“When designing the power section of any board, the most commonly used regulators are 78XX, 79XX, LM317, LM337 or similar. Engineers know these controllers are safe, reliable and easy to use, but they have limited current. If higher currents are required, a simple and affordable solution can be achieved using ADI’s LT1083 regulator.
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When designing the power section of any board, the most commonly used regulators are 78XX, 79XX, LM317, LM337 or similar. Engineers know these controllers are safe, reliable and easy to use, but they have limited current. If higher currents are required, a simple and affordable solution can be achieved using ADI’s LT1083 regulator.
A powerful regulator
The LT1083 regulator (see symbol and pinout in Figure 1) allows positive voltage regulation and can efficiently deliver currents up to 7.5 A. The internal circuitry is designed to operate with a voltage drop of up to 1 V between the input and output. At maximum output current, the maximum dropout voltage is 1.5 V. A 10 uF output capacitor is required. Here are some features worth noting:
● Adjustable output voltage;
● Current up to 7.5 A;
● TO220 package;
● Internally limit power consumption;
● Maximum 30 V differential voltage.
It can be used in various applications such as switching regulators, constant current regulators, high efficiency linear regulators and battery chargers. The models discussed in this tutorial have variable and configurable output voltages. There are two other models, the LT1083-5 and LT1083-12, whose outputs are regulated at 5 V and 12 V, respectively.
Figure 1: LT1083 regulator
Minimum application diagram for 5 V output voltage
Figure 2 shows an application reference diagram for a 5 V regulator. The input voltage must always be greater than 6.5 V. Of course, the supply voltage to the circuit cannot be too high, as all power ends up being dissipated unnecessarily in the form of heat, greatly reducing the efficiency of the system. The regulator is connected through its three pins to the input, output, and a resistive divider, which determines the value of the output voltage. It is strongly recommended to use two capacitors, one at the input and one at the output. This scheme has the function of regulating the output voltage at exactly 5 V. So 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 system can be achieved by replacing these two passive components with a regulator or potentiometer.
Figure 2: Minimal but fully functional application scenario for 5 V output voltage
Figure 3 shows the first measurements of load current and power dissipation of the integrated regulator. Simulations were performed by testing different load values with load impedances ranging from 1 Ω to 20 Ω. A very important fact is that the output voltage is very stable (always 5 V) even with large changes in the load. However, the current flowing through the load and the power dissipation of the integrated voltage regulator vary greatly. The regulator is very stable and safe as long as it is within the operating limits set by the manufacturer.
Figure 3: Measurement results of the 5 V regulator schematic
The regulator is designed to support dropout voltages up to 1 V. This dropout is independent of load current; due to its low value, the final system can be very efficient. Figure 4 shows the curves of input voltage (0 V to 8 V, red curve) and output voltage (blue curve). According to the manufacturer’s characterization, there is an effective “dropout” of approximately 1 V between these two voltages.
Figure 4: Curves of input, output and differential pressure
The output voltage of the integrated regulator (value for the resistive divider) is very stable even with different physical loads, as shown in the curves in Figure 5.
Figure 5: The curve shows the stability of the output, independent of the load used
Efficiency is much higher when the input voltage is close to the desired output voltage. The average efficiencies below were measured with different load values at three different supplies of 18 V, 12 V and 6.5 V.
● Input voltage: 18 V, the circuit efficiency is equal to 26.71%;
● Input voltage: 12 V, the circuit efficiency is equal to 40.84%;
● Input voltage: 6.5 V, the circuit efficiency is 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 as useless heat).
temperature effect
The regulators discussed in this tutorial are very stable even in the presence of temperature variations. Although the manufacturer certifies the stability as 0.5% in official documents, the results obtained in practice are more satisfactory. Now we study a simple application scheme equivalent to the first scheme above, which has the following static properties:
● Input voltage: 6.5 V;
● Output voltage: 5 V;
● The resistive impedance of the load connected to the output end: 5 Ω;
● Load current: 1 A;
● Voltage regulator power consumption: 1.51 W.
Now, we vary the temperature from -10 °C to +100 °C and run the simulation. From the curves shown in Figure 6, it can be seen that the output remains practically constant over a very wide temperature range (110 °C temperature difference). The IC is very stable, with a maximum change in output voltage of only 6.2 uV over two temperature extremes.
Figure 6: Curves showing output voltage variation at different operating temperatures
protection diode
The LT1083 regulator does not require any protection diodes, as shown in Figure 7. In fact, the new component design is able to limit the return current due to the use of internal resistors. In addition, internal diodes between the input and output of the IC are able to manage current peaks of 50 A to 100 A lasting several microseconds. Therefore, capacitors on the adjust pins are not strictly required either. Damage to the regulator is only possible if a capacitor with a value greater than 5000 uF is connected to the output while the input pin is shorted to ground, an unlikely event.
Figure 7: Protection diodes are no longer required between output and input
How to get different voltages
Between the output pin and the adjust pin, there is a reference voltage equal to +1.25 V. If a resistor is placed between these two terminals, a constant current will flow through the resistor. A second resistor connected to ground has the function of setting the overall output voltage. A current of 10 mA is sufficient for this precise regulation. By implementing a trimmer or potentiometer, a variable voltage power supply can be created. The current on the adjust pin is very low (about a few microamps) and can be ignored. For a 14 V supply, here are the steps to calculate these two resistors, which can be seen in the voltage divider diagram in Figure 8 and the equation shown in Figure 9:
● The input voltage Vin must always be at least 1 V higher than the desired 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 a current of 10 mA in the resistor R1 between the output pin and the reference pin;
● The value of R1 is equal to the ratio of the potential difference across the resistor 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 resistor 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 kΩ potentiometer in place of the R2 resistor, a variable supply voltage from 1 V to Vin can be obtained.
Figure 8: Calculation of voltage divider resistance required to obtain any voltage value
Figure 9: Equations for calculating these two resistors
in conclusion
The 3-pin LT1083 regulator is adjustable and very easy to use. It has a variety of protection features usually only provided by high-performance voltage regulators. These protection systems handle short-circuit conditions and thermal shutdown when temperatures exceed 165°C. Excellent stability supports the creation of high-quality power systems. To ensure full stability, a 150 uF electrolytic capacitor or a 22 uF tantalum output capacitor is required.
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