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News:  TL431 low dropout DC power supply parameters reference

Published: Wednesday 04 February, 2015

TL431 as a precision voltage source, is widely used in electronic circuit designs, due to the unique dynamic impedance, TL431 is also often used as a Zener diode. Voltage source for use in high-voltage power circuit is quite extensive, most use three-pin configuration, so the structure is simple and relatively easy to use. However, when only the low voltage battery supply, regulated power supply demand is likely to increase by 20% to 40% of the cost and volume. In view of this situation, this article introduces a design method of low dropout voltage regulated DC power supply circuit, the circuit devices use conventional devices, low cost, and has good load characteristics and voltage stability.

Circuit works

Figure 1 is a low-voltage DC power supply layer of high voltage power supply circuit diagram. The circuit is composed of a reference voltage, the voltage amplification and current amplification three aspects of the composition. Among them, the reference voltage generated by the TL431, according to the circuit in Figure 1 is connected, via R0 current to obtain a stable 2.5 V reference output at 0.5 ~ 10 mA when.

The output voltage is determined by the specific values UA op amp inverting amplifier superiority lies in its input impedance is great, can very well be TL431 output voltage of 2.5 V and the post-stage circuit isolation, it is not affected by changes in the load. Op amp and resistors R3 and R2 scaling links may reference voltage output scaled as required, but can not exceed the maximum output voltage of the op amp's power supply voltage.

Current amplification using two transistors, UA VQ2 control the regulator VQ1, consisting of the feedback link for current amplification, the output voltage is adjusted by driving the regulator, in order to achieve the regulated output. Diode VD UA voltage output of op amp when the adjusting tube VQ2 base-emitter voltage is negative, so that VQ2 immediately turned into an off state, current Ic2 rapidly reduced, VQ2 of VCE rises cause VQ1 base voltage increases, so that VQ1 base current IB reduced, thereby reducing the output current ICQ1 (βIB), and vice versa empathy. RL is the output load, C0, and C1 is a filter capacitor.

The main parameters of the circuit design

Control link design

Twenty one

Control link circuit equivalent diagram shown in FIG. 2 and FIG. 3, in which FIG. 2 is a proportional voltage gain diagram, Figure 3 is an enlarged schematic diagram of a current. According to Figures 2 and 3, can be drawn from the control link loop equation:


(2), Irg UA for the output of the op amp output a control current formula.

(2) From the formula, Irg VQ2 by controlling the current, IC2 control VQ1 base current, IB1, R8 control adjustment tube VQ2, then control VQ1 output current IC1, VQ2 series negative feedback and VQ1 is formed, without further amplification VQ1 output current IC1, IC1 with R8 for diversion. Circuit output voltage Vcc is 5 V, the drive rated load is 350 Ω, 7 V power supply is the standard output of the battery. Operational amplifier selected LM358, take R1, R2 is 10 kΩ, TL431 current range is 100 ~ 150 mA, the choice of R1 = 3 kΩ, to meet the requirements. VCC = (1 + R2 / R1) x2.5 = 5 V. Reasonable selection of resistance R8 and R9, so VQ1 and VQ2 are working in the linear region.

Under the grid and load fluctuations, Ib, Ie, Ucc as small as possible in order to reduce losses. Set static operating point to select the appropriate drive tube VQ1 and bias resistor R8, R9. VQ1 quiescent point is:


Where, Irg for controlling the output signal of the op amp, Vin is the supply voltage, Vcc is 5 V output voltage, RL is rated load 200Ω, VD is the diode turn-on voltage of 0.7V.

By formula (3) and (4) can be determined VQ2 parameters, and then calculate the resistor R9:


Use magnification β1, β2 between 30-80 of the regulator, the regulator greater magnification power consumption is small, but the stability decreased β chosen here is 50, designed the power supply fluctuates between 5.2 ~ 9 V in order to prevent the burning of the regulator VQ2 supply voltage is high, add about R8 1 kΩ resistor to limit protection.

Overcurrent protection circuit design

3, the resistance Ri of the transistor VQ3 composition overcurrent protection links. Output current is too large, the voltage sampling resistor Ri is greater than 0.7 V, VQ3 conduction, forced the regulator to reduce the base voltage Vbe until you close the power supply output. R4 = 0.7 / kIC. Which, LC is the output current, K is the maximum overcurrent factor, typically ranging from about 1.5. R7 = (Vcc-Uce3) / Ie3≈Vrg / Ic3, restrictions Ic3 should not be too large, so VQ3 overcurrent damage.




Figure 4 shows the design of a high-voltage power supply DC power supply module, the input power to DC 5 ~ 9 V battery pack, respectively, for power circuit design characteristics and load characteristics of the test, in which the load characteristics of the test to enter the 6.5 V battery to simulate the actual use work environment. Figure 5 for the test results recorded. Output ripple test data when the power supply input voltage is 5-11 V, output ripple is 5 ~ 8 mV.

As can be seen from the experiment, this design has a high-precision voltage regulator, load characteristics of a good range of features, the most important is a simple circuit structure, the interface can be used to monitor the actual power P0, this circuit has been put into production, through the practice test circuit design and reliable performance, low power consumption, can be very good to meet a single supply applications.

This article introduces a low dropout after the DC power supply design, which is too low to overcome the inconveniences caused by the power supply voltage, and saves the cost and time, I hope you read this article in the can this approach has a better understanding.

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