As electronic systems become more versatile, higher performance, and smaller packages, system cooling issues are increasingly a factor in design. Overheating of the system can degrade performance, damage components, or create a safety hazard. To track and reduce problems caused by system cooling, it is often necessary to monitor two parameters: continuous temperature measurement and overheat alarm.

Continuous temperature measurement allows the processor to monitor the rise or fall of the system temperature and take corrective action based on the measured temperature. For example, since the power amplifier (PA) is affected by the temperature rise of the system, it can show an increase in gain. The increase in gain causes the power amplifier to use more power, generate more heat, and then use higher power, which is called thermal runaway. For example, in wireless sensor network applications, excessive gain can cause the battery to consume more power than expected. By monitoring the temperature, the processor can adjust the gain of the amplifier to ensure that the power dissipation is consistent with the designer's expectations.

The processor receives a binary overheat warning signal when the system operating temperature exceeds the set limit. An example of an application is when the temperature in the system is about to exceed the maximum operating temperature of the component. At this point, the processor can suspend power to the component to prevent the system from being damaged due to overheating.

Discrete thermistor circuit

Conventional discrete component circuits for continuous temperature measurement and overheat alarm indication use a thermistor (thermistor) in the sensor element, typically using a negative temperature coefficient (NTC) thermistor. As the temperature increases, the resistance of the NTC thermistor decreases (Figure 1).

Figure 1: A circuit using a conventional thermistor. The processor's analog to digital converter is used to acquire the temperature analog voltage (VTEMP). When the temperature exceeds the threshold, the output of the digital comparator drives the input of the processor for a prompt.

The voltage divider directly derives the analog temperature signal as the voltage level of the thermistor temperature analog signal. The RBIAS resistor sets the circuit gain and keeps the thermistor operating within the allowed power, minimizing temperature-induced resistance errors. An overheat alarm is generated by connecting the output of the thermistor to the input of the comparator. The reference voltage is connected to the other input of the comparator to set the voltage value (overheat level) at which the comparator output is activated. A hysteresis feedback loop is used to prevent the comparator from switching back and forth quickly when VTEMP is equal to VREF.

But there are many design issues with discrete thermistor solutions. The LM57's integrated analog temperature sensor and temperature switch solve these design problems and improve system performance.

Integrated LM57 circuit

The LM57 not only integrates the functions of discrete thermistor circuits, but also improves its performance. As shown in Figure 2, we can see that the number of components has decreased, but the functionality has increased. For example, the low state trip point output and input pins allow the system to test the functionality of the LM57 in its home position.

Figure 2: LM57 integrated circuit application. The processor's analog to digital converter is used to acquire the temperature analog voltage (VTEMP). When the temperature exceeds the threshold, the overtemperature (TOVER) output drives the input of the processor for indication. The trip point is set by two passive resistors (RSENSE1 and RSENSE2) instead of the active reference and bias resistors.

Accuracy

One of the most important measurement parameters in any temperature sensor circuit is the accuracy (or error) of the overall circuit. When designing a discrete circuit solution, the error of each component is added to the maximum total error of the measured value. For example, the VTEMP analog temperature output in a discrete thermistor circuit (Figure 1) will be affected by both the thermistor and the resistance of the resistor RBIAS. The accuracy of the TOVER digital alarm is not only affected by the accuracy of the VTEMP, but also by the inherent errors of the comparator, feedback resistor, and hysteresis resistor. For example, if you use this circuit to control a large HVAC system, these errors can cause large systems to continue to operate when they do not need to work, causing the system to generate excessive power.

The LM57 is fully integrated (Figure 3), and all component inputs and outputs are included in the LM57's proofreading process, so the sources of error mentioned above are not generated. At the same time, the system designer does not need to accumulate the errors of the constituent components to arrive at the total error. The LM57 guarantees a maximum error of ±0.7°C for the VTEMP analog output and ±1.5°C for the TOVER alarm output.

