Abstract: This paper introduces the working principle and control point setting method of the TMP01 programmable temperature controller, and analyzes the typical application circuits of TMP01 in temperature control and transmission. Keywords: Temperature controller; AD654VFC; Darlington transistor 1 Introduction The TMP01 temperature controller is a low-power programmable temperature controller manufactured by Analog Devices (ADI). It is available in 8-pin dual in-line package and surface mount package, with an operating temperature range of -55℃ to +85℃ and a single voltage operation of 4.5V to 13.2V. This chip can generate a DC voltage signal proportional to temperature. When the measured temperature is higher or lower than the set temperature trigger point, a control signal is generated from one of the two output terminals. The temperature point can be set by selecting an external resistor. The temperature control accuracy of this controller can reach ±1℃, and the control signal output load capacity is up to 20mA. It can be used to control a variety of devices. 2. Working Principle The TMP01 is a linear voltage output temperature sensor. Its operating principle is shown in Figure 1. The voltage reference terminal VREF outputs a low-drift 2.5V reference voltage, and the VPTAT terminal outputs a voltage signal proportional to the absolute temperature with a temperature coefficient of 5mV/K, which is 1.49V at 25℃. Two comparators compare the external high and low temperature setpoint voltages (SETHIGH, SETLOW) with the internal temperature sensor voltage, respectively. When the temperature measured by the internal sensor is higher or lower than the temperature trigger point set by the external resistor, an open-circuit output signal is generated at the OVER or UNDER terminal, activating the external heating or cooling device, thus achieving dual-temperature automatic control. The TMP01 temperature controller incorporates a temperature hysteresis module. A hysteresis offset voltage is generated at the comparator input through a 1kΩ resistor to achieve temperature hysteresis. The magnitude of the internal hysteresis current (IHYS) is controlled by selecting an appropriate external resistor, thus setting the degree of hysteresis. After setting the hysteresis temperature, the comparator output can only be started or reset when the input of the internal comparator is equal to the sum of the temperature sensor voltage and the hysteresis offset voltage. The relationship between the internal hysteresis current (IHYS) and the external temperature setting reference terminal current (IVREF) is as follows: IHYS = IVREF = 5μA/℃ + 7μA. Since VREF = 2.5V, the temperature hysteresis is 0℃ when the reference load resistance is 357kΩ or greater (output current is 7μA or less). If the load resistance is larger, it will only further reduce the output current and the internal hysteresis current, without affecting the temperature setting. The 2.5V low drift reference output of the TMP01 temperature controller can be easily divided externally with a resistor or potentiometer to accurately set the heating/cooling point that is not affected by temperature. Alternatively, other voltage sources can be used to replace the reference voltage. 3. Method for setting the temperature control point To set the temperature control point of the TMP01, the following steps are taken: (1) Select the hysteresis temperature. (2) Calculate the hysteresis current IHYS. (3) Select the temperature control point. (4) Calculate the resistance distribution values. The following example illustrates the calculation method for temperature setting, with parameters shown in Figure 2. Design requirements: Set the hysteresis temperature to 2℃. When the temperature is higher than 25℃ or lower than 0℃, TMP01 outputs control signals respectively. Application example analysis and precautions: When directly driving a relay with TMP01, the current should not exceed 20mA (the current can be determined by dividing the relay coil voltage by the resistance). Driving high-power equipment should be achieved by driving relays, high-power MOSFETs, semiconductor thyristors, and Darlington transistors. To avoid voltage sparks generated by the relay coil induction, a diode should be connected across the relay coil. In DC circuits, high-power thyristors are usually used instead of relays. When TMP01 is used as a high-current switch, self-heating caused by the large load at the output end will bring temperature errors. An external transistor can be used to remove the load at the output end to avoid self-heating. By selecting an external transistor to shunt most of the current, TMP01 can be used to control many high-voltage equipment. Figure 3 shows a Darlington transistor circuit using TMP01 to control a high-current switch. Besides automatically controlling heating/cooling devices, the TMP01 can also output a voltage signal proportional to temperature. Converting this voltage to a digital frequency signal enables long-distance transmission of temperature measurements. Figure 4 shows a circuit that converts the temperature signal measured by the TMP01 into a frequency signal for long-distance transmission. The temperature-proportional analog voltage (VPTAT) is input to the non-inverting input of the AD654. The input stage, consisting of an input amplifier and an NPN transistor follower, converts the analog input voltage into a drive current proportional to it. This drive current simultaneously charges the timing capacitor CT, and the oscillation frequency (output frequency) of the multivibrator is proportional to this charging current. The AD654 outputs a square wave signal proportional to the input analog voltage signal, as shown in the following formula: A TMP01 voltage temperature coefficient of 5mV/℃ corresponds to an output frequency of 25Hz/℃, which is 7.5kHz at 25℃. At the signal receiving end, the AD650 can convert the frequency signal into a DC voltage signal, thus enabling long-distance temperature transmission. Error sources affecting the temperature control accuracy of the TMP01 include internal and external error sources. Internal error sources include initial tolerance, reference voltage temperature drift, set point comparator input offset voltage and bias current, and hysteresis current calibration coefficient. External error sources mainly come from resistor accuracy and grounding error voltage. In actual use, the main consideration is to reduce the influence of external error sources. External resistor error will directly affect the accuracy of the set point. Especially when setting a fixed temperature, a resistor with a suitable temperature coefficient should be selected, the resistor temperature drift should be fully considered, and the effects of circuit board layout, component placement and leakage current should be taken into account to reduce common thermal error sources. The bottom of the resistor divider should be as close to ground as possible to reduce voltage drop and coupling of external noise sources. When using an external power supply to power the chip, it is best to connect a 0.1μF bypass capacitor. References [1] LOW POWER PROGRAMMABLE TEMPERATURE CONTROLLERTMP01 [2] Song Jiayou. Integrated Electronic Circuit Design Manual [3] Li Hua. Practical Interface Technology of MCS-51 Series Microcontrollers