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TMP12 Datenblatt(PDF) 11 Page - Analog Devices

Teilenummer TMP12
Bauteilbeschribung  Airflow and Temperature Sensor
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TMP12
REV. 0
–11–
The on-board VREF output buffer is typically capable of 500
µA
output drive into as much as 50 pF load (max). Exceeding this
load will affect the accuracy of the reference voltage, could cause
thermal sensing errors due to excess heat build-up, and may induce
oscillations. External buffering of VREF with a low-drift voltage
follower will ensure optimal reference accuracy. Amplifiers which
offer low drift, low power consumption, and low cost appropriate
to this application include the OP284, and members of the OP113
and OP193 families.
With excellent drift and noise characteristics, VREF offers a good
voltage reference for data acquisition and transducer excitation ap-
plications as well. Output drift is typically better than
10 ppm/
°C,
with 315 nV/Hz (typ) noise spectral density at 1 kHz.
Preserving Accuracy Over Wide Temperature Range Operation
The TMP12 is unique in offering both a wide-range temperature
sensor and the associated detection circuitry needed to implement
a complete thermostatic control function in one monolithic device.
The voltage reference, setpoint comparators, and output buffer
amplifiers have been carefully compensated to maintain accuracy
over the specified temperature ranges in this application. Since the
TMP12 is both sensor and control circuit, in many applications the
external components used to program and interface the device are
subjected to the same temperature extremes. Thus, it is necessary
to place components in close thermal proximity minimizing large
temperate differentials, and to account for thermal drift errors
where appropriate, such as resistor matching temperature coeffi-
cients, amplifier error drift, and the like. Circuit design with the
TMP12 requires a slightly different perspective regarding the ther-
mal behavior of electronic components.
PC Board Layout Considerations
The TMP12 also requires a different perspective on PC board lay-
out. In many applications, wide traces and generous ground planes
are used to extract heat from components. The TMP12 is slightly
different, in that ideal path for heat is via the cooling system air
flow. Thus, heat paths through the PC traces should be minimized.
This constraint implies that minimum pad sizes and trace widths
should be specified in order to reduce heat conduction. At the
same time, analog performance should not be compromised. In
particular, the bottom of the setpoint resistor ladder should be
located as close to GND as possible, as discussed in the Under-
standing Error Sources section of this data sheet.
Thermal Response Time
The time required for a temperature sensor to settle to a
specified accuracy is a function of the thermal mass of the
sensor, and the thermal conductivity between the sensor and
the object being sensed. Thermal mass is often considered
equivalent to capacitance. Thermal conductivity is commonly
specified using the symbol Q, and is the inverse of thermal
resistance. It is commonly specified in units of degrees per
watt of power transferred across the thermal joint. Figures 3
and 5 illustrate the typical RC time constant response to a
step change in ambient temperature. Thus, the time required
for the TMP12 to settle to the desired accuracy is dependent
on the package selected, the thermal contact established in
the particular application, and the equivalent thermal con-
ductivity of the heat source. For most applications, the
settling-time is probably best determined empirically.
Switching Loads with the Open-Collector Outputs
In many temperature sensing and control applications some
type of switching is required. Whether it be to turn on a
heater when the temperature goes below a minimum value
or to turn off a motor that is overheating, the open-collector
outputs can be used. For the majority of applications, the
switches used need to handle large currents on the order of
1 Amp and above. Because the TMP12 is accurately mea-
suring temperature, the open-collector outputs should
handle less than 20 mA of current to minimize self-heating.
Clearly, the trip point outputs should not drive the equip-
ment directly. Instead, an external switching device is
required to handle the large currents. Some examples of
these are relays, power MOSFETs, thyristors, IGBTs, and
Darlington transistors.
This section shows a variety of circuits where the TMP12
controls a switch. The main consideration in these circuits,
such as the relay in Figure 23, is the current required to ac-
tivate the switch.
HYSTERESIS
GENERATOR
WINDOW
COMPARATOR
VPTAT
VREF
100
MOTOR
SHUTDOWN
2604-12-311
COTO
IN4001
OR EQUIV
+12V
R1
R2
R3
1
2
3
4
8
5
7
6
TMP12
NC
+12 V
140
TEMPERATURE
SENSOR &
VOLTAGE
REFERENCE
Figure 23. Reed Relay Drive
It is important to check the particular relay you choose to
ensure that the current needed to activate the coil does not
exceed the TMP12’s recommended output current of
20 mA. This is easily determined by dividing the relay coil
voltage by the specified coil resistance. Keep in mind that
the inductance of the relay will create large voltage spikes
that can damage the TMP12 output unless protected by a
commutation diode across the coil, as shown. The relay
shown has contact rating of 10 Watts maximum. If a relay
capable of handling more power is desired, the larger con-
tacts will probably require a commensurably larger coil,
with lower coil resistance and thus higher trigger current.
As the contact power handling capability increases, so does
the current needed for the coil, In some cases an external
driving transistor should be used to remove the current load
on the TMP12 as explained in the next section.


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