[Analog circuit] 78L05 three-terminal voltage regulator

1Project Introduction

1.1 Overview

      78L05 is a fixed voltage ( 5V ) three-terminal integrated voltage regulator. The 78 series is a positive voltage output. The letter in the middle of the model usually indicates the output current. L indicates that the maximum output current is 100mA . 05 indicates that the output voltage is 5V . With its small size, light weight, cheap price, stability and reliability, safety protection, thermal shutdown characteristics and other excellent qualities, it is deeply loved by engineers and enthusiasts and is widely used in various power circuits. So now let's DIY a chip of our own based on the internal structure of 78L05 . I named it LC78L05 .

Figure 1-1 LC78L05_TO-92 package

1.2 Design features

✅Pair it with the test base plate to complete the learning of 78L05 gauge experiment

1.3 Application circuit

✅Linear voltage regulator

✅Current regulator

✅Switching regulated power supply

✅Constant current voltage stabilizing circuit

✅Fixed output voltage stabilizing circuit

2 Overall design plan

2.1 Internal structure

According to the 78L05 data sheet   provided by the manufacturer , find the internal structure circuit diagram as shown below. As shown in the figure, the 78L05 three-terminal voltage regulator is composed of a startup circuit, a reference voltage circuit, an overheating protection circuit, an error amplifier circuit and a safe operating area protection circuit.

Figure 2-1 Internal structure circuit diagram of 78L05

Figure 2-2 Internal circuit block diagram of 78L05

2.2 Pin description

The circuit symbol of       78L05 is shown in Figure 2-3 . Pin 1 is the output terminal ( OUTPUT ), pin 2 is the ground ( GND ), and pin 3 is the input terminal ( INPUT ).

Figure 2-3 Circuit symbol of 78L05

2.3 Typical applications

      78L05 is a linear regulator, the output waveform has serious clutter, and the output will copy the input waveform, so capacitor filtering must be added before and after the input and output of 78L05 , as shown in Figure 2-4 .

Figure 2-4 Typical applications of 78L05

3 Circuit Principle

      The working principle of 78L05 is: the input voltage provides the initial bias for the reference voltage circuit through the startup circuit, and the bias is amplified by the error amplifier circuit. When the temperature is overheated, it will be protected. Finally, the trimming resistor is adjusted in the work area protection circuit to output a stable 5V. Voltage.

3.1 Startup circuit

  Field effect transistor Q1 , transistor Q2 , resistor R1 and voltage regulator tube VD1 form a startup circuit, which provides the initial bias Vbias for the reference voltage circuit .

Figure 3-1 Startup circuit

3.2 Reference voltage circuit

  In order to better understand the circuit, first introduce the basic circuit related to the 78L05 circuit - the zero temperature coefficient reference voltage circuit. As shown in Figure 3-2 , the left side of the figure is the zero temperature coefficient reference voltage circuit, and the right side of the figure is its equivalent circuit. The voltage regulator diode Dz has a positive temperature coefficient, the base-emitter voltage of the NPN transistor has a negative temperature coefficient, and the turn-on voltage UBE of n and m diodes (including transistor Q ) is basically the same.

Figure 3-2 Zero temperature coefficient reference voltage circuit

Since the materials of   resistors R1 and R2 are the same, R1 and R2 have the same temperature coefficient. When the temperature changes, their ratio will not change, so the temperature coefficient of UREF is

  可以看出，在m、n、稳压管的UDz和二极管UBE的温度系数确定的情况下，只要R1和R2按上式取值，就可以使基准电压UREF为零。

      78L05的基准电压电路与上面介绍的零温度系数基准电路相似，基准电压电路的稳压管随之启动电路提供的输出偏置上升到稳压工作值时，启动电路会与基准电压电路断开。

图3-3 基准电压电路

3.3 过热保护电路

  三极管Q1，Q2和电阻R1，R2组成过热保护电路。当78L05正常工作时，Q1基极与发射极压降Vbe1≈0.3V，Q1和Q2截止。随着温度的升高，Q1的BE结负温度系数的增加，使得Q1开始导通，进而Q2也导通，Q2从误差放大器负反馈电压调整电路的三极管中分流，形成过热保护。

图3-4 过热保护电路

3.4 误差放大器电路

  三极管Q1～Q4与电阻R1～R4组成误差放大器（差动放大器）电路，误差放大器的作用是通过比较取样电压(VQ)，与基准电压(VREF)之问的误差値来产生误差电压(Vσ)，Vσ负反馈给VREF进而调节使输出电压维持不变。

图3-5 误差放大器电路

3.5 工作区保护电路

  稳压管VD1、VD2，三极管Q1～Q3，电阻R1～R7和微调电阻RP1组成安全工作区保护电路,如图3-6所示，通过调节微调电阻RP1，使其输出平稳的5V电压。

图3-6 安全工作区保护电路

4 原理图设计

4.1 新建工程

  打开立创EDA，创建工程并命名为【模拟电路】78L05三端稳压器，将原理图文件命名为：SCH_78L05三端稳压器。根据以下电路进行绘制电路原理图。

图4-1 78L05三端稳压器电路图

4.2 器件选型

  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，例如2SK170缺货，通过管子特性及电路分析，可以选择用BS170替换，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_78L05三端稳压器。

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 78L05三端稳压器边框示意图

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

  Next is the third step of PCB design: PCB routing, which is called printed circuit board wiring ( PCB LAYOUT ). Since the circuit board has two sides, the top and the bottom, the PCB traces can be divided into top and bottom traces. The top trace is red by default and the bottom is blue. You can also set other colors according to personal preference. , wiring means connecting wires according to flying wires in the circuit board, and connecting the same networks.

  First select the layer to be routed among the layers and elements, and then click the wire tool to connect (the shortcut key is W ). The seemingly simple Lianliankan requires us to make adjustments patiently. The placement and layout of components will also affect the difficulty of wiring, so we need to further adjust the layout and optimize it during the wiring process. The PCB layout introduced earlier is equivalent to paving the way for wiring. Once the layout is done, the wiring will naturally be smooth. The following reference suggestions are provided in the wiring of this project: