Integrated temperature sensor LM94022 and its application

LM94022 is an integrated temperature sensor with analog output, mainly used in mobile phones, wireless transceivers, battery management, automobiles, office equipment and household appliances. The main features of this sensor include low operating voltage, which can work at 1.5V; wide operating voltage range of -1.5~5.5V; push-pull output at the end stage, with ±50μA output current capability; four sensitivities for users to choose; measurement range of -50~+150℃; low quiescent current, typical value is 5.4μA; accuracy (related to measurement range): 20~40℃ is ±1.5℃; -70~-50℃ is ±1.8℃; -50~90℃ is ±2.1℃; -50~150℃ is ±2.7℃; small size SO70 package is used.

Pin Arrangement and Function
The pin arrangement of LM94022 is shown in Figure 1, and the functions of each pin are shown in Table 1.

Figure 1 LM94022 pinout

Table 1 LM94022 pin functions

Sensitivity selection terminals GS0 and GS1
LM94022 has 4 sensitivities for users to choose according to the different levels applied to GS0 and GS1, as shown in Table 2. Users can make reasonable choices based on the temperature measurement range and the working voltage conditions of the interface circuit. The sensitivity is determined by the levels of GS0 and GS1: the high level is required to be greater than (VDD-0.5V); the low level is required to be less than 0.5V.

Table 2 LM94022 provides four sensitivities (typical values)

The output characteristics
of LM94022 are shown in Figure 2, which is the characteristic of measuring temperature and output voltage at different sensitivities. Since the output voltage decreases with increasing temperature, its sensitivity is negative. When VDD is 5V, the output voltage at several specific temperature values ​​with different sensitivities is shown in Table 3 (typical values).

Figure 2 Output characteristics of LM94022

Table 3 Output voltage values ​​when VDD is 5V and TA is 25℃

As can be seen from Figure 2, its linearity is excellent, which is the characteristic after linearization. The sensitivity value calculated according to the data in Table 3 is slightly different from the typical sensitivity given in Table 2. For example, when GS=00, the output voltage at -25℃ is 1168mV, and the output voltage at -50℃ is 1299mV, so its average sensitivity is -5.24mV/℃; the output voltage at 50℃ is 760mV, and the output voltage at 75℃ is 619mV, so its average sensitivity is 5.64mV/℃. When GS=00 in Table 2, the sensitivity is -5.5mV/℃.

Basic application circuit
Figure 3 is the basic application circuit of LM94022. In this circuit, GS0 and GS1 are both grounded (low level), so the sensitivity is -5.5mV/℃. LM94022 is generally used for temperature measurement and control with low accuracy requirements, and its output end is often connected to a comparator or microcontroller. If the temperature sensor is far away from the control circuit, the connecting wire should be a shielded wire.

Figure 3 Basic application circuit of LM94022

The circuits for connecting capacitive loads are shown in Figures 4 and 5. The difference between Figures 4 and 5 is the different load capacitance: when the load capacitance CLOAD is 1100pF, the circuit in Figure 5 is used. The RS value in Figure 5 is related to the CLOAD value, as shown in Table 4. The connections of the GS0 and GS1 terminals that determine the sensitivity are not drawn in Figures 4 and 5.

Figure 4 Circuit diagram of capacitive load (CLOAD 1100pF)

Table 4 Relationship between CLOAD value and RS

When the LM94022 is directly interfaced with an ADC (or the ADC in a microprocessor), at the start of operation, the push-pull output of the LM94022 can charge Cin in the ADC, as shown in Figure 6.

Figure 6 LM94022 and ADC interface circuit diagram

Application Circuit Example
1 Circuit with Shutdown Control Function
LM94022 is a low-power device. To achieve multi-channel temperature measurement, shutdown control can be used. When VDD is disconnected, the OUT terminal is high impedance. You can connect an inverter (see Figure 7) or a two-input AND gate to the VDD terminal of LM94022 to achieve shutdown (see Figure 8). The difference between the two is that the former is shut down when a high level is applied, while the latter is shut down when a low level is applied.

