Forget those three-terminal devices, you can design a high current regulator circuit like this

The LT1083 regulator (see symbol and pinout in Figure 1) allows regulation of positive voltages and can efficiently deliver currents up to 7.5 A. The internal circuitry is designed to operate with up to 1 V difference between input and output. The maximum dropout voltage is 1.5 V at maximum output current. A 10 uF output capacitor is required. Here are some features worth noting:

● Adjustable output voltage;

● Maximum current of 7.5 A;

● TO220 package;

● Internally limit power consumption;

● Maximum 30 V differential voltage.

It can be used in a variety of applications such as switching regulators, constant current regulators, high efficiency linear regulators, and battery chargers. The model discussed in this tutorial has a variable and configurable output voltage. There are two other models - LT1083-5 and LT1083-12, whose outputs are regulated at 5 V and 12 V respectively.

Figure 1: LT1083 regulator

Figure 2 shows the application reference diagram of a 5 V regulator. The input voltage must always be greater than 6.5 V. Of course, the supply voltage of the circuit cannot be too high, since all the power will eventually be dissipated unnecessarily in the form of heat, which greatly reduces the efficiency of the system. The regulator is connected through its three pins to the input, output and a resistor divider, the latter used to determine the value of the output voltage. It is highly recommended to use two capacitors, one at the input and one at the output. This solution has the function of stabilizing the output voltage at exactly 5 V. Therefore, the voltage divider consists of two 1% precision resistors, the first of which is 121 Ω and the second of which is 365 Ω. It is obvious that replacing these two passive components with trimmers or potentiometers will realize a power supply system with variable voltage.

Figure 2: Minimal but fully functional application scenario with 5 V output voltage

Figure 3 shows the first measurement results of the load current and the power consumption of the integrated regulator. The simulation was performed by testing different load values, with load impedance 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 consumption of the integrated regulator vary greatly. As long as they are within the operating limits set by the manufacturer, the regulator is very stable and safe.

Figure 3: Measurement results of the 5 V regulator schematic

The regulator is designed to support a voltage drop of up to 1 V. This voltage drop is independent of the load current; due to its low value, the efficiency of the final system can be very high. Figure 4 shows the curves for the input voltage (0 V to 8 V, red curve) and the output voltage (blue curve). The two voltages have an effective “voltage drop” of about 1 V between them, as specified by the manufacturer’s characterization.

Figure 4: Input, output and dropout curves

Even with loads of different physical dimensions, the output voltage of the integrated regulator (values ​​used for the resistor divider) is very stable, as shown by the curve in Figure 5.

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 desired output voltage, the efficiency is much higher. The following average efficiencies were measured with different load values ​​at three different supplies of 18 V, 12 V, and 6.5 V.

● Input voltage: 18 V, circuit efficiency equals 26.71%;

● Input voltage: 12 V, circuit efficiency equals 40.84%;

● Input voltage: 6.5 V, circuit efficiency equals 75.37%;

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

The voltage regulators discussed in this tutorial are very stable even in the presence of temperature variations. Although the stability certified by the manufacturer in the official documentation is 0.5%, the results obtained in practice are much more satisfactory. Now let's consider a simple application solution equivalent to the first one 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 terminal: 5 Ω;

● Load current: 1 A;

● Regulator power consumption: 1.51 W.

Now, we vary the temperature from -10 °C to +100 °C and run the simulation. The output remains virtually constant over a very wide temperature range (110 °C difference) as can be seen from the curves in Figure 6. The IC is very stable, with a maximum change of only 6.2 uV in the output voltage at the two temperature extremes.

Figure 6: Curve showing output voltage variation at different operating temperatures

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, the internal diodes between the input and output of the integrated circuit are able to manage current peaks of 50 A to 100 A lasting for a few microseconds. Therefore, the capacitor on the adjustment pin is not strictly required. The regulator can only be damaged if a capacitor with a capacitance value greater than 5000 uF is connected to the output while the input pin is shorted to ground, which is an unlikely event.

Figure 7: No more protection diodes between output and input

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 will flow through this resistor. The second resistor connected to ground has the function of setting the overall output voltage. A current of 10 mA is sufficient to obtain this precise regulation. By implementing a trimmer or potentiometer, a variable voltage supply can be created. The current on the adjustment pin is very low (on the order of a few microamperes) and can be ignored. For a 14 V supply, the following are the steps to calculate the two resistors, which can be seen in the voltage divider diagram in Figure 8 and in 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 1.25 V voltage 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 a fixed voltage of 1.25 V;

● A current of 10 mA must also flow through resistor R2, so this can be easily calculated using Ohm's law.

When R1 = 125 Ω and R2 = 1275 Ω, the output voltage is exactly 14 V. By replacing the R2 resistor with a 3.3 kΩ potentiometer, a variable power supply with a voltage from 1 V to Vin can be obtained.

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

Figure 9: Equations for calculating the two resistors

The 3-pin LT1083 regulator is adjustable and very easy to use. It has several protection features that are usually only available in high performance regulators. These protection systems handle short circuit conditions and thermal shutdown occurs when the temperature exceeds 165°C. Excellent stability allows 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.