When designing the power section of any board, the most used voltage regulators are 78XX, 79XX, LM317, LM337 or similar. Engineers know these controllers are safe. reliable and easy to use, but they are limited in current. If you need more power you can find simple and cheap solutions by using the Analog Devices LT1083 regulator
A powerful regulator
The LT1083 regulator (see the symbol and the pinout in figure 1) allows you to adjust a positive voltage and provides a current of up to 7.5 A with high efficiency. The internal circuits are designed to operate up to 1 V differential between input and output. The maximum dropout voltage is 1.5V at the maximum output current. A 10 uF output capacitor is required. Here are some of its noteworthy features:
- adjustable output voltage;
- current up to 7.5 Ampere;
- TO220 container;
- internally limited dissipation power;
- maximum differential voltage of 30V.
It can be used for various applications such as switching regulators, constant current regulators, high efficiency linear regulators and battery chargers. The model examined in this tutorial features a variable and configurable output voltage. There are two other models, LT1083-5 and LT1083-12 which stabilize the output at 5 V and 12 V, respectively.
Figure 1: the LT1083 regulator
Minimal application diagram for an output voltage of 5 V
Figure 2 shows the application diagram for a 5 V regulator. The input voltage must always be greater than 6.5 V. The supply voltage of the circuit, of course, must not be extremely high, since all the power would end up unnecessarily dissipated in heat, thus drastically lowering the efficiency of the system. The regulator is connected through its three pins to the input, to the output and to a resistive voltage divider that determines the value of the output voltage. The presence of two capacitors, one at the input and one at the output, is strongly recommended. The scheme has the function of stabilizing the output voltage at exactly 5 V. For this reason, the divider is made up of two 1% precision resistors, the first being 121 Ohm and the second being 365 Ohm. It is obvious that the replacement of the two passive components with a trimmer or a potentiometer implements a variable voltage power supply system.
Figure 2: the minimal but perfectly functional application scheme with 5-V output voltage
Figure 3 shows a first measurement of the results with the current on the load and the power dissipated by the integrated regulator. The simulations are carried out by testing different values of the loads, with an impedance between 1 Ohm and 20 Ohm. A very important fact is the extraordinary constancy of the output voltage (always exactly 5 V) even if the load undergoes drastic variations. The current flowing through the load, in fact, is extremely variable, together with the power dissipated by the integrated regulator. When remaining within the operating limits set by the manufacturer, the regulator is therefore extremely stable and safe.
Figure 3: The results of the measurements on the schematic of the 5-V regulator
Figure 4: the graph of the input, output and Dropout voltage
The output voltage of the integrated (with the values used for the resistive divider) is very stable even if a load of a different entity is used, as can be seen in the graph in figure 5.
Figure 5: the graph shows the stability of the output, which is independent of the load used
The efficiency is much higher as the input voltage approaches the desired output voltage. The following average efficiency measurements were made using different load values, with three different power sources, respectively at 18 V, 12 V and 6.5 V.
- Input voltage: 18 V with circuit efficiency equal to 26.71%;
- Input voltage: 12 V with circuit efficiency equal to 40.84%;
- Input voltage: 6.5 V with circuit efficiency equal to 75.37%;
The regulator, therefore, works more when the input voltage is much higher than the output voltage and therefore dissipates so much more energy that is lost in unused heat.
Effects of temperature
The regulator examined in this tutorial is extremely stable even under temperature variations. Although the manufacturer certifies a stability of 0.5%, in the official documentation, the results obtained are even more satisfactory. Now let’s examine a simple application scheme equivalent to the first examined, with the following static characteristics:
- input voltage: 6.5 V;
- output voltage: 5 V;
- resistive impedance of the load connected at the output: 5 Ohm;
- load current: 1 A;
- power dissipated by the regulator: 1.51 W.
Now let’s run a simulation by varying the temperature in the range between -10 ° C and + 100 ° C. By examining the graph of figure 6 we discover that in a very wide range of temperatures (110 ° C of excursion) the output it has practically remained constant. The integrated circuit is extremely stable and the maximum variation of the output voltage, at the two thermal extremes, is only 6.2 microVolts.
Figure 6: The graph shows the variation of the output voltage at different operating temperatures
Protection diode
The LT1083 regulator does not requireany protection diodes, as shown in the diagram in figure 7. The new component design, in fact, allows limiting the return currents thanks to the use of internal resistors. Furthermore, the internal diode, that is between the input and the output of the integrated circuit, is able to manage current peaks lasting microseconds from 50 A to 100 A. Therefore, even the capacitor on the regulation pin is not strictly necessary. The regulator can be damaged only if a capacitor with a capacity greater than 5000 uF is connected to the output and, at the same time, the input pin short-circuited to ground. And this is an unlikely event.
Figure 7: The protection diode between output and input is no longer necessary
How to get different tensions
Between the output pin and the adjustment pin there is a reference voltage equal to +1.25 V. If a resistor is placed between these two terminals, a constant current flows through this resistance. The second resistor, connected to ground, has the function of setting the overall output voltage. A current of 10 mA is enough to obtain this regulation, in a precise way. By implementing a trimmer or a potentiometer, a variable voltage power supply can be created. The current flowing on the regulation pin is very low, in the order of microAmperes, and can be ignored. Here are the steps for calculating the two resistances, for a 14 V power supply, and they can be seen in the diagram of the divider in figure 8 and the formulas shown in figure 9:
- the input voltage Vin must always be dimensioned to at least 1 V more than the desired output voltage, therefore Vin> 15;
- between the output pin and the reference pin there is always a voltage of 1.25 V;
- the resistance R1 between the output pin and the reference one must be crossed by a current of 10 mA;
- the value of R1 is equal to the ratio between the potential difference on the resistance and the current that must pass through it;
- the reference pin voltage is equal to the output voltage minus the fixed voltage of 1.25V;
- the resistance R2 must also be passed through by a current of 10 mA, therefore it can be easily calculated with Ohm’s law.
With the R1=125 Ohm and R2=1275 values, the output voltage is exactly 14 V. A variable power supply with voltage between 1 V and Vin can be obtained with a 3.3 kOhm potentiometer instead of the R2 resistor.
Figure 8: the calculation of the divider resistances to obtain any voltage value
Figure 9: equations to calculate the two resistances
Conclusion
The 3-terminal LT1083 regulator is adjustable and it is very simple to use. It is equipped with various protections which are usually provided in high performance regulators. These protection systems concern short circuits and thermal shutdowns above 165 ° C. The exceptional stability allows creating top quality power systems. A 150 uF electrolytic capacitor or a 22 uF tantalum output capacitor is required for complete stability.
