  We investigate equilibrium and steady-state non-equilibrium transport
properties of a spinless resonant level locally coupled to two conduction bands
of width ~\Gamma via a Coulomb interaction U and a hybridization t'. In order
to study the effects of finite bias voltages beyond linear response, a
generalization of the functional renormalization group to Keldysh frequency
space is employed. Being mostly unexplored in the context of quantum impurity
systems out of equilibrium, we benchmark this method against recently-published
time-dependent density matrix renormalization group data. We thoroughly
investigate the scaling limit \Gamma\to\infty characterized by the appearance
of power laws. Most importantly, at the particle-hole symmetric point the
steady-state current decays like J ~ V^{-\alpha_J} as a function of the bias
voltage V>>t', with an exponent \alpha_J(U) that we calculate to leading order
in the Coulomb interaction strength. In contrast, we do not observe a pure
power-law (but more complex) current-voltage-relation if the energy \epsilon of
the resonant level is pinned close to either one of the chemical potentials \pm
V/2.
