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## LM95231 Datasheet(PDF) 18 Page - National Semiconductor (TI)

 Part # LM95231 Description Precision Dual Remote Diode Temperature Sensor with SMBus Interface and TruTherm Technology Download 20 Pages Scroll/Zoom 100% Manufacturer NSC [National Semiconductor (TI)] Direct Link http://www.national.com Logo

## LM95231 Datasheet(HTML) 18 Page - National Semiconductor (TI)

 18 / 20 page3.0 Applications Hints (Continued)TruTherm should only be enabled when measuring the tem-perature of a transistor integrated as shown in the processorof Figure 3, because Equation (5) only applies to this topol-ogy.3.1.2 Calculating Total System AccuracyThe voltage seen by the LM95231 also includes the IFRSvoltage drop of the series resistance. The non-ideality factor,η, is the only other parameter not accounted for and de-pends on the diode that is used for measurement. Since∆VBE is proportional to bothη and T, the variations in ηcannot be distinguished from variations in temperature.Since the non-ideality factor is not controlled by the tempera-ture sensor, it will directly add to the inaccuracy of thesensor. For the Pentium 4 processor on 90nm process, Intelspecifies a +1.19%/−0.27% variation inη from part to partwhen the processor diode is measured by a circuit thatassumes diode equation, Equation (4), as true. As an ex-ample, assume a temperature sensor has an accuracyspecification of ±0.75˚C at a temperature of 65 ˚C (338Kelvin) and the processor diode has a non-ideality variationof +1.19%/−0.27%. The resulting system accuracy of theprocessor temperature being sensed will be:TACC = ± 0.75˚C + (+1.19% of 338 K) = +4.76 ˚CandTACC = ± 0.75˚C + (−0.27% of 338 K) = −1.65 ˚CTrueTherm technology uses the transistor equation, Equa-tion (5), resulting in a non-ideality spread that truly reflectsthe process variation which is very small. The transistorequation non-ideality spread is ±0.1% for the Pentium 4processor on 90nm process. The resulting accuracy whenusing TruTherm technology improves to:TACC = ±0.75˚C + (±0.1% of 338 K) = ± 1.08 ˚CThe next error term to be discussed is that due to the seriesresistance of the thermal diode and printed circuit boardtraces. The thermal diode series resistance is specified onmost processor data sheets. For the Pentium 4 processor on90 nm process, this is specified at 3.33Ω typical. TheLM95231 accommodates the typical series resistance of thePentium 4 processor on 90 nm process. The error that is notaccounted for is the spread of the Pentium’s series resis-tance, that is 3.242Ω to 3.594Ω or +0.264Ω to −0.088Ω. Theequation to calculate the temperature error due to seriesresistance (TER) for the LM95231 is simply:(6)Solving Equation (6) for RPCBequal to +0.264Ω and−0.088Ω results in the additional error due to the spread inthe series resistance of +0.16˚C to −0.05˚C. The spread inerror cannot be canceled out, as it would require measuringeach individual thermal diode device. This is quite difficultand impractical in a large volume production environment.Equation (6) can also be used to calculate the additionalerror caused by series resistance on the printed circuitboard. Since the variation of the PCB series resistance isminimal, the bulk of the error term is always positive and cansimply be cancelled out by subtracting it from the outputreadings of the LM95231.Processor FamilyDiode EquationηD,non-idealitySeriesRmintypmaxPentium III CPUID 67h11.0065 1.0125Pentium III CPUID68h/PGA370Socket/Celeron1.00571.0081.0125Pentium 4, 423 pin0.9933 1.0045 1.0368Pentium 4, 478 pin0.9933 1.0045 1.0368Pentium 4 on 0.13micron process,2-3.06GHz1.0011 1.0021 1.0030 3.64ΩPentium 4 on 90 nmprocess1.00831.0111.0233.33ΩPentium M Processor(Centrino)1.00151 1.00220 1.00289 3.06ΩMMBT39041.003AMD Athlon MP model61.0021.0081.016AMD Athlon 641.0081.0081.096AMD Opteron1.0081.0081.096AMD Sempron1.002610.93Ω3.1.3 Compensating for Different Non-IdealityIn order to compensate for the errors introduced by non-ideality, the temperature sensor is calibrated for a particularprocessor. National Semiconductor temperature sensors arealways calibrated to the typical non-ideality and series resis-tance of a given processor type. The LM95231 is calibratedfor two non-ideality factors and series resistance values thus20120243FIGURE 3. Thermal Diode Current Pathswww.national.com18

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