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Speed and Accuracy
Time Step Control
More About Time Step Control
Usage of a Fixed Time Step
Control of the Local Truncation Error (LTE)
When LVLTIM equals 2 or 3, LTE estimates are used to monitor the time step selection.
Truncation error is the part of the solution error that originates from the approximation of the
time-derivative terms (for example, in the case of a simple capacitor, the dv/dt term in the
I=C.dv/dt equation) with finite-differences expressions. LTE can be estimated and monitored
using the dv/dt terms or the v term itself.
Tip
See “.OPTION LVLTIM” in the Eldo Reference Manual.
When using LTE control to determine the acceptable time steps, Eldo may use the voltage
quantities and/or the charge/flux quantities.
By default Eldo uses voltage quantities for LTE estimates. There are, however, situations where
LTE on charge/flux is used:
• If .OPTION QTRUNC is specified, LTE will be computed on charges/flux, not on
voltages. This corresponds to the way SPICE operates. This is sometimes more efficient
and/or accurate for PCB applications, but often seems indirect to IC designers.
• If Eldo detects ‘PCB-like’ devices such as those shown in the list below, then LTE will
be computed on charges/flux:
o
o
o
o
o
Current sources above 1A
Voltage sources above 500V
Negative capacitors or capacitors above 900pF
Inductors above 0.1μH
Magnetic core devices
If option QTRUNC is set, LTE is computed using charges and flux quantities. In this case
parameters NGTOL, CHGTOL, and FLUXTOL designate the absolute tolerances on voltages,
charges and fluxes respectively, and RELTRUNC is the associated relative tolerance.
Tip
See “.OPTION QTRUNC” in the Eldo Reference Manual.
Related Topics
More About Time Step Control
1250
Eldo® User's Manual, 15.3