hspice book hspice ch11 1 Thu Jul 23 19 10 43 1998 Chapter 11 Optimizing Performance Optimization the automatic generation of model parameters and component values from a given set of electrical specifications or measured data is available in Star Hspice With a user defined optimization program and a known circuit topology Star Hspice automatically selects the design components and model parameters to meet DC AC and transient electrical specifications Star Hspice optimization is the result of more than ten years of research in both the optimizing algorithms and user interface The optimizing function is integrated into the core of Star Hspice resulting in optimum efficiency The circuit result targets are part of the MEASURE command structure and the parameters to be optimized are Star Hspice defined parameter functions A MODEL statement sets up the optimization The most powerful feature of the Star Hspice approach is its incremental optimization technique Incremental optimization allows you to solve the DC parameters first then the AC parameters and finally the transient parameters A set of optimizer measurement functions not only makes the task of transistor optimization easy but significantly improves cell and whole circuit optimization This chapter covers the following topics Performing Optimization Understanding the Statements Syntax Examining Optimization Examples Performing Timing Analysis Star Hspice Manual Release 1997 2 11 1 hspice book hspice ch11 2 Thu Jul 23 19 10 43 1998 Performing Optimization Optimizing Performance Performing Optimization To perform optimization create an input netlist file specifying Minimum and maximum parameter and component limits The variable parameters and components An initial estimate of the selected parameter and component values The circuit performance goals or model versus data error function Given the input netlist file optimization specifications component limits and initial guess the optimizer reiterates the circuit simulation until the target electrical specification is met or an optimized solution is found For improved optimization and simulation time and to increase the likelihood of a convergent solution the initial estimate of the component values should produce a circuit with specifications near those of the original target This reduces the number of times the optimizer reselects component values and resimulates the circuit Optimization Control The length of time to complete an optimization is a function of the number of iterations allowed the relative input tolerance the output tolerance and the gradient tolerance The default values are satisfactory for most applications Generally 10 to 30 iterations are sufficient to get accurate optimizations Simulation Accuracy Set the simulator with tighter convergence options than normal for optimization The following options are suggested For DC MOS model optimizations absmos 1e 8 relmos 1e 5 relv 1e 4 For DC JFET BJT and diode model optimizations 11 2 Star Hspice Manual Release 1997 2 hspice book hspice ch11 3 Thu Jul 23 19 10 43 1998 Optimizing Performance Performing Optimization absi 1e 10 reli 1e 5 relv 1e 4 For transient optimizations relv 1e 4 relvar 1e 2 Curve Fit Optimization Use optimization to curve fit user defined DC AC or transient data In a curve fit optimization the desired numeric data for curves is stored in the data file as in line data using the DATA statement The variable circuit components and parameter values of the netlist are specified in the PARAM xxx OPTxxx statement The optimization analysis statements call the in line data using the DATA keyword The MEASURE statement uses the simulation result and compares it with the values given in the data file The MEASURE statement controls the comparison of simulation results to the values given in the data file This is usually done with the ERR1 keyword If the calculated value is not within the error tolerances specified in the optimization model a new set of component values are selected and the circuit is resimulated This is repeated until the closest fit to the curve is obtained or the error tolerances set is satisfied Avant s Device Model Builder DMB product automates curve fit optimization for the development of most DC MOSFET models Goal Optimization Goal optimization differs from curve fit optimization in that it usually only applies to the optimization of a particular electrical specification such as rise time or power dissipation Goal optimizations are specified using the GOAL keyword with a choice of relational operator in the MEASURE statement where GOAL is the target electrical specification being measured This choice of relational operator is useful in multiple constraint optimizations when the absolute accuracy of some criteria is less important than for others Star Hspice Manual Release 1997 2 11 3 hspice book hspice ch11 4 Thu Jul 23 19 10 43 1998 Understanding the Statements Syntax Optimizing Performance Understanding the Statements Syntax Several Star Hspice statements are required for optimization MODEL modname OPT PARAM parameter OPTxxx init min max A DC AC or TRAN analysis statement with MODEL modname OPTIMIZE OPTxxx and RESULTS measurename MEASURE measurename GOAL val note that a space is required on either side of the relational operator or The PARAM statement lets you specify initial lower and upper bound values The types of MEASURE statements available for optimization are described in Chapter 4 Specifying Simulation Output Output statements PRINT PLOT and GRAPH must be associated with the analysis statements DC AC or TRAN An analysis statement with the keyword OPTIMIZE is used for optimization only To generate output for the optimized circuit another analysis statement DC AC or TRAN must be specified along with the output statements The proper specification order is 1 Analysis statement with OPTIMIZE 2 MEASURE statements specifying optimization goals or error functions 3 Ordinary analysis statement 4 Output statements 11 4 Star Hspice Manual Release 1997 2 hspice book hspice ch11 5 Thu Jul 23 19 10 43 1998 Optimizing Performance Understanding the Statements Syntax Analysis Statement Syntax DC TRAN AC The syntax is DC DATA filename SWEEP OPTIMIZE OPTxxx RESULTS ierr1 ierrn MODEL optmod or AC DATA filename SWEEP OPTIMIZE OPTxxx RESULTS ierr1 ierrn MODEL optmod or TRAN DATA filename SWEEP OPTIMIZE OPTxxx RESULTS ierr1 ierrn MODEL optmod where DATA specifies the in line file of parameter data to use