This paper proposes a cascade or multiloop digital control strategy for the voltage regulation of a three-phase islanded single distributed generation (DG) unit microgrid and its dedicated load. The cascade control strategy consists of an inner dq-currents control loop and an outer dq-voltages control loop responsible for providing the set-points of the inner loop. To design the outer loop, a constrained optimization-based loop shaping method is adopted. It uses multi-input multi-output (MIMO) nonparametric or spectral models of the system for an arbitrary number of desired operating points along with a linearly parameterized MIMO controller to form a set of open-loop transfer function matrices. Minimizing the summation of the second norm of the errors between the open-loop transfer function matrices and a desired one, the coefficients of the controller are optimally determined. Taking several load change scenarios, the performance of the proposed controller is evaluated based on time-domain simulation studies in MATLAB/PLECS.