TL431 Circuits – A Guide for Beginners

Do you need power supply circuits for your open-loop configuration or linear regulator project? Or are you looking for a circuit that serves as a comparator for all kinds of voltage? Then, the TL431 circuits are the perfect go-to. Plus, they are ideal for regular bandgap voltage reference work or programmable shunt voltage reference.

The TL431 circuit is crucial for voltage control and power supply control chip—even though its setup is a bit tricky. Plus, it comes with the temperature compensated bandgap voltage reference. 

In other words, the design of the TL431 circuit is pretty complex. Plus, it can get you confused—especially if you're a beginner.

Thankfully, we've created this detailed article to give you in-depth information about TL431 circuits.  

Are you ready? Then, let's begin!

1. What are TL431 Circuits?

A TL431 circuit, which comes in switch-mode power supplies loops, is a three-terminal integrated circuit. And you can use it as an adjustable precision shunt voltage regulator. So, you can join a TL431 actual power supply feedback circuit with an external voltage divider. Also, you can regulate 2.5 - 36 voltage ratings—with a maximum standby power supply current of 100mA and junction capacitor.

2. What are the Characteristics of TL431 Circuits?

Before we dive into how this circuit works, here are the features of the TL431 circuit:

It has a good reference voltage tolerance at a temperature of 25°C for:

• Standard grade tolerance and output capacitor (2%)
• A grade tolerance and traditional capacitor (2%)
• B grade tolerance (2%)

• The TL431 also has a Vref to 36 v adjustable output voltage
• Can withstand the effects of temperature from −40°C to 125°C
• Has a typical temperature drift (TL43xB):
• 14 mV (I Temp, Q Temp)
• Six mV (C Temp)
• Generates a low output noise
• Has a typical output impedance of 0.2-Ω 
• Has a 1mA to 100 mA sink-current capability
• The type of capacitor in TL431 has a varying capacitance with voltage

3. TL431 Circuits Working Principle

As we mentioned earlier, the TL431 is essentially a voltage controller with an 8-pin IC package. But that's only at a fundamental level. Going deeper, we find that the TL431 is an alternative for the adjustable Zener voltage regulator. It also supports surface-mounted SOT23-3 package and transistor-like package.

Plus, you can set the output voltage with:

• A variety of packages
• Larger package
• An external precision divider resistor

But there's more!

The first capacitor of the circuit also works with a reverse-biased diode and a diode for reference.

How the TL431 Circuits Works?

So, how does it work?

The circuit determines the resistance values of both R1 and R2 resistors. Plus, it generates feedback and poor resistor tolerances—which is dependent on the partial pressure of Vo.

So, when Vo increases, the feedback will also increase and the shunt of TL431. In essence, the increase in shunt reduces the pressure and detailed schematic of the Vo.

Additionally, you need to do something when the REF terminal voltage and previous schematics is equal to the reference voltage. Here, it's ideal to keep the negative feedback and internal schematic of the circuit stable. At this point, you'll have Vo = (1+R1/R2)Vref.

Also, you can get any output voltage and maximum currents between 2.5v to 36v. And it happens when you choose different values for your R1 and R2 resistors.

Note, there are some necessary conditions you've to meet before the TL431 can work. One of them involves selecting a suitable resistor, copper anode and basic level of internal anode. Hence, the current passing through the anode pins and cathode of your TL431 should be greater than 1mA.

To sum it up, the circuit's output voltage and output sampling increase when the input voltage increases. In short, it's the working principle and the level of chip decapping of the TL431.

Furthermore, you can adjust the internal circuit to increase the current that flows through it. Plus, the current limit circuit also increases the voltage drop of the current limiting resistor.

So, to achieve voltage regulation;

The output voltage = input voltage - current-limiting resistance.

4. 9 Applications That Use TL431 Circuits

Here are nine applications using the TL431 circuit.

1. Adjustable Regulator Circuit Using TL431

The adjustable regulator circuit application and switching frequency is quite simple when you're using the TL431 integrated circuit.

Circuit diagram for adjustable regulator

Source: Wikimedia Commons

So, the circuit can regulate the poor gain and voltage between the range of 2.5v - 36v voltage rating. Plus, it depends on the following:

• Input supply voltage
• The board layout
• Change of the component values of R2 and R1

Also, the adjustable regulator circuit uses the following formula and block diagram to calculate;

V0 = Vref(1+R1/R2), Vref = 2.5v.

However, the current has a limit of 100mA. Therefore, you can enhance the current with a transistor, booster transistor, or couple of transistors if you choose.

Can you relate this circuit's voltage to (Vi - Vo)? Then, the power consumption of R increases when the voltage difference is immense. Then, it becomes a programmable shunt regulator with semiconductor technology with temperature-stabilized bandgap.

2. TL431 Precision Reference Voltage Source

The precision reference voltage source uses the TL431, which is an unusual choice, on the control circuit of isolated power supplies. Hence, you can use the TL431 to provide a precision reference voltage and configure it as an analog circuits controller.

