[Analog circuit] 431 reference voltage source

1 Project Introduction

1.1 Overview

       The 431 voltage reference chip is a 3-pin voltage stabilizing integrated circuit. The 431 reference voltage source has good thermal stability and a three-terminal adjustable shunt. It is also called a voltage regulator or a three-terminal sampling integrated circuit. With its small size, light weight, high precision, stability and reliability, precision and adjustable reference voltage, large output current, and low price, it is deeply loved by engineers and enthusiasts and is widely used in various power circuits. At present, the 431 chips we see with different names such as TL431, KA431, μA431, LM431, etc. are 431 voltage reference chips launched by different manufacturers. Now let’s DIY a chip of our own based on the internal structure of 431. I will It is named LC431.

Figure 1-1 LC431_TO-92 package

1.2 Design features

✅Build with discrete components and learn circuits in depth

✅Use direct plug-in components to facilitate soldering and debugging for beginners

✅Onboard banana header and pin header interface for easy debugging and expansion

✅Paired with the test base plate, complete the learning of 431 routine experiments

1.3 Application circuit

✅Precision reference voltage source

✅Adjustable voltage stabilizing circuit

✅Constant current source circuit

✅Voltage comparator

✅Overvoltage protection circuit

2 Overall design plan

2.1 Internal structure

     According to the 431 data sheet provided by the manufacturer, find the internal structure circuit diagram as shown below. As shown in the figure, the 431 reference voltage source is composed of a reference voltage circuit, an error amplifier circuit, a Darlington output circuit and a diode protection circuit.

Figure 2-1 Internal structure circuit diagram of 431

       The circuit symbol of 431 is shown in Figure 2-2. Its use is the same as that of a voltage regulator tube. The cathode (K) is connected to high level, the anode (A) is connected to low level, and R is the reference terminal.

Figure 2-2 Circuit symbol of 431

       Figure 2-3 is a schematic diagram of the equivalent function of the 431, which is composed of an operational amplifier, a protection diode, an NPN transistor and a 2.5V precision reference voltage source Vref. The reference terminal (R) is connected to the non-inverting input terminal of the op amp, and the 2.5V (Vref) reference voltage source is connected to the inverting input terminal of the op amp. The op amp is equivalent to an error amplifier, which converts the voltage difference between the non-inverting input terminal and the inverting input terminal. Amplified many times, the voltage at the reference terminal (R) is compared with the 2.5V (Vref) voltage of the precision reference voltage source. When the voltage at the reference terminal (R) is greater than 2.5V (Vref), the voltage at the output terminal of the op amp is high level. When the transistor is turned on, the output voltage of the op amp will increase as the voltage of the reference terminal (R) increases, forming a negative feedback. The transistor plays a role in regulating the load current, and the protection diode can prevent the polarity connection of the power supply between KA and reverse attack. Wear a triode.

Figure 2-3 Equivalent functional block diagram of 431

2.2 Pin description

3 Circuit Principle

     The working principle of 431 is: when the input voltage increases, the output voltage increases, causing the output sampling to increase. At this time, the internal circuit adjusts to increase the current flowing through it, which also increases the resistance flowing through the current limiter. , the voltage drop increases, and the output voltage is equal to the input voltage minus the current limiting resistor. The increase in voltage drop causes the output voltage to decrease, achieving the voltage stabilization function.

3.1 Reference voltage circuit

     In the early 1970s, Widlar first proposed the concept of bandgap reference voltage source, referred to as bandgap voltage. The reference voltage source is a high-stability voltage source used as a voltage standard. Currently, , it has been widely used in various integrated linear regulators.

     Transistors Q1~Q4 and resistors R1~R4 form a 2.5V reference voltage circuit, as shown in Figure 3-1. When the circuit is working, there is a voltage difference of 2.5V between the reference terminal (R) and the anode (A). The circuit will be described below. analyze.

Figure 3-1 Reference voltage circuit

     At 27°C or 300K, the voltage drop between the base and emitter of the triode VBE = VTln (IC/IS), where VT is the voltage equivalent of temperature, IC is the collector current, and IS is the saturation current (consistent with the emitter maximum). Proportional). Temperature voltage equivalent VT=KT/q, where K is Bollman's constant (1.38×10^-23J/K), T is the thermodynamic temperature, that is, absolute temperature (300K), and q is the electron charge (1.6×10^- 19C). At normal temperature, VT≈26mV.

    The expression of the reference voltage source is: VREF=VR1+VR2+VBE1+VBE3

    Since the base voltages of transistors Q3 and Q4 are the same, and the resistance value of R3 is three times that of R2, the current flowing through Q3 is three times that of Q4. Assume the current of Q4 is I, so the current flowing through Q3 is 3I, and the current flowing through R1 is the sum of the currents of R2 and R3, which is 4I.

