  We introduce a Langevin equation describing the pinning-depinning phase
transition experienced by Kardar-Parisi-Zhang interfaces in the presence of a
bounding ``lower-wall''. This provides a continuous description for this
universality class, complementary to the different and already well documented
one for the case of an ``upper-wall''. The Langevin equation is written in
terms of a field that is not an order-parameter, in contrast to standard
approaches, and is studied both by employing a systematic mean-field
approximation and by means of a recently introduced efficient integration
scheme. Our findings are in good agreement with known results from microscopic
models in this class, while the numerical precision is improved. This Langevin
equation constitutes a sound starting point for further analytical
calculations, beyond mean-field, needed to shed more light on this poorly
understood universality class.
