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LTM4639

4639f

For more information www.linear.com/LTM4639

Temperature Monitoring

Measuring the absolute temperature of a diode is possible

due to the relationship between current, voltage and

temperature described by the classic diode equation:

ID =IS • e

η • VT













VD = η• VT •ln

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

the ideality factor (typically close to 1.0) and IS (saturation

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

APPLICATIONS INFORMATION

temperature sensors. The IS term in the equation above

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

whereKD=8.62−5,andknowingln(ID/IS)isalwayspositive

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

equation that:

VD = T(KELVIN)•KD •ln

where VD appears to increase with temperature. It is

common knowledge that a silicon diode biased with a

current source has an approximate –2mV/°C temperature

relationship (Figure 6), which is at odds with the equation.

Infact,theIStermincreaseswithtemperature,reducingthe

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

composite diode voltage slope.

Figure 6. Diode Voltage VD vs Temperature T(°C)

for Different Bias Currents

4639 F06

TEMPERATURE (°C)

–173

–73

127

0.4

0.6

0.8

1.0

∆VD

ID = 100µA

ID = 10µA

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