Datasheet

[ ]
T
J
= T
A
+ T
JA
(V
DD
I
Q
) + (V
DD
- V
TEMP
) I
L
R
S
V
TEMP
SM72480
V
DD
GND
C
LOAD
>
1100 pF
OPTIONAL
BYPASS
CAPACITANCE
SM72480
SNIS156C –NOVEMBER 2010–REVISED APRIL 2013 www.ti.com
Figure 11.
C
LOAD
Minimum R
S
1.1 nF to 99 nF 3 kΩ
100 nF to 999 nF 1.5 kΩ
1 μF 800 Ω
VOLTAGE SHIFT
The SM72480 is very linear over temperature and supply voltage range. Due to the intrinsic behavior of an
NMOS/PMOS rail-to-rail buffer, a slight shift in the output can occur when the supply voltage is ramped over the
operating range of the device. The location of the shift is determined by the relative levels of V
DD
and V
TEMP
. The
shift typically occurs when V
DD
− V
TEMP
= 1.0V.
This slight shift (a few millivolts) takes place over a wide change (approximately 200 mV) in V
DD
or V
TEMP
. Since
the shift takes place over a wide temperature change of 5°C to 20°C, V
TEMP
is always monotonic. The accuracy
specifications in the Electrical Characteristics table already includes this possible shift.
Mounting and Temperature Conductivity
The SM72480 can be applied easily in the same way as other integrated-circuit temperature sensors. It can be
glued or cemented to a surface.
The best thermal conductivity between the device and the PCB is achieved by soldering the DAP of the package
to the thermal pad on the PCB. The temperatures of the lands and traces to the other leads of the SM72480 will
also affect the temperature reading.
Alternatively, the SM72480 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 SM72480 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. If moisture creates a short circuit from the V
TEMP
output to
ground or V
DD
, the V
TEMP
output from the SM72480 will not be correct. Printed-circuit coatings are often used to
ensure that moisture cannot corrode the leads or circuit traces.
The thermal resistance junction-to-ambient (θ
JA
) is the parameter used to calculate the rise of a device junction
temperature due to its power dissipation. The equation used to calculate the rise in the SM72480's die
temperature is
(8)
where T
A
is the ambient temperature, I
Q
is the quiescent current, I
L
is the load current on the output, and V
O
is
the output voltage. For example, in an application where T
A
= 30 °C, V
DD
= 5 V, I
DD
= 9 μA, Gain 4, V
TEMP
= 2231
mV, and I
L
= 2 μA, the junction temperature would be 30.021 °C, showing a self-heating error of only 0.021°C.
Since the SM72480's junction temperature is the actual temperature being measured, care should be taken to
minimize the load current that the V
TEMP
output is required to drive. If The OVERTEMP output is used with a 100
k pull-up resistor, and this output is asserted (low), then for this example the additional contribution is [(152°
C/W)x(5V)
2
/100k] = 0.038°C for a total self-heating error of 0.059°C. Table 2 shows the thermal resistance of the
SM72480.
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