  Within the framework of a general three-level problem, the dynamics of the
nitrogen-vacancy (NV) spin is studied for the case of a special type of
external driving consisting of a set of continuous fields with decreasing
intensities. Such a set has been proposed for minimizing coherence losses. Each
new driving field with smaller intensity is designed to protect against the
fluctuations induced by the driving field at the preceding step with larger
intensity. We show that indeed this particular type of external driving
minimizes the loss of coherence, using purity and entropy as quantifiers for
this purpose. As an illustration, we study the coherence loss of an NV spin due
to a surrounding spin bath of $^{13}$C nuclei.
