  We formulate a d-p multiband charge transfer model for Cu$_2$O$_5$ coupled
spin ladders, relevant for Cu$_2$O$_3$ plane of
Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$, and solve it using Hartree-Fock
approximation. The results explain that (i) the charge density wave (CDW) with
its periodicity dependent on doping is stabilized by purely electronic
many-body interactions in a single spin ladder and (ii) the inclusion of the
interladder interactions favors (disfavors) the stability of the CDW with odd
(even) periodicity, respectively. This stays in agreement with recent
experimental results and suggests the structure of the minimal microscopic
model which should be considered in future more sophisticated studies.
