Rainbow Electronics LM19 - Manual (Page 5)

1.0 LM19 Transfer Function
(Continued)
2.0 Mounting
The LM19 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 LM19 is
sensing will be within about +0.02˚C of the surface tempera-
ture to which the LM19’s leads are attached.
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
LM19 die is directly attached to the GND pin. The temper-
tures of the lands and traces to the other leads of the LM19
will also affect the temperature that is being sensed.
Alternatively, the LM19 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 LM19 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
ensure that moisture cannot corrode the LM19 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 LM19 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 LM19’s junction temperature is the
actual temperature being measured care should be taken to
minimize the load current that the LM19 is required to drive.
The tables shown in Figure 3 summarize the rise in die
temperature of the LM19 without any loading, and the ther-
mal resistance for different conditions.
3.0 Capacitive Loads
The LM19 handles capacitive loading well. Without any pre-
cautions, the LM19 can drive any capacitive load less than
300 pF as shown in Figure 4. Over the specified temperature
range the LM19 has a maximum output impedance of 160
Ω.
In an extremely noisy environment it may be necessary to
add some filtering to minimize noise pickup. It is recom-
mended that 0.1 µF be added from V
+ to GND to bypass the
power supply voltage, as shown in Figure 5. In a noisy
environment it may even be necessary to add a capacitor
from the output to ground with a series resistor as shown in
Figure 5. A 1 µF output capacitor with the 160
Ω maximum
output impedance and a 200 Ω series resistor will form a 442
Hz lowpass filter. Since the thermal time constant of the
LM19 is much slower, the overall response time of the LM19
will not be significantly affected.
Temperature Range Linear Equation
V
O
=
Maximum Deviation of Linear Equation
from Parabolic Equation (˚C) T
min
(˚C) T
max
(˚C)
−55 +130 −11.79 mV/˚C x T + 1.8528 V ± 1.41
−40 +110 −11.77 mV/˚C x T + 1.8577 V ± 0.93
−30 +100 −11.77 mV/˚C x T + 1.8605 V ± 0.70
-40 +85 −11.67 mV/˚C x T + 1.8583 V ± 0.65
−10 +65 −11.71 mV/˚C x T + 1.8641 V ± 0.23
+35 +45 −11.81 mV/˚C x T + 1.8701 V ± 0.004
+20 +30 −11.69 mV/˚C x T + 1.8663 V ± 0.004
FIGURE 2. First Order Equations Optimized For Different Temperature Ranges.
TO-92 TO-92
no heat sink small heat fin
θ
JA
T
J
− T
A
θ
JA
T
J
− T
A
(˚C/W) (˚C) (˚C/W) (˚C)
Still air 150 TBD TBD TBD
Moving air TBD TBD TBD TBD
FIGURE 3. Temperature Rise of LM19 Due to
Self-Heating and Thermal Resistance ( θ
JA
)
20004015
FIGURE 4. LM19 No Decoupling Required for
Capacitive Loads Less than 300 pF.
LM19
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