8 typical application examples of quad voltage comparator LM339

　　Abstract: There are four independent voltage comparators inside the LM339 integrated block. The characteristics of this voltage comparator are: 1) small offset voltage, typical value is 2mV; 2) wide power supply voltage range, single power supply is 2-36V, dual power supply The power supply voltage is ±1V-±18V; 3) The internal resistance limit of the comparison signal source is wider; 4) The common mode range is large, 0 ~ (Ucc-1.5V) Vo; 5) The differential input voltage range is large , large enough to be equal to the power supply voltage; 6) The output potential can be selected flexibly and conveniently.

　　Keywords: Voltage comparator LM339 Typical application example 
    
     LM339 integrated block adopts C-14 package. Figure 1 shows the appearance and pin arrangement. Due to the flexible use and wide application of LM339, major IC manufacturers and companies in the world have launched their own four comparators, such as IR2339, ANI339, SF339, etc. Their parameters are basically the same and can be used interchangeably.

figure 1

　　 The LM339 is similar to an op amp with non-adjustable gain. Each comparator has two inputs and one output. One of the two input terminals is called the non-inverting input terminal, represented by "+", and the other is called the inverting input terminal, represented by "-". When used to compare two voltages, add a fixed voltage to any input end as the reference voltage (also called the threshold level, which can select any point in the LM339 input common mode range), and add a signal voltage to be compared to the other end. . When the voltage at the "+" terminal is higher than the "-" terminal, the output tube is cut off, which is equivalent to an open circuit at the output terminal. When the "-" terminal voltage is higher than the "+" terminal, the output tube is saturated, which is equivalent to the output terminal being connected to a low potential. A voltage difference of more than 10mV between the two input terminals can ensure that the output can be reliably converted from one state to another. Therefore, it is ideal to use LM339 in situations such as weak signal detection. The output terminal of LM339 is equivalent to a transistor without a collector resistor. When used, a resistor (called a pull-up resistor, choose 3-15K) must be connected from the output terminal to the positive power supply. Choosing pull-up resistors with different resistance values ​​will affect the value of the high potential at the output end. Because when the output transistor is turned off, its collector voltage basically depends on the value of the pull-up resistor and the load. In addition, the output terminals of each comparator are allowed to be connected together.

　　Single limit comparator circuit

　　Figure 1a shows a basic single-limit comparator. The input signal Uin, which is the voltage to be compared, is added to the non-inverting input terminal, and a reference voltage (threshold level) Ur is connected to the inverting input terminal. When the input voltage Uin>Ur, the output is high level UOH. Figure 1b shows its transmission characteristics.

    Figure 3 shows the overheat detection and protection circuit in an instrument. It is powered by a single power supply, and a fixed reference voltage is added to the inverting input of 1/4LM339, and its value depends on R1 and R2. UR=R2/(R1+R2)*UCC. The voltage at the non-inverting terminal is equal to the voltage drop of the thermal element Rt. When the temperature inside the machine is below the set value, the "+" terminal voltage is greater than the "-" terminal voltage, and Uo is at high potential. When the temperature rises above the set value, the "-" terminal voltage is greater than the "+" terminal, the comparator reverses, and the Uo output is zero potential, causing the protection circuit to operate. Adjusting the value of R1 can change the threshold voltage, which sets the temperature The size of the value.

image 3

　　hysteresis comparator

　　The hysteresis comparator can also be understood as a single-limit comparator with positive feedback. For the single-limit comparator introduced earlier, if the input signal Uin has slight interference near the threshold value, the output voltage will produce corresponding jitter (fluctuation). Introducing positive feedback into the circuit can overcome this shortcoming.

　　Figure 1a shows a hysteretic comparator. The familiar "Schmitt" circuit is a comparator with hysteresis. Figure 1b shows the transfer characteristics of the hysteresis comparator.

figure 1

　　It is not difficult to see that once the output state is converted, as long as the interference near the jump voltage value does not exceed the value of ΔU, the value of the output voltage will be stable. But what comes with it is a reduction in resolution. Because for the hysteresis comparator, it cannot distinguish two input voltage values ​​whose difference is less than ΔU. Adding positive feedback to the hysteresis comparator can speed up the response speed of the comparator, which is one of its advantages. In addition, since the positive feedback added by the hysteresis comparator is very strong, which is much stronger than the parasitic coupling in the circuit, the hysteresis comparator can also avoid self-oscillation caused by the parasitic coupling in the circuit.

　　If a trip point needs to be fixed at a certain reference voltage value, a nonlinear component, such as a crystal diode, can be connected to the positive feedback circuit and the unidirectional conductivity of the diode can be used to achieve the above requirements. Figure 2 is its schematic diagram.

figure 2

　　Figure 3 shows the grid overvoltage detection circuit part of an induction cooker circuit. When the grid voltage is normal, U4 of 1/4LM339 is <2.8V, U5=2.8V, the output is open circuit, the overvoltage protection circuit does not work, and the emitter follower BG1 as a positive feedback is turned on. When the grid voltage is greater than 242V, U4>2.8V, the comparator flips, the output is 0V, BG1 is cut off, and the voltage of U5 is completely determined by the voltage division value of R1 and R2, which is 2.7V, making U4 larger than U5, which means The state after flipping is extremely stable, and instability caused by small fluctuations in the grid voltage near the overvoltage point is avoided. Due to the creation of a certain hysteresis (hysteresis), after overvoltage protection, when the grid voltage drops to 242-5=237V, U4

image 3

　　Double limit comparator (window comparator)

　　The circuit in Figure 1 consists of two LM339s forming a window comparator. When the compared signal voltage Uin is between the threshold voltages (UR1UR2 or Uin

　　Use LM339 to form an oscillator

　　Figure 1 shows the circuit of an audio square wave oscillator composed of 1/4LM339. Changing C1 can change the frequency of the output square wave. In this circuit, when C1=0.1uF. f=53Hz; when C1=0.01uF, f=530Hz; when C1=0.001uF, f=5300Hz.

　　 LM339 can also form high-voltage digital logic gate circuits and can directly interface with TTL and CMOS circuits.

figure 1