LTM4646 datasheet(17/30 Pages) AD

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LTM4638

Rev. C

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APPLICATIONS INFORMATION

Temperature Monitoring

Measuring the absolute temperature of a diode is possible

due to the relationship between current, voltage and tem-

perature described by the classic diode equation:

ID =IS •e

η• VT

⎛

⎝⎜

⎞

⎠⎟

VD = η• VT •In

where ID is the diode current, VD is the diode voltage, η

is the ideality factor (typically close to 1.0) and IS (satura-

tion current) is a process dependent parameter. VT can be

broken out to:

VT =

k • T

where T is the diode junction temperature in Kelvin, q is

the electron charge and k is Boltzmann’s constant. VT is

approximately 26mV at room temperature (298K) and

scales linearly with Kelvin temperature. It is this linear

temperature relationship that makes diodes suitable tem-

perature sensors. The IS term in the previous equation is

the extrapolated current through a diode junction when

the diode has zero volts across the terminals. The IS term

varies from process to process, varies with temperature,

and by definition must always be less than ID. Combining

all of the constants into one term:

KD =

η•k

where KD = 8.62−5, and knowing ln(ID/IS) is always posi-

tive because ID is always greater than IS, leaves us with

the equation that:

VD = T KELVIN

(

)•KD •In

where VD appears to increase with temperature. It is com-

mon knowledge that a silicon diode biased with a current

source has an approximate –2mV/°C temperature rela-

tionship (Figure 8), which is at odds with the equation. In

fact, the IS term increases with temperature, reducing the

ln(ID/IS) absolute value yielding an approximate –2mV/°C

composite diode voltage slope.

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

4638 F08

125

Figure 8. Diode Voltage VD vs Temperature T(°C)

To obtain a linear voltage proportional to temperature

we cancel the IS variable in the natural logarithm term to

remove the IS dependency from the equation 1. This is

accomplished by measuring the diode voltage at two cur-

rents I1, and I2, where I1 = 10 • I2) and subtracting we get:

VD =T(KELVIN)•KD•IN

−

T(KELVIN)•KD•IN

Combining like terms, then simplifying the natural log

terms yields:

ΔVD = T(KELVIN) • KD • lN(10)

and redefining constant

K'D = KD •IN(10) =

198µV

yields

ΔVD = K’D • T(KELVIN)

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