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LM95235 Datasheet(PDF) 22 Page - National Semiconductor (TI) |
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LM95235 Datasheet(HTML) 22 Page - National Semiconductor (TI) |
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22 / 25 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 7, because Equation (5) only applies to this topol- ogy. 3.1.2 Calculating Total System Accuracy The voltage seen by the LM95235 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 for Intel processor on 65nm process, Intel specifies a +4.06%/−0.897% 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 ±1.0˚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: T ACC = + 1.0˚C + (+4.06% of 353 K) = +15.3 ˚C and T ACC = - 1.0˚C + (−0.89% of 353 K) = −4.1 ˚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 ±0.39% for the Pentium 4 processor on 90 nm process. The resulting accuracy when using TruTherm technology improves to: T ACC = ±0.75˚C + (±0.39% of 353 K) = ± 2.16 ˚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 Intel processors in 65 nm process, this is specified at 4.52 Ω typical. The LM95235 accommodates the typical series resistance of Intel Proces- sor on 65 nm process. The error that is not accounted for is the spread of the processor’s series resistance, that is 2.79 Ω to 6.24 Ω or ±1.73Ω. The equation to calculate the tempera- ture error due to series resistance (T ER) for the LM95235 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 LM95235. Processor Family Transistor Equation η D, non-ideality Series R, Ω min typ max Intel Processor on 65 nm process 0.997 1.001 1.005 4.52 Processor Family Diode Equation η D, non-ideality Series R, Ω 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 20174943 FIGURE 7. Thermal Diode Current Paths www.national.com 22 |
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