Features of SMD NTC Thermistor

　　An NTC thermistor is a thermistor device that utilizes a semiconductor ceramic whose resistance and temperature have a negative temperature coefficient (NTC: Negative Temperature Coefficient) relationship and have a large rate of change. Taking advantage of this property, it is not only used as a temperature sensor, but also as a temperature protection device such as temperature detection and temperature compensation.

　　Temperature compensation refers to compensating for changes in electronic components and electronic circuits whose characteristics vary with temperature. For example, the operation of electronic circuits using transistors or crystal oscillators can be slightly unstable due to temperature changes. At this time, by embedding an NTC thermistor whose resistance value decreases as the temperature rises, the circuit can be kept stable.

　　NTC thermistors are divided into disc type, SMD, glass-encapsulated diodes, resin-encapsulated film wires, etc., and SMD-shaped SMD NTC thermistors manufactured by lamination process are embedded in circuits as temperature protection devices. The following introduces application examples of chip NTC thermistors used as temperature protection devices such as temperature detection and temperature compensation.

　　※The chip NTC thermistor in the text and the figure is referred to as the NTC thermistor for short. In addition, the circuit diagram is a simplified circuit diagram.

　　1 Temperature detection and temperature compensation in smartphones and tablets

　　There are multiple NTC thermistors used in smartphones or tablets for temperature detection and temperature compensation.

　　Its basic circuit is a voltage divider circuit connected in series with an NTC thermistor and a fixed resistor. The resistance value of NTC thermistors installed near heat-generating parts such as CPUs and power modules decreases as the temperature rises, so the output voltage of the voltage divider circuit changes. This change is sent to the microcontroller to protect circuit components from overheating, or to compensate for temperature.

　　2 Temperature detection in mobile device battery charging

　　In addition to the + terminal and the - terminal, there is another terminal in the battery pack (lithium-ion battery) of a mobile device such as a smartphone. That is, the terminal for temperature monitoring with names such as T terminal, and it is equipped with an NTC thermistor inside. When the battery temperature rises, the temperature of the NTC thermistor will also rise, so the resistance value will drop. When the upper limit charging temperature is exceeded, the charging control IC will stop charging. The figure below is an example of a basic circuit. Protection ICs within the battery pack measure the battery voltage to prevent overcharging or overdischarging.

　　When fast charging and other situations require more precise charging control, the NTC thermistor will be connected to the charging control IC to measure the ambient temperature.

　　3 Microcontroller temperature detection

　　Since microcontrollers such as smartphones need to be reliable, they need to be protected from overheating. The figure below shows an example of a microcontroller temperature protection circuit combining an NTC thermistor and a fixed resistor. The NTC thermistor consists of a fixed resistor RS and a voltage divider circuit. If excessive flow is passed through the circuit, the temperature of the NTC thermistor will rise and the resistance value will drop, which will suppress the drive voltage of the microcontroller. The circuit components used are small SMD chip NTC thermistors and resistors, so they can be directly mounted on the circuit board or the heating part to play an effective temperature protection role.

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