  We present the dependence of $D$ production on the charged particle
multiplicity in proton-proton collisions at LHC energies. We show that, in a
framework of source coherence, the open charm production exhibits a growth with
the multiplicity which is stronger than linear in the high density domain. This
departure from linearity was previously observed in the $J/\psi$ inclusive data
from proton-proton collisions at 7 TeV and was successfully described in our
approach. Our assumption, the existence of coherence effects present in
proton-proton collisions at high energy, applies for high multiplicity
proton-proton collisions in the central rapidity region and should affect any
hard observable.
