We present a general framework for the design of a damping controller to tame lightly damped internal dynamics and to assess the stability of an approximate inputoutput linearised closed-loop system. In nonlinear systems, conclusions cannot be drawn regarding the stability of internal dynamics even though the zero dynamics is globally stable. Here, we investigated the sufficient conditions with a parameter-dependent Lyapunov (PDL) function for semi-globally exponential stability of damped internal dynamics. Using a composite Lyapunov function based on the PDL function, the uniform boundedness of the output regulation error is established within a region of interest. Controllability and observability analyses offer a theoretical guideline as to how a damping controller can be designed in the view of control effort and transient performance.