    In order to obtain temperature compensation, Q4 is connected in parallel with two transistors, that is, the relative area of ​​Q3 and Q4 is 1:2. Let the saturation current of Q3 be IS, so the saturation current of Q4 is 2IS. From the formulas VBE=VTln(IC/IS) and VBE3=VBE4+IR4, the voltage of Q3 collector VBE3=VTln(3I/IS)=VTln(I/2IS)+IR4 can be obtained through simplification:

Set the transistor voltage drop to 580mV, and you can get the reference voltage of the 431 reference terminal (R): VREF=VR1+VR2+VBE1+VBE3=818+446+580+580=2.43V

During actual production, there are some differences in the reference voltage value, which is generally around 2.5V.

3.2 Error amplifier circuit

   Transistors Q1~Q4 and resistors R1~R4 form an error amplifier (differential amplifier) ​​circuit. The function of the error amplifier is to generate an error voltage (Vσ) by comparing the error value between the sampling voltage (VQ) and the reference voltage (VREF). , Vσ is negatively fed back to VREF and then adjusted to maintain the output voltage unchanged.

Figure 3-2 Error amplifier circuit

3.3 Darlington output circuit

     A Darlington structure is used to form a Darlington tube output circuit. The Darlington tube is also called a composite triode. It uses a composite connection method to connect the collectors of two or more triodes together. The emitter of the first triode is directly coupled to the base of the second triode, which is connected in turn. , and finally lead to three electrodes B, C, and E.

Figure 3-3 Darlington tube structure

     The Darlington tube composed in this way has the advantages of high gain, fast switching speed, and good stability. When used, the Darlington tube can be directly regarded as a high-performance triode with a high current amplification factor. If the gain of a single transistor is 10, then the gain of a Darlington composed of 2 transistors will be 10×10=100 times. As the number of transistors increases, the Vbe conduction voltage of the Darlington tube will also increase.

Figure 3-4 Darlington output circuit and diode protection circuit

    It should be noted that since the Darlington tube is internally composed of multiple tubes and resistors, when tested with a multimeter, the forward and reverse resistance values ​​of the BE junction are different from those of ordinary triodes. For high-speed Darlington tubes, some tubes have an input diode connected in anti-parallel to the front-end be junction. At this time, the measured forward and reverse resistance values ​​of the be junction are very close, and it is easy to misjudge it as a bad tube. Please pay attention.

3.4 Diode protection circuit

    According to the one-way conduction characteristics of the diode, a diode is used in this project to form a protection circuit. In the Darlington output circuit, a damping diode is connected in reverse parallel between the collector and emitter of the final triode to prevent the triode from suddenly losing power. It can be broken down and avoid damage to the 431 chip when the cathode and anode are connected reversely.

4 Schematic design

4.1 New construction

    Open Easy EDA, create a project and name it [Analog Circuit] 431 Reference Voltage Source, and name the schematic file: SCH_431 Reference Voltage Source. Draw the circuit schematic diagram based on the following circuit.

Figure 4-1 SCH_431 reference voltage source

4.2 Device selection

    在本项目的元器件选型中，三极管使用的NPN型的9014以及PNP型的9012，电阻选择1/4W的直插电阻即可，芯片引脚用排针与香蕉头接口引出，便于安装与测试。所有器件可直接在立创EDA的元件库中进行搜索，如果对元器件不熟悉，也可以通过复制物料中的商品编号进行搜索（每一个元器件在立创商城都有唯一的商品编号），如果出现物料缺货情况，亦可选择其他可替换物料，通过以上电路的分析，相信聪明的你对各个元器件在电路中的作用有所了解，那么更换个别物料也不会影响到电路的工作性能的，了解电路工作特性后，电路选型也就变得简单了。

图4-2 元器件搜索示意图

图4-3 通过商品编号搜索示意图

4.3 物料清单

5 PCB设计

     完成原理图设计后，经过检查电路与网络连接正确后点击顶部菜单栏的 “设计”→ “原理图转PCB”（快捷键为Alt+P），随即会生成一个PCB设计界面，可先暂时忽略弹出的边框设置，然后将PCB文件保存到工程文件中，并命名为：PCB_431基准电压源设计。

5.1 边框设计

     在绘制PCB前需根据个人意愿以及元器件数量所占空间确定PCB的形状及边框大小，若无特殊外壳要求，一般设计成矩形、圆形以及正方形。在设计该项目时，秉承着大小合适，美观大方的原则，我们在顶部工具菜单栏下的边框设置选型中设定了一个长为100mm、宽70mm、圆角半径为2mm的圆角矩形。实际板框大小会随着布局布线中进行调整，如果太小可适当放大，太大也可缩小边框，风格样式可自由发挥，但尽量控制在10cm*10cm之内，这样就可以到嘉立创免费打样啦～

