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LM95241 Datasheet(PDF) 17 Page - National Semiconductor (TI) |
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LM95241 Datasheet(HTML) 17 Page - National Semiconductor (TI) |
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17 / 19 page  3.0 Applications Hints (Continued) TruTherm should only be enabled when measuring the tem- perature of a transistor integrated as shown in the processor of Figure 3, because Equation (5) only applies to this topol- ogy. 3.1.2 Calculating Total System Accuracy The voltage seen by the LM95241 also includes the I FRS voltage 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 ∆V BE is proportional to both η and T, the variations in η cannot 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 the sensor. For the Pentium D processor on 65nm process, Intel specifies a +4.06%/−0.89% variation in η from part to part when the processor diode is measured by a circuit that assumes diode equation, Equation (4), as true. As an ex- ample, assume a temperature sensor has an accuracy specification of ±1.25˚C at a temperature of 65 ˚C (338 Kelvin) and the processor diode has a non-ideality variation of +4.06%/−0.89%. The resulting system accuracy of the processor temperature being sensed will be: T ACC = + 1.25˚C + (+4.06% of 338 K) = +14.97 ˚C and T ACC = - 1.25˚C + (−0.89% of 338 K) = −4.26 ˚C TrueTherm technology uses the transistor equation, Equa- tion (5), resulting in a non-ideality spread that truly reflects the process variation which is very small. The transistor equation non-ideality spread is ±0.4% for the Pentium D processor on 65nm process. The resulting accuracy when using TruTherm technology improves to: T ACC = ±1.25˚C + (±0.4% of 338 K) = ± 2.60 ˚C The next error term to be discussed is that due to the series resistance of the thermal diode and printed circuit board traces. The thermal diode series resistance is specified on most processor data sheets. For the Pentium D processor on 65 nm process, this is specified at 4.52 Ω typical. The LM95241 accommodates the typical series resistance of the Pentium D processor on 65nm process. The error that is not accounted for is the spread of the Pentium’s series resis- tance, that is 2.79 Ω to 6.24Ω or ±1.73Ω. The equation to calculate the temperature error due to series resistance (T ER) for the LM95241 is simply: (6) Solving Equation (6) for R PCB equal to ±1.73 Ω results in the additional error due to the spread in the series resistance of ±1.07˚C. The spread in error cannot be canceled out, as it would require measuring each individual thermal diode de- vice. This is quite difficult and impractical in a large volume production environment. Equation (6) can also be used to calculate the additional error caused by series resistance on the printed circuit board. Since the variation of the PCB series resistance is minimal, the bulk of the error term is always positive and can simply be cancelled out by subtracting it from the output readings of the LM95241. Processor Family Transistor Equation n D, non-ideality Series R min typ max Intel processor on 65nm process 0.997 1.001 1.005 4.52 Ω Processor Family Diode Equation η D, non-ideality Series R min typ max Pentium III CPUID 67h 1 1.0065 1.0125 Pentium III CPUID 68h/PGA370Socket/ Celeron 1.0057 1.008 1.0125 Pentium 4, 423 pin 0.9933 1.0045 1.0368 Pentium 4, 478 pin 0.9933 1.0045 1.0368 Pentium 4 on 0.13 micron process, 2-3.06GHz 1.0011 1.0021 1.0030 3.64 Ω Pentium 4 on 90 nm process 1.0083 1.011 1.023 3.33 Ω Pentium on 65 nm porcess 1.000 1.009 1.050 4.52 Ω Pentium M Processor (Centrino) 1.00151 1.00220 1.00289 3.06 Ω MMBT3904 1.003 AMD Athlon MP model 6 1.002 1.008 1.016 AMD Athlon 64 1.008 1.008 1.096 AMD Opteron 1.008 1.008 1.096 20199743 FIGURE 3. Thermal Diode Current Paths www.national.com 17 |
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