Throughout the design flow, constraints should be propagated
down the hierarchy for consistency as it evolves through the design
stages, thus ensuring that the top-level block meets the target
specifications. The ultimate advantage of such top–down design is
that you can catch problems early in the design flow and better your
chances of first-time success with a better overall system design.
In analog design, it is rare to have one clean top–down design path.
Second-order effects are sometimes hard to quantify as the design
evolves. Libraries of selected standard cells are not the answer,
unfortunately, because a library can only partially cover the wide
ranging circuit specifications demanded by different applications.