LM95235DIMMSLASHNOPB datasheet(25/36 Pages) TI

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MMBT3904

LM95235

100 pF

PROCESSOR

IE = IF

LM95235

100 pF

D+

D-

q x

x k

x ln

¸¸

¨¨

§ I 2

I 1

¸¸

¨¨

T =

q x

'VBE

K x k x ln

IF2

IF1

'VBE

IF2

IF1

LM95235

LM95235-Q1

www.ti.com

SNIS142F – APRIL 2006 – REVISED MARCH 2013

(3)

Solving Equation 3 for temperature yields:

(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 18 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.

TruTherm technology uses the transistor equation, Equation 5, which is a more accurate representation of the

topology of the thermal diode found in an FPGA or processor.

(5)

Figure 18. Thermal Diode Current Paths

TruTherm should only be enabled when measuring the temperature of a transistor integrated as shown in the

processor of Figure 18, because Equation 5 only applies to this topology.

Calculating Total System Accuracy

The voltage seen by the LM95235 also includes the IFRS 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.

Since

ΔVBE 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 temperature 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

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:

TACC = + 1.0°C + (+4.06% of 353 K) = +15.3°C

(6)

and

TACC = - 1.0°C + (-0.89% of 353 K) = -4.1°C

(7)

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