Why? Because it features an onboard error amplifier.

Circuit diagram of Precision reference voltage source

Source: Wikimedia Commons

Additionally, the precision reference voltage source circuits feature a huge output transistor, stable reference, and good temperature stability. However, ensure you watch out for the value of CL when connecting capacitive loads. By doing so, you can prevent self-excitation and gain a stable voltage reference (Vref).

3. Voltage Detector Circuit Using TL431

The voltage detector circuit is another simple pressure level circuit you can build with the integrated circuit, TL431. So, you can use a 5v power supply in a digital circuit, bipolar transistors and real transistor. Plus, the general feeding signal input will become a high-class logic—releasing a 5v output.

So, When the level logic is low, the output level reduces to 1.8v. Thus, it's easy to assemble this circuit with adjustable shunt regulator to achieve feedback loop and the results you want.

4. TL431 Overvoltage Protection Circuit

Overvoltage Protection circuit diagram

Source: Wikimedia Commons

As the name implies, the circuit offers protection from high voltages and it handles temperature compensation for analog chips. The equipment with this pin input circuit automatically shuts down when its power crosses the fixed voltage value. The IC balanced comparator voltage references serve as a low-temperature adjustable Zener diode. Plus,  you can program it from Vref to 36v—with the aid of two external resistors.

This single layer circuit has a substantial current range of 1.0 mA to 100 mA for operations and a typical dynamic impedance of 0.22 W. So, when Vi passes the set feedback voltage limit, it triggers the TL431. While this happens, the thyristor turns on to generate a sizable pulsating current. This larger variety current blows a fuse to protect the rear circuit. Hence, the V protection point is equal to (1+R1/R2)Vref.

5.TL431 Constant Current Source Circuit

You can use the TL431 shunt regulator in a pass-series constant current regulator. The most significant factor in this output is the RCL and—not the R1. Though R1 has its formula, it's not that important.

The formula is Vref = 2.5 V.

The value of the constant minimum voltage depends on the external resistance and positive voltage references.

Circuit Diagram of Constant Current Source

Source: Wikimedia Commons

Therefore, it's essential to consider the margin when selecting a power transistor for this circuit. Moreover, you can use this current source as a current limiter if you don't connect it to a stabilized circuit.

6. TL431 Comparator

The TL431 Comparator conducts and switches on an optocoupler. And it happens when the voltage across it passes a limit.

Circuit diagram of Comparator

Source: Wikimedia Commons

But remember that the TL431 has three pins. VT measures the voltage across—which is proportional to the output voltage. Thus, it cleverly utilizes the critical voltage of Vref = 2.5v. Also, the output and input waveforms track well because of the distance of the TL431.

7. TL431 Voltage Monitor

The TL431 voltage monitor is another application with a single purpose. Here, the circuit lights up an LED when it reaches the target voltage rating. Hence, it's perfect for battery chargers, like the laptop power adapter—indicating when the batteries get fully charged.

Also, phone chargers are good examples of power devices with this circuit.

So, the voltage monitor uses a simple High limit = Vref (1+R1/R2). Here, the high limit here is the target voltage that lights up the LED with emitter voltages when hit.

Circuit diagram for Voltage Monitor

Source: Wikimedia Commons

The voltage reference is at 2.5 volts in the TL431. Also, the R1 and R2 form a voltage divider that allows you to set the desired high limit range.

8. TL431 Controllable Shunt Features

For this application, something happens when the voltage at the REF terminal goes through a slight change. It changes the shunt from the cathode voltage. Also, the process changes the anode within 1 - 100 mA. So, it affects both the cathode current and the anode current.

With the controllable shunt characteristics, you can use small voltage changes to control indicator light, relays, etc. Plus, you can even drive current audio loads directly.

Controllable Shunt Features circuit diagram

Source: Wikimedia Commons

9. TL431 Switching Power Supply

Previous generation switching power supply designs featured a system that did one thing.

The TL431 sent back the output current to the alternating current input terminal after error amplification. However, recent technology allows most power supply industries to adopt a new scheme.

Circuit diagram for switching power supply

Source: Wikimedia Commons

Here, the TL431 sends back the output as voltage feedback so it can amplify the error. Then, the sinking end of the TL431 drives the light-emitting section of the optocoupler. With this, you can obtain voltage feedback from the photocoupler. Also, you can use it to adjust the time of the PWM controller current mode. Thus, making the direct current output voltage stable.

Final Words

To sum it up, TL431 circuits have various uses—not restricted to the nine applications listed above. For example, the circuit helps you control the input voltage of your devices, like the programmable Zener diode. So, if you need a voltage comparator, go for the TL431.

Before wrapping this article, you must know this:

The accuracy of your resistors determines the accuracy of your voltage monitor. Hence, you can fine-tune this using the series resistor R2. And you can find it in series with a low-value variable resistor and other electronic components.

Are you still finding it hard to grasp a hold of things when it comes to TL431 circuits? Then, feel free to contact us. We'd love to help!