  An effective spin-orbit Hamiltonian is derived for a spin-1/2 trimerized
kagome antiferromagnet in the second-order of perturbation theory in the ratio
of two coupling constants. Low-energy singlet states of the obtained model are
mapped to a quantum dimer model on a triangular lattice. The quantum dimer
model is dominated by dimer resonances on a few shortest loops of the
triangular lattice. Characteristic energy scale for the dimer model constitutes
only a small fraction of the weaker exchange coupling constant.
