, and is also used to solve for the maximal stability range of singularly perturbed systems. In both cases closed formulae are obtained with lower dimensions than existing formulae.