x

PCB

R

¸

¹

·

:

62

.

0

=

ER

T

C

º

¨

©

§

MMBT3904

LM96063

100 pF

PROCESSOR

IR

IF

IC IR

IE = IF

LM96063

100 pF

2

2

3

3

D+

D+

D-

D-

LM96063

www.ti.com

SNIS149B – NOVEMBER 2011 – REVISED MARCH 2013

Equation 8 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 14 it may 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 8 is an approximation.

Figure 14. Thermal Diode Current Paths

Calculating Total System Accuracy

The voltage seen by the LM96063 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. 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 ±0.39%. The

resulting system accuracy of the processor temperature being sensed will be:

TACC = ±0.75°C + (±0.39% of 353 K) = ± 2.16 °C

(9)

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 and is in the range

of several ohms. The LM96063 does not need to be compensated for series resistance errors if the manufacturer

of the thermal diode says that the diode they provide matches the performance of an MMBT3904. If they provide

a typical series resistance number that does not have to be compensated for because it is taken into account.

The error that is not accounted for is the spread of the processor's series resistance and additional PCB trace

resistance. The equation used to calculate the temperature error (TER) due to thermal diode series resistance

variation is simply:

(10)

Solving Equation 10 for RPCB equal to -1.5Ω to 2.5Ω results in the additional error due to the spread in this series

resistance of -0.93°C to +1.55°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 10 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 LM96063 using the Remote

Temperature Offset register.

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