Figure 7 LM94022 connected to an inverter to achieve shutdown function

Figure 8 LM94022 connected to two input AND gates to achieve shutdown function

Figure 9 Digital thermometer circuit

2 Digital display thermometer
Figure 9 is a digital thermometer with a measuring temperature range of -40 to +125°C. The temperature detected by LM94022 is converted into an analog signal voltage output. Its output voltage is directly connected to the microprocessor with ADC interface. The digital signal converted to ADC is processed by the microprocessor and converted into the corresponding seven-segment code, which is sent to the temperature display (digital tube). If the microprocessor is used to perform software linear compensation on the sensor, the temperature measurement accuracy can be improved. The digital key can input the alarm temperature to the microprocessor. If the detected temperature exceeds the alarm temperature, the microprocessor outputs a signal to make the alarm circuit emit an audible and visual alarm. The I/O port of the microprocessor can also output a switch control signal to achieve simple switch control of the temperature (this part is not drawn in Figure 9).

Figure 10 Simple over-threshold temperature alarm circuit

3 Simple over-threshold temperature alarm circuit
Figure 10 is a simple over-threshold temperature alarm circuit. The circuit consists of a temperature sensor, a comparator, a 4.1V reference voltage source, a transistor, a buzzer, and resistors R1 to R5.

Working principle of the circuit: If the sensitivity of the LM94022 temperature sensor has been set, the voltage value VT corresponding to the set threshold temperature TTH can be obtained from Figure 2 (or Table 3). If the influence of R3, which produces hysteresis, is not considered first, the values ​​of R1 and R2 can be obtained based on the known VT value (the R2 value is obtained after the R1 value is determined first), VT=4.1V×R2/(R1+R2).

In order to prevent the comparator output from oscillating due to the influence of noise voltage in the sensor output signal when the temperature is near the threshold temperature, a positive feedback resistor R3 is added to the comparator circuit, which generates a hysteresis voltage VHYS, and the VT value is also affected by R3 and becomes VT2. The temperature characteristics and output waveform of the improved over-threshold temperature alarm circuit are shown in Figure 11.

Figure 11 Temperature characteristics and output waveform

VHYS=VT2-VT1, where VT1 and VT2 can be calculated according to the following formula.

VT2=4.1V×R2/(R1+R2//R3)

VT1=4.1V×R2/(R2+R1//R3)

In the above formula, 4.1V is the reference voltage value. In order to reduce the influence of R3 on VT value, R3 is generally set to a larger value (such as 470kΩ～2MΩ).

The reference voltage 4.1V is divided by R1 and R2 to form a voltage VT2 which is added to the comparator’s in-phase terminal. The voltage VTEMP output by the LM94022 to measure the temperature is added to the comparator’s inverting terminal.

4 Simple temperature control circuit
If you want to control the temperature TTH in Figure 10 to remain basically stable (about ±3~±5℃ change), you can use the circuit in Figure 12 to automatically control TTH. When the temperature exceeds TTH, the comparator's VOUT outputs a high level, and after the inverter outputs a low level, the N-channel MOSFET is turned off, and the heater stops heating; when the temperature drops to (TTH-THYS), VOUT jumps from a high level to a low level, the N-channel MOSFET is turned on, and the heater heats up again, causing the temperature to rise. In this way, the temperature fluctuates in a wave-like manner above and below TTH.

Figure 12 Simple temperature control circuit

Application Notes
The design notes for applying LM94022 are as follows.
● To ensure the accuracy of the sensor output voltage, the VDD value is VDD=VOUT+1V;

● The sensitivity should be as large as possible during design to reduce the impact of noise on the output signal voltage;

● To reduce the impact of noise, a high-frequency bypass capacitor can be added to the output of the LM94022;

● When the connection between the sensor and the interface circuit is long, the connecting wire should be shielded.