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AGBell 1 LM117 Regulator Risk Assessment - SPICE Why are you here? Ans: To provide feedback and information on how to verify that a design is truly flight.

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Presentation on theme: "AGBell 1 LM117 Regulator Risk Assessment - SPICE Why are you here? Ans: To provide feedback and information on how to verify that a design is truly flight."— Presentation transcript:

1 AGBell 1 LM117 Regulator Risk Assessment - SPICE Why are you here? Ans: To provide feedback and information on how to verify that a design is truly flight worthy. Ans: To provide feedback on you convections about quality. Ans: To review how we validate performance or design quality now. Why am I here? Ans: To gather your feedback Ans: To formulate concepts which will expand our understand of quality Ans: To propose and challenge you with some “new” or “old” ideas. Ans: To determine if there is a clear consensus

2 AGBell 2 LM117 Regulator Risk Assessment - SPICE General Questions 1.) Why do we need to do WCA? 2.) How do you evaluate the quality of a design? 3.) When do we do EVA versus Monte Carlo, Sensitivity, RSS, etc? 4.) How many samples is enough? 5.) Does parameter distribution matter? 6.) Does seed number matter? 7.) Why do sensitivity analysis? 8.) When do we say that the design is “good enough”? Other questions we should address?

3 AGBell 3 LM117 Regulator Risk Assessment - SPICE Background I wrote a paper for SYS611 last year entitled “Risk Assessment of a LM117 Voltage Regulator Circuit Design Using Crystal Ball and Minitab (Part 1)” I would like to write Part 2 of the paper which will focus on conventional circuit analysis of the same circuit. Design Requirements A 5VDC regulated voltage is required which can regulate +9VDC to +5VDC ±2.5%. To verify that the design will meet the requirements a risk assessment (worst case analysis) must be performed to demonstrate performance. The design must regulate to the desired voltage after five years of use and in an environment of -25°C to +75°C. The output load will be 100mA and the input voltage will be 9VDC. The design will use standard available parts and no tailoring will be allowed. Do we have all the info we need?

4 AGBell 4 LM117 Regulator Risk Assessment - SPICE Initial Design – Nominal Performance The 5VDC regulator circuit will be designed using equation 1 which describes how the circuit shown in Figure 1 functions. The resistors that will be used are M55342 type surface mount resistors which have a ±1% tolerance and have been used in other designs. Table 1 - Initial Design Equation 1 Figure 1 Are we happy with the design?

5 AGBell 5 LM117 Regulator Risk Assessment - SPICE Do we meet the requirements? Supplemental Part 1 results 5V±2.5% = 4.875V to 5.125V All part distributions are Gaussian All part distributions are Flat

6 AGBell 6 LM117 Regulator Risk Assessment - SPICE * *** Voltage regulators (positive/adjustable) * * LM117 voltage regulator "macromodel" subcircuit * created using Parts release 5.3 on 04/08/93 at 11:33 * modified 2/26/07 by AGBell (ITT SSD) * * Vref has 4% tolerance * Iadj has a 100% tolerance * * connections: input * | adjustment pin * | | output * | | |.SUBCKT LM117AGB IN ADJ OUT * * POSITIVE ADJUSTABLE VOLTAGE REGULATOR * JADJ IN ADJ ADJ JADJMOD;ADJUSTMENT PIN CURRENT *VREF 4 ADJ 1.25 E1 4 ADJ IREF e-3 RREF 0 40 RMOD 1e3 DBK IN 13 DMOD * * ZERO OF RIPPLE REJECTION * CBC e-010 RBC * QPASS 13 5 OUT QPASSMOD RB RB * * CURRENT LIMITING * DSC 6 11 DMOD ESC 11 OUT VALUE { *V(6,5)*V(13,5)} * * FOLDBACK CURRENT * DFB 6 12 DMOD EFB 12 OUT VALUE { *V(13,5) *V(13,5)*V(13,5) *V(13,5)*V(6,5)} * EB 7 OUT 8 OUT * * ZERO OF OUTPUT IMPEDANCE * RP CPZ 10 OUT 3.979e-006 * DPU 10 OUT DMOD;POWER-UP CLAMPLING DIODE RZ EP 9 OUT 4 OUT 100 RI OUT 4 100MEG *.MODEL QPASSMOD NPN (IS=30F BF=50 VAF=8.891 NF=2.612).MODEL JADJMOD NJF (BETA=50e-006 DEV=100% VTO=-1).MODEL DMOD D (IS=30F N=2.612).MODEL RMOD RES (R=1 DEV=4%).ENDS How do you get a good model? Ok, lets use PSPICE and see what we get. To use PSPICE you will need to overcome some tool and model limitations

