Description
These keywords specify the transit time parameters used to estimate the transit time capacitances or to develop transit time capacitance tables for the [GND Clamp] and [POWER Clamp] tables. The transit time capacitance is added to C_comp. It is in a Spice reference model as
Ct = TT * d(Id)/d(Vd)
where d(Id)/d(Vd) defines the direct current (DC) conductance at the incremental DC operating point of the diode and TT is the transit time.This expression does not include any internal series resistance. Such a resistance is assumed to be negligible in practice.
Assume that the internal diode current (Id) and voltage (Vd) relationship is
Id = Is * (exp(q(Vd)/kT) - 1)
where Is is the saturation current, q is electron charge, k is Boltzmann's constant, and T is temperature in degrees Kelvin. Then d(Id)/d(Vd) is approximately (q/kT) * Id when the diode is conducting, and zero otherwise. This yields the simplification of
Ct = TT * (q/kT) * Id. The Id is found from the [GND Clamp] and [POWER Clamp] operating points, and the corresponding TTgnd or TTpower is used to calculate the Ct value. If the [Temperature Range] keyword is not defined, then use the default ""typ"" temperature for all Ct calculations.