Data Sheet

LTM4640

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Rev. 0

23 of 38

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 12.

������������ = ������������ × ������

������������

η×������������

or

������������ = η × ������������ × ������������

������������

������������

(12)

where ID is the diode current, VD is the diode voltage, η is the ideality factor (typically close to 1), and IS (saturation

current) is a process-dependent parameter. The VT is broken out with Equation 13.

������������ =

������ × ������

q

(13)

where T is the diode junction temperature in Kelvin, q is the electron charge, and K is Boltzmann’s constant. The VT

is approximately 26mV at room temperature (298K) and scales linearly with Kelvin temperature. It is this linear

temperature relationship that makes diodes suitable temperature sensors. The IS term in Equation 13 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 must always be less than ID. Combining all the

constants into one term (see Equation 14).

������������ =

η × k

������

(14)

where KD = 8.62 × 10−5 V/K, assuming the ideality factor is 1 and knowing ln (ID/IS) is always positive because ID is

always greater than IS, leaves us with the results given in Equation 15.

������������ = ������(������������������������������������) × ������������ × ������������

������������

������������

(15)

It should be noted that as the temperature increases, the IS term increases more quickly, and thus, the ln (ID/IS)

absolute value reduces faster, yielding a negative coefficient of diode voltage vs. temperature, which is an

approximate –2mV/°C temperature relationship as shown in Figure 24.

Figure 24. Diode Voltage VD vs. Temperature T(°C)

TEMPERATURE (°C)

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–25

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