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TMP12 Datenblatt(PDF) 11 Page - Analog Devices |
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TMP12 Datenblatt(HTML) 11 Page - Analog Devices |
11 / 14 page 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 Ω 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. |
Ähnliche Teilenummer - TMP12 |
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Ähnliche Beschreibung - TMP12 |
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