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LTM4671

Rev. B

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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 7), 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.

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:

∆VD = T(KELVIN) • KD • lN(10)

and redefining constant

K'D = KD •IN(10) =

198µV

yields

∆VD = K’D • T(KELVIN)

Solving for temperature:

T(KELVIN) =

∆VD

K'D

(°CELSIUS) = T(KELVIN)– 273.15

where

300°K = 27°C

means that is we take the difference in voltage across the

diode measured at two currents with a ratio of 10, the

resulting voltage is 198μV per Kelvin of the junction with

a zero intercept at 0 Kelvin.

The diode connected NPN transistor across the TSENSEn+

and pin and TSENSEn− pins can be used to monitor the

internal temperature of the LTM4671 channel 0 and 3.

The 5A Channels (CH1, CH2):

The LTM4671 produces a voltage at the TMON pin

proportional to the measured junction temperature. The

junction temperature-to-voltage scaling factor is 200°K/V.

Thus, to obtain the junction temperature in degrees Kelvin,

simply multiply the voltage provided at the TMON pin by

the scaling factor. To obtain the junction temperature in

degrees Celsius, subtract 273 from the value obtained in

degrees Kelvin.

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

4671 F07

125

Figure 7. Diode Voltage VD vs Temperature T(°C)

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