Ngspice checks the convergence on the non-linear functions that describe the non-linear branches in circuit elements. In semiconductor devices the functions defines currents through the device and thus the name of the criterion.

Ngspice computes the difference between the value of the nonlinear function computed for the last voltage and the linear approximation of the same current computed with the actual voltage

[\begin{array}{ll} {\left| {\hat{i_{branch}^{({k + 1})}} - i_{branch}^{(k)}} \right| \leq \mathtt{RELTOL} i_{br_{max}} + \mathtt{ABSTOL},} & \ \end{array}]

where

[\begin{array}{ll} {i_{br_{max}} = \max\left( {\hat{i_{branch}^{({k + 1})}},i_{branch}^{(k)}} \right).} & \ \end{array}]

In the two expressions above, the (\hat{i_{branch}}) indicates the linear approximation of the current.