图5-1 边框设置

图5-2 431基准电压源边框示意图

5.2 PCB布局

     在绘制完板框外形后，接下来进行PCB设计的第二步，对元器件进行分类和布局，分类指的是按照电路原理图的功能模块把各个元器件进行分类，图中有很多三极管和电阻，但哪一个三极管和电阻是连到一起的呢，这里需要我们用到立创EDA所提供的布局传递功能，首先确保PCB工程已保存到原理图文件的同一个工程文件夹中，然后框选原理图中的某一电路模块，比如选中二极管保护电路，然后点击顶部菜单栏中的 ”工具” → ”布局传递“ （快捷键为Ctrl+Shift+X），PCB页面所对应的元器件就好进行选中并按照原理图布局进行摆放，使用这个方法将各个电路模块进行分类后依次摆放在前面所放置的边框中。

     在布局的时候注意摆放整齐，可根据飞线的指引进行摆放，按照原理图信号的流向和器件连接关系进行摆放，是可以把原理图器件摆放非常整齐的，在布局的过程中注意接口位置，比如我们把排针以及香蕉头接口按照左右下摆放，布局参考如图5-3所示。

图5-3 PCB布局参考图（飞线已隐藏）

5.3 PCB走线

     接下来进行PCB设计的第三步：PCB走线，全称为印刷电路板布线（PCB LAYOUT）。由于电路板有顶面与底面两个面，在PCB走线也就可以分为顶层和底层走线，其中顶层走线默认是红色线，底层为蓝色线，也可按照个人喜好设置其他颜色，走线也就是在电路板中按照飞线连接导线，将相同的网络连接起来即可。

     首先选择层与元素中要走线的层，然后点击导线工具进行连线（快捷键为W）。看似简单的连连看，其中需要我们耐心的进行调整，元器件的摆放布局也会影响走线的难度，所以还需要在走线过程中进一步调整布局，进一步优化。前面所介绍的PCB布局相当于是在给走线做铺垫，布局好了，走线也就自然顺畅了。在该项目的走线中提供以下几点参考建议：

（1）电源线设置为35mil，信号线设置为20mil宽度

（2）走线以顶层走线为主，走不通的可以切换到底层进行连接

（3）走线过程中优先走直线，需要拐弯的地方以钝角或圆弧拐弯为主

（4）最后加上泪滴，添加丝印标记该PCB板的尺寸以及接口功能

布线参考如图5-4所示，初次设计可参考下图进行走线，也可自由设计，属于你的431基准电压源芯片。

图5-4 PCB走线参考图

图5-5 PCB-3D预览图

6 焊接与调试

6.1 硬件焊接

     拿到板子和元器件后应先检查物料是否有缺失和遗漏，检查无误后再进行焊接。焊接原则是先低后高，首先把电阻，电容和二极管焊接到板子上，然后再焊接三极管，排针，最后安装香蕉头接口。直插器件的焊接方法如下图所示，注意焊接时对准位置，检查元器件型号是否正确，锡线是否虚焊，避免影响电路性能，导致电路不能正常工作。

图6-1 电阻焊接操作图

图6-2 三极管焊接操作图

图6-3 PCB装配图

图6-4 未焊接PCB板

图6-5 PCBA实物图

图6-6 PCB-3D预览图

6.2 硬件调试

     完成焊接第一步，切勿直接上电测试，即使你很兴奋，顺利完成了元器件的焊接，但也不能心急。焊接完成后需要使用万用表检查电源与地是否短路，焊接过程中有没有出现短路以及断路的情况，检查无误后方能进行上电测试。

     首先测试是否有基准电压。将电路板上K端引线和R端引线接在一起，串接1个1K电阻，再接12V直流电源正极，A端接电源负极，电路如图6-4所示，实物连接图如图5所示。测输出电压(K、A端之间电压）为2.5V左右，如偏离太多，应检查电路板元件是否焊错，锡线是否虚焊等。

图6-7 基准电压测试图

图6-8 基准电压测试连接图

     基准电压精度测试按图6-8电路实物装置，测5V~10V的不同输入电压，如图6-9所示，得到一组基准电压的值如表6-1所示。可以看出基准电压在2.45V不变。

表6-1 5V~10V输入电压测基准电压表

图6-9 基准电压测试图

     最后测试5V输出电压，将电路板上K端串联1个2K电阻和R端接在一起，K端串联1个470Ω电阻，再接直流电源正极，R端串联1个2K电阻和A端接在一起，A端接直流电源负极，如图6-10所示。测输出电压(K、A端之间电压）为5V左右，如偏离太多，应检查电路板元件是否焊错，锡线是否虚焊等。

Figure 6-10 Regulated voltage output test chart

Figure 6-11 Stabilized output test connection diagram

     The voltage stabilization accuracy test is based on the physical device of the circuit in Figure 6-11, and different input voltages from 7V to 12V are measured, as shown in Figure 6-12. A set of reference voltage values ​​are obtained as shown in Table 6-2. It can be seen that the regulated voltage is around 4.9V.

Table 6-2 7V~12V input voltage measurement stabilized output voltmeter

Figure 6-12 Regulated voltage output test chart

7 project information

     Special thanks to Teacher Yu Hong for providing information and support for this project~