The remote temperature sensor has all the advantages of other IC temperature sensors, including linear response, no calibration, and low power consumption, making it ideal for industrial applications.
To understand the concept of a remote temperature sensor and how it differs from a local temperature sensor, let's first review some of the basics of temperature sensor integrated circuits (ICs). The silicon temperature sensor integrated circuit utilizes the basic operation of the silicon PN junction as a function of temperature.
Therefore, if current is passed through two PN junctions in different regions, they will produce two different forward voltage values. When the current is constant, the voltage changes with the temperature change at the PN temperature junction. The difference between these two voltages is proportional to the absolute temperature:
After understanding the basic workings of all temperature sensor ICs, the second issue is about the difference between the remote temperature sensor and the local temperature sensor. The PN junction of the local temperature sensor is typically thermally connected to the ground pin of the package. According to the principle of thermal balance, the temperature on the PN junction is substantially the same as the temperature experienced by the printed circuit board (PCB) to which the ground pin is connected.
The remote temperature sensor senses the local temperature as it does the local temperature sensor. In addition, they sense one or more remote diodes or transistors in the system. In many cases, these remote components are located in other ICs, such as processors, application specific integrated circuits (ASICs), or field programmable gate arrays (FPGAs). The "remote diode" here is actually a PNP transistor whose collector is connected to the substrate of the device.
PNP is an intrinsic device for most CMOS processing, so it can be easily applied in any complex, high-power circuit that is prone to overheating. Or in systems that must monitor multiple point temperatures, a standard transistor (such as 2N3904 or 2N3906) can be used in each "remote" location for high efficiency.
In addition, the remote temperature sensor integrates an analog-to-digital converter (ADC) with an analog multiplexer on the input to switch between different temperature sources. This allows direct interface to the controller via a standard serial interface, such as I2C or Serial Peripheral Interface (SPI), and can transfer some of the thermal management functions, such as overheat alarms, to the temperature sensor for processing.
Remote junction temperature transistor
The remote temperature sensor can be adjusted to work with a specific type of transistor connection. Because of the different characteristics of these remote transistors/diodes in geometry processing and other variable processing, remote temperature sensors should include some methods to account for these differences when calculating remote temperatures. If different connection types are used, different transistor types must be compensated for using the "non-ideal" factor:
among them:
TCF = temperature correction factor
TCR = center of the required temperature range
ηTS = remote temperature sensor non-ideal specifications
ηProcess = non-ideal specifications for remote transistors/diodes
Many new temperature sensors, such as the TMP451 from Texas Instruments, have multiple options for non-ideal factors so that they can be used with different discrete transistors or processing node application specific integrated circuit (ASIC) integrated temperature diodes.
Another common signal conditioning feature included with remote temperature sensors is series resistance cancellation. The series resistance in the application is usually caused by the PCB trace resistance. This remote line length can be automatically offset by the remote temperature sensor to prevent possible temperature drift. Typical devices can offset serial line resistances totaling up to 1kΩ. This eliminates the need for additional characterization and temperature offset correction.
Once the processing geometry of the processor, ASIC, and FPGA reaches 90 nm or less, the characteristics of the remote sensing diode/transistor will change. Early remote temperature sensors worked by controlling the current of the emitter. The degree of change in the physical properties of these transistors as they process smaller geometries causes VBE to become a function of the collector and emitter currents. If the BETA characteristics of the transistor are independent of the collector current, then the control of the emitter current can provide sufficient accuracy.
However, these newer, smaller geometry transistors exhibit BETA characteristics associated with collector currents. Like many newer remote temperature sensors, such as TI's TMP435, an automatic BETA compensation function is achieved by controlling the collector current instead of the emitter current. The TMP435 automatically detects and selects the correct range based on the BETA factor of the external transistor.
Choose a remote temperature junction or a thermistor?
Thermistors are the most commonly used temperature measuring devices. Essentially, a thermistor is a resistive component whose resistance varies proportionally with the temperature on the device. Since the thermistor is a relatively small component, it can monitor the temperature of the object very close to the object.
In some applications, this can be a limiting factor for local temperature sensors. By replacing the thermistor with a remote transistor, a package size very similar to the thermistor can be achieved. However, this also brings many of the advantages of temperature sensor ICs, including reduced overall power consumption, reduced noise-tolerant data transmission, and simplified implementation and signal processing.
The graph above shows a comparison between the linear response of the temperature sensor and the nonlinear response of the thermistor.
The simplification of the silicon temperature sensor is better than that of the thermistor (see chart). No additional processing is required to calculate the actual temperature.
to sum up
Many industrial applications require temperature monitoring at multiple locations within the system. Whether it is to ensure system reliability or to achieve closed-loop control of some systems, accurate temperature monitoring can optimize system performance.
Remote temperature sensors have all the advantages of other IC temperature sensors, including linear response, no calibration required, and low power consumption. They can monitor the temperature of multiple locations without the need for multiple signal processing paths. Depending on the selected remote diode or transistor, the sensor can be used in different assembly configurations, and the local temperature sensor IC mounted on the board is dwarfed at this point.
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