  We investigate inflation with a class of concave inflaton potentials of the
form $\sim \phi^n$ $(0<n<1)$ in the Randall-Sundrum model with an infinite
extra spatial dimension. We show that this class of models is much more in good
agreement with observations compared to the standard inflation. We also find
the range of the five-dimensional Planck scale ($M_5$) and show that large
tensor-to-scalar ratios do not eliminate small-field inflation in braneworld
cosmology.