7 AGBell 7 LM117 Regulator Risk Assessment - SPICE This appears to agree with the Crystal Ball simulation but we have used Uniform distributions which should widen the spread. Does this look ok?

8 AGBell 8 LM117 Regulator Risk Assessment - SPICE Something is wrong? This appears not to agree with the Crystal Ball simulation. The spread is much wider than the Uniform… PSPICE has a problem with the interpretation of Gaussian and Cadence is working on it.

9 AGBell 9 LM117 Regulator Risk Assessment - SPICE Device name: LM117AGB _____________________________________________________ Device Information: Device Placement: /LM117AGB Device Type: Adjustable Positive Regulator Body: /appmnt/prod/ECAD/cadence/cadence_psd_14.1/share/library/bodies/reg_adjust/sym_1/symbol.css SimName: SimLib: Parameter Information: Reference Voltage (VR): 1.25 (4 %; 4 %; gauss0.4) V 1st TempCo of VR (TC1VR): -15 u ( ; ; ) /C 2nd TempCo of VR (TC2VR): u ( ; ; ) /C^2 Line Regulation (LINREG):.01 ( ; ; ) %/V Load Regulation (LDREG):.1/1.5 ( ; ; ) %/A Quiescent Current (IB): 50 u (50 u; 50 u; gauss0.4) A Minimum Dropout Voltage (VDMIN): 1.5 ( ; ; ) V Dropout Voltage at IVD1 (VD1): 2.25 ( ; ; ) V Current for Dropout (IVD1): 1.5 ( ; ; ) A 1st TempCo of Dropout Voltage (TC1VD): u ( ; ; ) /C 2nd TempCo of Dropout Voltage (TC2VD): 7.9 u ( ; ; ) /C^2 Maximum Current (IMAX): 2.3 ( ; ; ) A Maximum VI-VO for max current (VMAIP): 11 ( ; ; ) V Linear slope of current limit (M1ILIM): -128 m ( ; ; ) A/V Quadratic slope of current limit (M2ILIM): 2.2 m ( ; ; ) A/V^2 1st TempCo of current limit (TC1IMAX): 0 ( ; ; ) /C 2nd TempCo of current limit (TC2IMAX): 0 ( ; ; ) /C^2 Timing Resistor (RT):.5 k ( ; ; ) Ohm Timing Capacitor (CT): 800 p ( ; ; ) Farad Max input-output voltage (VINMAX ): 40 ( ; ; ) V Max junction temp (TJ): 150 ( ; ; ) C J-C thermal resistance (RJC): 2.3 ( ; ; ) C/W C-A thermal resistance (RCA): 38 ( ; ; ) C/W Reference Voltage (VR): 1.25 (4 %; 4 %; FLAT) V Quiescent Current (IB): 50 u (50 u; 50 u; FLAT) A Model looks like it has lots of bells and whistles. Ok, lets use AWB and see what we get. Do we understand the model template?

10 AGBell 10 LM117 Regulator Risk Assessment - SPICE This appears to agree with the Crystal Ball simulation but we have used Uniform distributions which should widen the spread. Have we taken enough samples?

11 AGBell 11 LM117 Regulator Risk Assessment - SPICE This also appears to agree with the Crystal Ball simulation. However, AWB is no longer supported. We have good results but the tool is obsolete!

12 AGBell 12 LM117 Regulator Risk Assessment - SPICE Lets try the Advance Analysis in PSPICE. Results are not correct. Not all tolerances are accounted for. Cadence is working on it … We have bad results, now what?

13 AGBell 13 LM117 Regulator Risk Assessment - SPICE We are not done. The tools look like they have issues that need to be solved. We can’t trust the models. We have not answered our “general questions” yet. Not only do models need to be validated but maybe the tools as well.


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