With the development of electronic systems towards multifunctionality, higher performance, and smaller packaging, system heat dissipation has gradually become a factor that must be considered in the design process. Overheating the system can reduce performance, damage components, or create safety hazards. To track and reduce issues caused by system heat dissipation, it is usually necessary to monitor two parameters: continuous temperature measurement and overheating alarm.

Continuous temperature measurement allows the processor to monitor the rise or fall of system temperature and take remedial measures based on the measured temperature. For example, due to the influence of system heating, a power amplifier (PA) can display an increase in gain. The increase in gain causes the power amplifier to use more power, generate more heat, and then use higher electrical energy, which is called thermal dissipation. For example, in wireless sensor network applications, excessive gain can cause the battery to consume power faster than expected. By monitoring temperature, the processor can adjust the gain of the amplifier to ensure that the power dissipation matches the designer's expectations.

When the operating temperature of the system exceeds the set limit, the processor will receive a binary overheat alarm signal. An application example is when the temperature in the system is about to exceed the operating temperature of the component. At this point, the processor can suspend power supply to the components to prevent damage to the system due to overheating.

Discrete thermistor circuit

The traditional separated component circuit used for continuous temperature measurement and overheating alarm indication uses thermistors (thermistors) in sensor components, typically using negative temperature coefficient (NTC) thermistors. As the temperature increases, the resistance value of the NTC thermistor decreases (Figure 1).

The voltage divider directly generates an analog temperature signal as the voltage level of the thermistor temperature analog signal. RBIAS resistors can set circuit gain and keep thermistors operating within allowable power, thereby minimizing resistance errors caused by temperature. The overheating alarm is generated by connecting the output terminal of the thermistor to the input terminal of the comparator. The reference voltage is connected to the other input terminal of the comparator to set the voltage value (overheat level) at which the comparator output terminal is activated. By using a hysteresis feedback loop, the comparator is avoided from switching back and forth quickly when VTEMP is equal to VREF.

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

Integrated LM57 circuit

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 LM57 in its original position.

[Sensitive Words] Degree

One of the important measurement parameters in any temperature sensor circuit is the sensitivity (or error) of the overall circuit. When designing discrete circuit solutions, the errors of each component will accumulate to obtain the total error of the measured values. For example, the VTEMP analog temperature output in a discrete thermistor circuit (Figure 1) will be affected by the sensitivity of both the thermistor and the resistor RBIAS. The sensitivity of TOVER digital alarms is not only affected by the sensitivity of VTEMP, but also by the inherent errors of comparators, feedback resistors, and hysteresis resistors. For example, if this circuit is used to control a large HVAC system, these errors may cause the system to continue operating when it is no longer needed, resulting in excessive power generation.