LM20EP Transfer Function

The LM20EP’s transfer function can be described in different

ways with varying levels of precision. A simple linear transfer

function, with good accuracy near 25˚C, is

V

Over the full operating temperature range of −55˚C to

+130˚C, best accuracy can be obtained by using the para-

bolic transfer function

V

solving for T:

A linear transfer function can be used over a limited tempera-

ture range by calculating a slope and offset that give best

results over that range. A linear transfer function can be

calculated from the parabolic transfer function of the

LM20EP. The slope of the linear transfer function can be

calculated using the following equation:

m = −7.76 x 10

where T is the middle of the temperature range of interest

and m is in V/˚C. For example for the temperature range of

T

T=35˚C

and

m = −11.77 mV/˚C

The offset of the linear transfer function can be calculated

using the following equation:

b=(V

,

where:

•

V

the parabolic transfer function for V

O

•

V

parabolic transfer function for V

Using this procedure the best fit linear transfer function for

many popular temperature ranges was calculated in Figure

2. As shown in Figure 2 the error that is introduced by the

linear transfer function increases with wider temperature

ranges.

Mounting

The LM20EP can be applied easily in the same way as other

integrated-circuit temperature sensors. It can be glued or

cemented to a surface. The temperature that the LM20EP is

sensing will be within about +0.02˚C of the surface tempera-

ture to which the LM20EP’s leads are attached to.

This presumes that the ambient air temperature is almost the

same as the surface temperature; if the air temperature were

much higher or lower than the surface temperature, the

actual temperature measured would be at an intermediate

temperature between the surface temperature and the air

temperature.

To ensure good thermal conductivity the backside of the

LM20EP die is directly attached to the pin 2 GND pin. The

tempertures of the lands and traces to the other leads of the

LM20EP will also affect the temperature that is being

sensed.

Alternatively, the LM20EP can be mounted inside a sealed-

end metal tube, and can then be dipped into a bath or

screwed into a threaded hole in a tank. As with any IC, the

LM20EP and accompanying wiring and circuits must be kept

insulated and dry, to avoid leakage and corrosion. This is

especially true if the circuit may operate at cold temperatures

where condensation can occur. Printed-circuit coatings and

varnishes such as Humiseal and epoxy paints or dips are

often used to ensure that moisture cannot corrode the

LM20EP or its connections.

The thermal resistance junction to ambient (

θ

rameter used to calculate the rise of a device junction tem-

perature due to its power dissipation. For the LM20EP the

equation used to calculate the rise in the die temperature is

as follows:

T

θ

where I

the output. Since the LM20EP’s junction temperature is the

actual temperature being measured care should be taken to

minimize the load current that the LM20EP is required to

drive.

The tables shown in Figure 3 summarize the rise in die

temperature of the LM20EP without any loading, and the

thermal resistance for different conditions.

Temperature Range

Linear Equation

V

Maximum Deviation of Linear Equation

from Parabolic Equation (˚C)

T

T

−55

+130

−11.79 mV/˚C x T + 1.8528 V

−40

+110

−11.77 mV/˚C x T + 1.8577 V

−30

+100

−11.77 mV/˚C x T + 1.8605 V

-40

+85

−11.67 mV/˚C x T + 1.8583 V

−10

+65

−11.71 mV/˚C x T + 1.8641 V

+35

+45

−11.81 mV/˚C x T + 1.8701 V

+20

+30

−11.69 mV/˚C x T + 1.8663 V

FIGURE 2. First order equations optimized for different temperature ranges.

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