8.2.1 Design Requirements

The LM95214 operates only as a slave device and communicates with the host through the SMBus serial interface essentially compatible with I²C. SMBCLK is the clock input pin, SMBDATA is a bidirectional data pin, and TCRIT1, TCRIT2, TCRIT3 are the output pins. The LM95214 requires a pullup resistor on the SMBCLK, SMBDATA, and TCRIT1, TCRIT2, TCRIT3 pins due to an open-drain output. It is very important to consider the pullup resistor for the I²C systems. The recommended value for the pullup resistors is in Figure 25. Use a ceramic capacitor type with a temperature rating from –40°C to +125°C, placed as close as possible to the V_DD pin of the LM95214. The decoupling capacitor reduces any noise induced by the system. A0 (pin 6) can be connected to either Low, Mid-Supply or High voltages for address selection for configuring three possible unique slave ID addresses; SMBus Interface explains the addressing scheme.

8.3.1 Diode Non-Ideality Factor Effect on Accuracy

When a transistor is connected as a diode, the following relationship holds for variables V_BE, T and I_F:

Equation 1.

where

• 
• q = 1.6 × 10⁻¹⁹ Coulombs (the electron charge)
• T = Absolute Temperature in Kelvin
• k = 1.38 × 10⁻²³ joules/K (Boltzmann's constant)
• η is the non-ideality factor of the process the diode is manufactured on
• I_S = Saturation Current and is process dependent
• I_f = Forward Current through the base-emitter junction
• V_BE = Base-Emitter Voltage drop

In the active region, the –1 term is negligible and may be eliminated, yielding Equation 2

Equation 2.

In Equation 2,  η and I_S are dependant upon the process that was used in the fabrication of the particular diode. By forcing two currents with a very controlled ratio(I_F2 / I_F1) and measuring the resulting voltage difference, it is possible to eliminate the I_S term. Solving for the forward voltage difference yields the relationship:

Equation 3.

Solving Equation 3 for temperature yields:

Equation 4.

Equation 4 holds true when a diode connected transistor such as the MMBT3904 is used. When this diode equation is applied to an integrated diode such as a processor transistor with its collector tied to GND as shown in Figure 26 it will yield a wide non-ideality spread. This wide non-ideality spread is not due to true process variation but due to the fact that Equation 4 is an approximation.

Texas Instruments invented TruTherm beta cancellation technology that uses the transistor equation, Equation 5, which is a more accurate representation of the topology of the thermal diode found in some sub-micron FPGAs or processors.

Equation 5.

Figure 26. Thermal Diode Current Paths

TruTherm technology can be found in the LM95234 four channel remote diode sensor that is pin and register compatible with the LM95214. The LM95214 does not support this technology.

8.3.2 Calculating Total System Accuracy

The voltage seen by the LM95214 also includes the I_FR_S voltage drop of the series resistance. The non-ideality factor, η, is the only other parameter not accounted for and depends on the diode that is used for measurement. Because ΔV_BE is proportional to both η and T, the variations in η cannot be distinguished from variations in temperature. Because the non-ideality factor is not controlled by the temperature sensor, it will directly add to the inaccuracy of the sensor. For the for Intel processor on 65 nm 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 example, 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 processor temperature being sensed will be:

and

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 MMBT3904 transistor, this is specified at 0 Ω typical. The LM95214 accommodates the typical series resistance of a circuit with the offset register compensation. The error that is not accounted for is the spread of the thermal diodes series resistance. If a circuit has a series resistance spread that is 2.79 Ω to 6.24 Ω or 4.515 Ω ±1.73 Ω, the 4.515 Ω can be cancelled out with the offset register setting. The ±1.73 Ω spread cannot be cancelled out. The equation to calculate the temperature error due to series resistance (T_ER) for the LM95214 is simply:

Equation 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 bulk of the error caused by the 4.515 Ω will cause a positive offset in the temperature reading of 2.79°C, which can be cancelled out by setting the offset register to –2.75°C. The spread in error cannot be canceled out, as it would require measuring each individual thermal diode device. 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. Because 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 LM95214.

8.3.3 Compensating for Different Non-Ideality

To compensate for the errors introduced by non-ideality, the temperature sensor is calibrated for a particular processor. Texas Instruments temperature sensors are always calibrated to the typical non-ideality and series resistance of a given transistor type. The LM95214 is calibrated for the non-ideality factor and series resistance values of the MMBT3904 transistor without the requirement for additional trims. When a temperature sensor calibrated for a particular thermal diode type is used with a different thermal diode type, additional errors are introduced.

Temperature errors associated with non-ideality of different processor types may be reduced in a specific temperature range of concern through use of software calibration. Typical Non-ideality specification differences cause a gain variation of the transfer function, therefore the center of the temperature range of interest must be the target temperature for calibration purposes. The Equation 7 can be used to calculate the temperature correction factor (T_CF) required to compensate for a target non-ideality differing from that supported by the LM95214.

Equation 7.

where

• η_S = LM95214 non-ideality for accuracy specification
• η_PROCESSOR = Processor thermal diode typical non-ideality
• T_CR = center of the temperature range of interest in °C

The correction factor must be directly added to the temperature reading produced by the LM95214. For example when using the LM95214, with the 3904 mode selected, to measure a AMD Athlon processor, with a typical non-ideality of 1.008, for a temperature range of 60°C to 100°C the correction factor would calculate to:

Equation 8.

Therefore, 1.75°C must be subtracted from the temperature readings of the LM95214 to compensate for the differing typical non-ideality target.