  We consider the constraints on the physical parameters of a homogeneous SSC
model that can be derived from the spectral shape and variability of TeV
blazars. Assuming that the relativistic electron spectrum is a broken power
law, where the break energy $\gamma_b$ is a free parameter, we write the
analytical formulae that allow to connect the physical parameters of the model
to observable quantities. The constraints can be summarized in a plane where
the coordinates are the Doppler factor and the magnetic field. The consistency
between the break energy and the balance between cooling and escape and the
interpretation of the soft photon lags measured in some sources as radiative
cooling times are treated as additional independent constraints.
  We apply themethod to the case of three well known blazars, PKS 2155-304, Mrk
421 and Mrk 501.
