LM99 datasheet(21/30 Pages) TI

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Vbe = K

ln (N)

k T

IF = IS e

Vbe

KVt

Vt =

k T

IF = IS e

- 1

Vbe

KVt

LM99

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SNIS129D – JANUARY 2003 – REVISED MARCH 2013

DIODE NON-IDEALITY

Diode Non-Ideality Factor Effect on Accuracy

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

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,

•

IS = Saturation Current and is process dependent,

•

If = Forward Current through the base-emitter junction

•

VBE = Base-Emitter Voltage drop

(1)

In the active region, the -1 term is negligible and may be eliminated, yielding the following equation

(2)

In the above equation,

η and IS are dependant upon the process that was used in the fabrication of the particular

diode. By forcing two currents with a very controlled ratio (N) and measuring the resulting voltage difference, it is

possible to eliminate the IS term. Solving for the forward voltage difference yields the relationship:

(3)

The voltage seen by the LM99 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°C at

room temperature of 25 °C and the process used to manufacture the diode has a non-ideality variation of ±0.1%.

The resulting accuracy of the temperature sensor at room temperature will be:

TACC = ± 1°C + (±0.1% of 298 °K) = ±1.4 °C

(4)

The additional inaccuracy in the temperature measurement caused by

η, can be eliminated if each temperature

sensor is calibrated with the remote diode that it will be paired with.

Compensating for Diode Non-Ideality

In order 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 of a

given processor type. The LM99 is calibrated for the non-ideality of the NVIDIA GeForceFX family thermal diode.

When a temperature sensor calibrated for a particular processor type is used with a different processor type or a

given processor type has a non-ideality that strays from the typical, errors are introduced.

Temperature errors associated with non-ideality may be reduced in a specific temperature range of concern

through use of the offset registers (11h and 12h). See Table 17 below.

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