Experiments with TL431A Shunt Regulator
by Lewis Loflin
In simple terms the TL431A acts a temperature compensated variable/adjustable Zener diode. It can also act as a voltage reference.
The TL431A, B integrated circuits are three−terminal programmable shunt regulator diodes. These monolithic IC voltage references operate as a low temperature coefficient Zener diode.
Related - see Transistor-Zener Diode Regulator Circuits.
The 2.5 V internal reference makes it convenient to obtain a stable reference for 5.0 V logic supplies. Since the TL431A, B operates as a shunt regulator, they can be used as either a positive or negative voltage references.
The TL431A is programmable from Vref to 36 V with only two external resistors. These devices exhibit a wide operating current range of 1.0mA to 100 mA with a typical dynamic impedance of 0.22 Ohms.
The characteristics of these references make them excellent replacements for zener diodes in many applications such as digital voltmeters, power supplies, and op amp circuitry.
In addition to good temperature compensation and stability, the TL431A has the following features:
Programmable Output Voltage to 36 V
Voltage Reference Tolerance: +-0.4%, Typ @ 25°C (TL431B)
Low Dynamic Output Impedance, 0.22 Ohms Typical
Sink Current Capability of 1.0 mA to 100 mA
Note: the values used below were experimental and may not reflect actual values in finished products. These circuits were built and tested and come with no warranty.
In Fig. 2 we use the TL431A as a simple 2.5 volt reference. This is far more temperature stable than a Zener diode. Choose a resistor values to limit Ik to between 20mA and 40mA.
In Fig. 3 we use a voltage divider to create a 5 volt reference. The output voltage equals (1 + R1 / R2) * 2.5V. Because finding the right values for R1 and R2 we can use a potentiometer.
In this case it would be 5 / 2.5 = 2; 2 - 1 = 1. Two equal resistors for R1 and R2 would work: 10K for each.
Fig. 4 uses a 100K potentiometer to create a variable voltage reference. for 5 volts the 100K potentiometer would be set dead center. This makes it easy to create a stable variable sort of Zener diode.
In Fig.5 we use a series pass transistor to boost current. Calculate Rk to limit Ik to no more than 10-40 mA. The voltage on the base of the transistor must be calculated at Vout + 0.6V to make up for the EB voltage drop.
Fig. 6 works exactly like Fig. 5 except for using a Darlington pass transistor. The transistor based voltage must be Vout + 1.2 volts.
This produces a higher current output.
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