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TMP01ES Datasheet(PDF) 10 Page - Analog Devices |
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TMP01ES Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page TMP01 REV. C –10– 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 Over and Under 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 TMP01 is accurately measuring tempera- ture, the open-collector outputs should handle less than 20 mA of current to minimize self-heating. Clearly, the Over-temp and Under-temp outputs should not drive the equipment directly. Instead, an external switching device is required to handle the large currents. Some examples of these are relays, power MOSFETs, thyristors, IGBTs, and Darlingtons. Figure 15 shows a variety of circuits where the TMP01 controls a switch. The main consideration in these circuits, such as the relay in Figure 15a, is the current required to activate the switch. MOTOR SHUTDOWN 2604-12-311 COTO IN4001 OR EQUIV. +12V R1 R2 R3 TEMPERATURE SENSOR & VOLTAGE REFERENCE 1 2 3 4 7 HYSTERESIS GENERATOR WINDOW COMPARATOR TMP01 VPTAT VREF 8 5 6 Figure 15a. 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 TMP01’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 TMP01 out- put unless protected by a commutation diode across the coil, as shown. The relay shown has a contact rating of 10 watts maxi- mum. If a relay capable of handling more power is desired, the larger contacts will probably require a commensurately 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 TMP01 as explained in the next section. Power FETs are popular for handling a variety of high current DC loads. Figure 15b shows the TMP01 driving a p-channel MOSFET transistor for a simple heater circuit. When the out- put transistor turns on, the gate of the MOSFET is pulled down to approximately 0.6 V, turning it on. For most MOSFETs a gate-to-source voltage or Vgs on the order of –2 V to –5 V is suf- ficient to turn the device on. Figure 15c shows a similar circuit for turning on an n-channel MOSFET, except that now the gate to source voltage is positive. Because of this reason an external transistor must be used as an inverter so that the MOSFET will turn on when the “Under Temp” output pulls down. NC = NO CONNECT NC NC IRFR9024 OR EQUIV. HEATING ELEMENT 2.4k Ω (12V) 1.2k Ω (6V) 5% V+ R1 R2 R3 TEMPERATURE SENSOR & VOLTAGE REFERENCE 1 2 3 4 7 HYSTERESIS GENERATOR WINDOW COMPARATOR TMP01 VPTAT VREF 8 5 6 Figure 15b. Driving a P-Channel MOSFET IRF130 NC = NO CONNECT NC NC 2N1711 HEATING ELEMENT V+ R1 R2 R3 4.7k Ω 4.7k Ω TEMPERATURE SENSOR & VOLTAGE REFERENCE 1 2 3 4 HYSTERESIS GENERATOR WINDOW COMPARATOR TMP01 VPTAT VREF 7 8 5 6 Figure 15c. Driving a N-Channel MOSFET Isolated Gate Bipolar Transistors (IGBT) combine many of the benefits of power MOSFETs with bipolar transistors, and are used for a variety of high power applications. Because IGBTs have a gate similar to MOSFETs, turning on and off the devices is relatively simple as shown in Figure 15d. The turn on voltage for the IGBT shown (IRGBC40S) is between 3.0 and 5.5 volts. This part has a continuous collector current rating of 50 A and a maximum collector to emitter voltage of 600 V, enabling it to work in very demanding applications. IRGBC40S NC = NO CONNECT NC NC 2N1711 V+ R1 R2 R3 4.7k Ω 4.7k Ω TEMPERATURE SENSOR & VOLTAGE REFERENCE 1 2 3 4 HYSTERESIS GENERATOR WINDOW COMPARATOR TMP01 VPTAT VREF 7 8 5 6 MOTOR CONTROL Figure 15d. Driving an IGBT |
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