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## LM95235 Datasheet(PDF) 22 Page - National Semiconductor (TI)

 Part No. LM95235 Description Precision Remote Diode Temperature Sensor with SMBus Interface and TruTherm Technology Download 25 Pages Scroll/Zoom 100% Manufacturer NSC [National Semiconductor (TI)] Direct Link http://www.national.com Logo

## LM95235 Datasheet(HTML) 22 Page - National Semiconductor (TI)

 22 / 25 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 7, because Equation (5) only applies to this topol-ogy.3.1.2 Calculating Total System AccuracyThe voltage seen by the LM95235 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 for Intel processor on 65nm process, Intelspecifies a +4.06%/−0.897% 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 ±1.0˚C at a temperature of 80˚C (353 Kelvin)and the processor diode has a non-ideality variation of+1.19%/−0.27%. The resulting system accuracy of the pro-cessor temperature being sensed will be:TACC = + 1.0˚C + (+4.06% of 353 K) = +15.3 ˚CandTACC = - 1.0˚C + (−0.89% of 353 K) = −4.1 ˚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 ±0.39% for the Pentium 4processor on 90 nm process. The resulting accuracy whenusing TruTherm technology improves to:TACC = ±0.75˚C + (±0.39% of 353 K) = ± 2.16 ˚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 Intel processors in 65 nmprocess, this is specified at 4.52Ω typical. The LM95235accommodates the typical series resistance of Intel Proces-sor on 65 nm process. The error that is not accounted for isthe spread of the processor’s series resistance, that is 2.79Ωto 6.24Ω or ±1.73Ω. The equation to calculate the tempera-ture error due to series resistance (TER) for the LM95235 issimply:(6)Solving Equation (6) for RPCB equal to ±1.73Ω results in theadditional error due to the spread in the series resistance of±1.07˚C. The spread in error cannot be canceled out, as itwould require measuring each individual thermal diode de-vice. This is quite difficult and impractical in a large volumeproduction 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 LM95235.Processor FamilyTransistor EquationηD,non-idealitySeriesR,ΩmintypmaxIntel Processor on65 nm process0.9971.0011.0054.52Processor FamilyDiode EquationηD,non-idealitySeriesR,ΩmintypmaxPentium III CPUID67h11.00651.0125Pentium III CPUID68h/PGA370Socket/Celeron1.00571.0081.0125Pentium 4, 423 pin0.99331.00451.0368Pentium 4, 478 pin0.99331.00451.036820174943FIGURE 7. Thermal Diode Current Pathswww.national.com22

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