Figure 3: Functional block diagram of the LM57 integrated analog temperature sensor and temperature switch.

Another source of error for NTC circuits is the error of VTRIP. One way to minimize this error is to use a high precision reference. However, the input from the comparator collects noise from the reference. The trip point of the comparator will vary with the level of the signal produced by the noise. The LM57 solves this problem with a patented technology. The user can set the value of VTRIP by selecting the values ​​of the two resistors RSENSE1 and RSENSE2. The LM57 uses a digital to analog converter to determine the trip voltage range. As long as the voltage in the sensing line is within the specified range, the trip temperature will not change. This means that the LM57 sense input is not affected by the right amount of noise at the input. This also means that as long as the tolerance of the resistor is 1% or lower, the trip point of each resistor does not change.

Linearity and conversion noise

Maximum accuracy in sensor measurements requires attention to quantify noise errors, which are errors resulting from the conversion of analog signals to binary data. The analog signal is digitized to produce a digital value that is close to the actual measured analog value. The minimum increment of digital measurement (LSB) is the voltage obtained by dividing the analog-to-digital converter reference voltage by the countable number of codes of the analog-to-digital converter. For example, an 8-bit analog-to-digital converter using a 2.56V reference produces an LSB value of 2.56V ÷ 28 = 10mV. Any difference between the measured analog and digital values ​​will be referred to as the error in the conversion, which is referred to as the conversion noise or conversion error. For example, if an attempt is made to acquire a 1.384V signal, this signal is digitized to obtain a value close to 10mV, assuming a value of 1.380V, the sampled value has a converted noise value of 4mV. For a more detailed discussion of conversion noise, see the article "The ABCs of ADCs" on National.com.

So what does this noise mean in temperature error? The answer depends on the gain of the sensor output. The greater the gain of the sensor, the less it is affected by noise—the higher the sensor gain, the less error the quantization noise produces. As shown in Figure 4, it can be seen that when the trip temperature is set to 100 °C, the VTEMP analog output of the LM57 has a good linear relationship with the typical gain value of -10.4 mV/°C (actually, the LM57 has 4 possible gains). This depends on the selected trip point value, but in this case we choose 100 °C). This means that the effect of noise per millivolt of noise is 0.097 ° C / mV. Also at 100 ° C, the 1 mV noise at the thermistor output will produce an error of 1.7 ° C (the NCP15XH103 thermistor and the 6.2 kΩ bias resistor are used in this simulation).

Figure 4: Noise sensitivity comparison of the LM57 and NTC thermistors (Murata NCP15XH103F).

range of working temperature

Another advantage of the LM57 is that it has a wider usable operating temperature range than a thermistor. As shown in Figure 4, the LM57 can operate in the temperature range of -50 ° C to 150 ° C. This thermistor is specified over the -40°C to 125°C temperature range, but its usable range is close to -20°C to 100°C. With a linear output value in this range, there is no need to optimize the circuit for a narrower, higher temperature range; the LM57 has excellent accuracy and noise tolerance at 140 °C.

Design time and board space

In today's shorter product development cycle, the integrated LM57 can add value by reducing design time. The LM57 can be integrated into the circuit and connected to the processor using a simple design optimization method. No component matching, sequence error, etc. are considered.

Due to the single package, the small size saves board space and production costs and improves quality. Combining multiple components in a discrete solution will take up more board space because of the need to maintain a minimum spacing between components. Each time a new component is added to the design, the cost of placing the component in the circuit is added to the cost of the product. Each add-on requires the addition of one device and two or more wires, so more issues need to be considered in the design.

Summary of this article

The integrated LM57 analog temperature sensor and temperature switch combine not only the advantages of traditional temperature sensors and comparator circuits, but also more features and performance than discrete solutions. To improve system performance and reduce design time, the LM57 is the best choice.

Concerned about surprises

Label: Integrated temperature sensor solves heat dissipation problems

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