LM20
1.0 LM20 Transfer Function
The LM20’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
O
= −11.69 mV/˚C x T + 1.8663 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
O
= (−3.88x10
−6
xT
2
) + (−1.15x10
−2
xT) + 1.8639
solving for T:
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
min
= −30 to T
max
= +100˚C:
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
OP
(T
max
) + V
OP
(T) + m x (T
max
+T))/2,
where:
•
V
OP
(T
max
) is the calculated output voltage at T
max
using
the parabolic transfer function for V
O
•
A linear transfer function can be used over a limited tempera-
ture range by calculating a slope and offset that give best re-
sults over that range. A linear transfer function can be calcu-
lated from the parabolic transfer function of the LM20. The
slope of the linear transfer function can be calculated using
the following equation:
m = −7.76 x 10
−6
x T − 0.0115,
V
OP
(T) is the calculated output voltage at T using the
parabolic transfer function for V
O
.
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 lin-
ear transfer function increases with wider temperature
ranges.
Temperature Range
T
min
(˚C)
−55
−40
−30
-40
−10
+35
+20
T
max
(˚C)
+130
+110
+100
+85
+65
+45
+30
Linear Equation
V
O
=
−11.79 mV/˚C x T + 1.8528 V
−11.77 mV/˚C x T + 1.8577 V
−11.77 mV/˚C x T + 1.8605 V
−11.67 mV/˚C x T + 1.8583 V
−11.71 mV/˚C x T + 1.8641 V
−11.81 mV/˚C x T + 1.8701 V
−11.69 mV/˚C x T + 1.8663 V
Maximum Deviation of Linear
Equation from Parabolic Equation
(˚C)
±
1.41
±
0.93
±
0.70
±
0.65
±
0.23
±
0.004
±
0.004
FIGURE 2. First order equations optimized for different temperature ranges.
2.0 Mounting
The LM20 can be applied easily in the same way as other
integrated-circuit temperature sensors. It can be glued or ce-
mented to a surface. The temperature that the LM20 is sens-
ing will be within about +0.02˚C of the surface temperature to
which the LM20’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 ac-
tual temperature measured would be at an intermediate tem-
perature between the surface temperature and the air tem-
perature.
To ensure good thermal conductivity the backside of the
LM20 die is directly attached to the pin 2 GND pin. The tem-
pertures of the lands and traces to the other leads of the
LM20 will also affect the temperature that is being sensed.
Alternatively, the LM20 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 LM20 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 conden-
sation can occur. Printed-circuit coatings and varnishes such
as Humiseal and epoxy paints or dips are often used to en-
sure that moisture cannot corrode the LM20 or its connec-
tions.
The thermal resistance junction to ambient (θ
JA
) is the pa-
rameter used to calculate the rise of a device junction tem-
perature due to its power dissipation. For the LM20 the
equation used to calculate the rise in the die temperature is
as follows:
T
J
= T
A
+
θ
JA
[(V
+
I
Q
) + (V
+
− V
O
) I
L
]
where I
Q
is the quiescent current and I
L
is the load current on
the output. Since the LM20’s junction temperature is the ac-
tual temperature being measured care should be taken to
minimize the load current that the LM20 is required to drive.
The tables shown in
Figure 3
summarize the rise in die tem-
perature of the LM20 without any loading, and the thermal
resistance for different conditions.
5
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