User`s guide

13
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32 Monte Carlo and Sensitivity/Worst-Case Analyses
Hints and Other Useful
Information
VARY BOTH, VARY DEV, and VARY LOT
When VARY BOTH is specified in the .WC statement and a
model parameter is specified with both DEV and LOT
tolerances defined, the worst-case analysis may produce
unexpected results. The sensitivity of the collating function is
only tested with respect to LOT variations of such a parameter;
for example, during the sensitivity analysis, the parameter is
varied once affecting all devices referring to it and its effect on
the collating function is recorded. For the worst-case analysis,
the parameter is changed for all devices by LOT + DEV in the
determined direction. Consider the example schematic in
Figure 13-17 and circuit file in Figure 13-18.
Fi
g
ure 13-18
Circuit File Demonstrating Use of VARY BOTH
In this case, V(3) is maximized if:
• Rwc1 and Rwc2 are both increased by 10% per the LOT
tolerance specification, and
• Rwc1 is decreased by 5% and Rwc2 is increased by 5% per
the DEV tolerance specification.
The final values for Rwc1 and Rwc2 should be 105 and 115,
respectively. However, because Rwc1 and Rwc2 are varied
together during the sensitivity analysis, it is assumed that both
must be increased to their maximum for a maximum V(3).
Therefore, both are increased by 15%.
Here again, the purpose of the technique is to reduce the number
of simulations. For a more accurate worst-case analysis, you
should first perform a worst-case analysis with VARY LOT,
Fi
g
ure 13-17
Schematic
Demonstratin
g
Use of VARY
BOTH
W
CASE VARY BOTH Test
V
in 1 0 10V
Rs121K
Rwc12 3 Rmod 100
Rwc23 0 Rmod 100
.MODEL Rmod RES(R=1 LOT 10% DEV 5%)
.DC Vin LIST 10
.WC DC V(3) MAX VARY BOTH LIST OUTPUT ALL
.ENDS