Practical calculation of the whole process of operational amplifier hysteresis comparator

Hysteresis of comparator （ Hysteresis ） Actual combat calculation of comparator

Next, we will design a circuit of battery low-voltage protection , It will introduce the actual use of hysteresis comparator and the detailed calculation and derivation process .

Suppose we decide 18.5V It is the battery undervoltage protection voltage , That is to say, under the current load, the battery voltage is lower than 18.5V when , Other functions will be limited or unable to carry the current load , And after the load disappears or stops , The battery voltage will slowly recover and will be higher than the undervoltage protection voltage 18.5V, Then it will be turned on and off again 、 switch 、 The state of the switch , Then to solve this phenomenon, we need to use a double threshold circuit , Turn off below the low threshold , Open when it is higher than the high threshold , It can not only avoid repeated oscillation of the switch, but also improve the reliability of the circuit .

Suppose there is a battery with a nominal voltage of 24V After the actual filling is 29V, We are lower than 18.5V Is the low threshold voltage , High threshold is 18.5V To 24V Between , The choice of the 18.5V For low threshold 21V For high threshold , The high and low threshold can be set according to the specific situation .

In the process of circuit design , Power design is the top priority , A good power supply can provide a stable voltage and current for the circuit , Suppose we have designed a 5V LDO The power supply , Here LDO The power supply is not built with discrete devices , Need to use LDO chip , The purpose is to ensure the accuracy and stability of the voltage value .

Figure 1 ▲

As shown in Figure 1 , It is a double threshold hysteresis comparator circuit , The negative input terminal is connected to the battery voltage , Positive input termination 5V The power supply is subject to resistance voltage dividing treatment , So how can we determine the value of parameters in the figure ？

First look at the negative input of the comparator , Divide the voltage through two voltage dividing resistors , The value of these two voltage dividing resistors should not be too large or too small , The threshold setting of our battery is 18.5V—21V, The nominal voltage of the battery 24V Maximum 29V, Therefore, the comprehensive consideration is to choose 21V Ensure the current flowing through the voltage dividing resistor 1mA about , So it won't be 18.5V When the current is too small, it is easy to be disturbed , It's not as good as 29V When the current consumption is too large , When selecting resistance, select the resistance value of common nominal value , Then take R1=20K,R6=1K, As shown in Figure 2 .

Figure 2 ▲

When BATT=18.5V when ,U1=18.5*R6/R1+R6=0.88V

When BATT=21.0V when ,U1=21.0*R6/R1+R6=1V

According to the calculation results ：BATT When the threshold voltage changes U1 Will change synchronously , And the threshold is 0.88V—1V, That is to say, when U1=0.88V When the comparator outputs low level , When U1=1V When the comparator outputs high level （ Figure 3 ）, The back pole circuit can control other operations according to the high and low levels

Figure 3 ▲

When U1=0.88V The equivalent circuit of the time comparator is shown in Figure 5 When U1=1V The equivalent circuit of the time comparator is shown in Figure 6

Figure 5 ▲

Figure 6 ▲

because R3 When the output of the comparator is high or low, it acts as a pull-up resistor , Let's ignore R3、R4 Access Rd , Then the equivalent circuit with high output of the comparator can be changed to figure 7 , At this time, the power supply voltage is 5V, Hold circuit 1mA The current can be determined R5+R2 be equal to 5K Up and down , Then carry out resistance distribution ,R5=1K,R2=4K3 Figure 8 shows （ The value of resistance is not unique , Other resistance values are also ok ）

Figure 7 ▲

Figure 8 ▲

Sure R2、R5 After the resistance value, see the equivalent circuit when the output of the comparator is low （ Figure nine ）, At this time, the unknown quantity in the equivalent circuit with low output of the comparator is only R3, According to the superposition theorem, an equation can be listed

Figure nine ▲

equation ：

hold V2=0.88V、R2=4300R、R5=1000R Substitute into the formula for simplification

After simplification, we get R3=1.126*10^4Ω（ In fact, there is no 11.26K The resistance of , We choose a nominal value of 11.2K The resistance of , Then the threshold voltage will not affect the normal use when it is slightly adjusted in the actual design ） obtain R3 After the resistance value, look back when U2=1V when , The equivalent circuit when the comparator outputs high level （ Figure ten ）

Figure ten ▲

Also write the formula according to the superposition theorem (VCC=5V,V1=V2)：

Come to a conclusion ：R1=20K、R2=4K3、R3=11K2、R4=46K、R5=R6=1K, All resistance accuracy selections 1% Accuracy or higher

Figure eleven ▲

Final hysteresis comparison circuit diagram （ Comparator in the figure 2 The feet should be grounded ）

This article comes from Fan Yi Education , Reference to the original ：《 Hysteresis （ Hysteresis ） The actual calculation of the whole process of the comparator （ Surprise at the end of the article ）》