Abstract TL431 adjustable shunt reference the characteristics of the chip and its application in a number of functional circuit.
Keywords adjustable shunt reference constant current PWM switching power supply
1 TL431’s profile
Texas Instruments (TI) TL431 is the first production of a good thermal stability of three-terminal adjustable shunt reference. Its output voltage with two resistors can be arbitrarily set to from Vref (2.5V) to 36V range of any value (see Figure 2). The device’s typical dynamic impedance of 0.2Ω, in many applications can use it instead of Zener diodes, for example, digital voltage meter, amplifier circuit, voltage regulator power supply, switching power supply and so on.
The left is the symbol of the device.3 pins are: the cathode ( Cathode ), anode ( aNode ) and the reference terminal ( REF ).TL431 specific functions can be used as shown in Figure 1 indicate the function modules.
Can be seen from the figure, VI is an internal 2.5V reference, connected to the inverting input of op amp. Shows the characteristics of the op amp, and only when the REF terminal (inverting input) voltage is very close to the VI (2.5V), the transistor will have a stable in the non-saturation current through, and with small changes in voltage REF, through the transistor in Figure 1 from 1 to 100mA of current changes. Of course, the figure is by no means the actual internal structure of the TL431, it can not simply replace it with this combination. However, if in the design, analytical applications TL431 circuit, this block diagram of open ideas, are helpful to understand the circuit, some analysis of this article based on this module will also be initiated.
2. Constant voltage circuit applications
TL431 mentioned earlier contains a 2.5V internal reference voltage, so when the end of the introduction of the REF output feedback, the device from the cathode to the anode through a wide range of streaming, control the output voltage. The circuit shown in Figure 2, when the resistance R1 and R2 to determine when the partial pressure of both the introduction of Vo feedback, if V o increases, the amount of feedback increases, TL431 shunt will increase, which in turn led to Vo decline. Obviously, the depth of the negative feedback circuit must equal the reference voltage VI at the stable, then Vo = (1 + R1/R2) Vref. Choose different values of R1 and R2 can be from 2.5V to 36V output voltage range of any in particular, when R1 = R2 when, Vo = 5V. Note that, the choice of resistance TL431 must ensure the necessary conditions of work, that is, the current through the cathode is greater than 1 mA.
Of course, this circuit is not very practical, but it clearly shows the working principle of the device in the application of the method. This circuit is modified, you can get in a lot of practical power circuit, as shown in Figure 3,4.
Figure 3, high current shunt regulator circuit 4 Precision 5V regulator
3, Experimental Notes
Resistance values: R0 take 1.5K, R1, R2 were taken 10K, according to the results of the output voltage should be 5V. Vin with 12V, the measured voltage is 5V. Vin with 24V, the measured voltage 5V (my 3 1 / 2 meter of the display), so the accuracy of such devices is very high. Access to the load, in the C, A-side and connected load resistance, Vin with 12V. When the load resistance is greater than 2K, the output voltage almost do not see any change. When the resistance is less than 2K, the output voltage begins to decrease, this time should be mentioned in front of the cathode current conditions do not meet the.
Made with a TL431 precision regulated DC power supply
Circuit of the ripple is extremely small, high precision, can be used for high-grade electrical power supply.