When designing the power supply part of any circuit board, the most commonly used voltage regulators are 78xx, 79xx, LM317, lm337 or similar devices. Engineers know that these controllers are safe, reliable and easy to use, but their current is limited. If more current is needed, ADI’s lt1083 regulator can be used to realize a simple and affordable solution.

A powerful voltage regulator

The lt1083 voltage regulator (see the symbol and pin arrangement in Figure 1) allows positive voltage adjustment and can efficiently provide a current of up to 7.5 a. The internal circuit is designed to operate at a voltage difference of up to 1 V between input and output. Under the condition of maximum output current, the maximum differential voltage is 1.5 v. A 10 UF output capacitor is required. Here are some notable features:

● adjustable output voltage;

● current up to 7.5 a;

● to220 package;

● internal limited power consumption;

● differential voltage up to 30 v.

It can be used in various applications, such as switching regulator, constant current regulator, high efficiency linear regulator and battery charger. The models discussed in this tutorial have variable and configurable output voltages. There are two other models, lt1083-5 and lt1083-12, whose output is stable at 5 V and 12 V respectively.

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Figure 1: lt1083 voltage regulator

Minimum application diagram of 5 V output voltage

Figure 2 shows the application reference diagram of 5 V regulator. The input voltage must always be greater than 6.5 v. Of course, the power supply voltage of the circuit can not be too high, because all power will eventually dissipate unnecessarily in the form of heat, which greatly reduces the efficiency of the system. The regulator is connected to the input, output and resistance voltage divider through its three pins, which is used to determine the value of the output voltage. It is strongly recommended to use two capacitors, one at the input and one at the output. The scheme has the function of stabilizing the output voltage at exactly 5 v. Therefore, the voltage divider consists of two 1% precision resistors, the first is 121 Ω and the second is 365 Ω. Obviously, a variable voltage power supply system can be realized by replacing these two passive components with a regulator or potentiometer.

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Figure 2: minimum but complete of 5 V output voltage

Application scheme that can work normally

Figure 3 shows the first measurement results of load current and integrated regulator power consumption. The simulation is carried out by testing different load values, and the load impedance is in the range of 1 Ω to 20 Ω. A very important fact is that even if the load changes greatly, the output voltage is very stable (always 5 V). However, the current flowing through the load and the power consumption of the integrated regulator vary greatly. The regulator is very stable and safe as long as it is within the working limits set by the manufacturer.

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Figure 3: measurement results of schematic diagram of 5 V voltage regulator

The regulator is designed to support a differential pressure of up to 1 v. This voltage difference is independent of the load current; Due to its low value, the efficiency of the final system may be very high. Figure 4 shows the curve of input voltage (0 V to 8 V, red curve) and output voltage (blue curve). According to the manufacturer’s characteristics, there is an effective “voltage difference” of about 1 V between the two voltages.

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Figure 4: curve of input, output and differential pressure

Even if loads of different entities are used, the output voltage of the integrated voltage regulator (the value is used for the resistance voltage divider) is very stable, as shown in the curve in Figure 5.

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Figure 5: the curve shows the stability of the output, which is independent of the load used

When the input voltage is close to the required output voltage, the efficiency is much higher. The following average efficiencies were measured using different load values at three different power supplies of 18 V, 12 V and 6.5 v.

● input voltage: 18 V, circuit efficiency equal to 26.71%;

● input voltage: 12 V, circuit efficiency equal to 40.84%;

● input voltage: 6.5 V, circuit efficiency equal to 75.37%;

Therefore, when the input voltage is much higher than the output voltage, the regulator needs to work harder and consume more energy (lost with useless heat).

Temperature effect

The voltage regulator discussed in this tutorial is very stable even if there are temperature changes. Although the stability certified by the manufacturer in the official documents is 0.5%, the actual results are more satisfactory. Now we study a simple application scheme equivalent to the first scheme above, which has the following static characteristics:

● input voltage: 6.5 V;

● output voltage: 5 V;

● resistive impedance of the load connected to the output end: 5 Ω;

● load current: 1 A;

● power consumption of voltage regulator: 1.51 W.

Now let’s change the temperature in the range of – 10 ° C to + 100 ° C and run the simulation. From the curve shown in Figure 6, it can be found that the output actually remains constant over a very wide temperature range (110 ° C temperature difference). The IC is very stable. Under the two temperature extremes, the maximum change of output voltage is only 6.2 UV.

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Figure 6: curve showing the change of output voltage under different operating temperatures

Protective diode

The lt1083 regulator does not require any protective diodes, as shown in Figure 7. In fact, the new element design can limit the return current due to the use of internal resistance. In addition, the internal diode between the input and output of the integrated circuit can manage 50 A to 100 a current peaks lasting for several microseconds. Therefore, the capacitor on the regulating pin is not strictly required. Only when a capacitor with a capacitance greater than 5000 UF is connected to the output and the input pin is short circuited to ground can the voltage regulator be damaged, which is an unlikely event.

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Figure 7: protective diode is no longer required between output and input

How to obtain different voltages

There is a reference voltage equal to + 1.25 V between the output pin and the regulation pin. If a resistor is placed between the two terminals, a constant current flows through the resistor. The second resistor connected to the ground has the function of setting the overall output voltage. A current of 10 Ma is sufficient to achieve this precise adjustment. By implementing a regulator or potentiometer, a variable voltage power supply can be created. The current on the regulating pin is very low (about a few microamps) and can be ignored. For the 14 V power supply, the following are the steps to calculate these two resistances, which can be seen in the voltage divider diagram in Figure 8 and the formula shown in Figure 9:

● the input voltage Vin must always be at least 1 V higher than the required output voltage, so VIN > 15 V;

● there is always a voltage of 1.25 V between the output pin and the reference pin;

● there must be 10 mA current in the resistance R1 between the output pin and the reference pin;

● the value of R1 is equal to the ratio of the potential difference on the resistance to the current that must flow through it;

● the reference pin voltage is equal to the output voltage minus the fixed voltage of 1.25 V;

● a current of 10 Ma must also flow through resistance R2, so it can be easily calculated by Ohm’s law.

When R1 = 125 Ω and R2 = 1275 Ω, the output voltage is exactly 14 v. Using a 3.3 K Ω potentiometer instead of the R2 resistor, a variable power supply with a voltage of 1 V to VIN can be obtained.

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Figure 8: Calculation of voltage divider resistance required to obtain any voltage value

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Figure 9: equation for calculating these two resistances

conclusion

The 3-pin lt1083 regulator is adjustable and very easy to use. It has a variety of protection functions that are usually provided only by high-performance voltage regulators. These protection systems can cope with short circuit conditions and thermal shutdown when the temperature exceeds 165 ° C. Excellent stability supports the creation of high-quality power systems. To ensure complete stability, a 150 UF electrolytic capacitor or a 22 UF tantalum output capacitor is required.

Reviewed and edited by: Li Qian

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