Monolithic, or integrated, linear voltage regulators make it possible to build a stable power supply with a small number of parts, saving both time and space. Additionally, a lot of the modern devices on the market incorporate cutting-edge features like short-circuit, overvoltage, and overcurrent protection. Since they were first used, linear regulators have dominated the power supply industry—at least until switching technology appeared. They continue to be employed in many applications, particularly low-power ones, due to their simplicity. Efficiency is the primary constraint on linear regulators since the lower it is, the bigger the disparity between the input and output voltages.
The efficiency trend as a function of the VOUT/VIN ratio is shown in Figure 1. When the VOUT/VIN ratio leans toward 1, the best outcomes are attained. Because extra energy is lost as heat when output and input voltages range noticeably, it is also vital to offer suitable thermal management solutions.
Figure 1: Efficiency vs. VOUT/VIN ratio
The dropout voltage, or the difference between the input voltage and the output voltage, is a crucial component of every linear regulator. The input voltage should never drop below 6.5 V, for instance, in the case of a regulator with an output voltage VOUT of 5 V and a dropout voltage VDROPOUT of 1.5 V. The following list of some of the most well-liked linear regulators on the market includes their key characteristics and application circuits.
78xx and 79xx series
The 78xx series of monolithic linear regulators (for positive voltages) and 79xx (for negative voltages) are among the longest-lived on the market. With a wide range of output voltages VOUT and input voltage VIN up to 40 V, these regulators can deliver an output current of up to 1 A, with plastic or metallic package depending on the model. Regulators include internal circuits for thermal and short-circuit protection.
The dropout voltage, depending on the model, varies between 1.7 V and 2.5 V.
By using a center-tapped transformer, the two circuits based on 78xx and 79xx linear voltage regulators can be easily combined, creating a dual power supply (for instance, 12 V and –12 V).
We have decided to include in our selection this regulator and not the LM317 (very popular in past years) because, compared with the latter, LM1117 has a lower dropout voltage (1.2 V at the maximum deliverable current of 800 mA) and it is more suitable for generating a 3.3-V output voltage (widely used by today’s microcontrollers). The LM1117 is available both in the version with fixed output voltage (1.8 V, 2.5 V, 3.3 V, and 5 V) and in the version with variable output voltage, adjustable through a pair of external resistors.
The LM1117 has integrated features for current limiting and thermal shutdown, as well as an internal Zener diode, which ensures an accuracy of the output voltage of ±1%. Although not strictly necessary, it is advisable to insert at the output a tantalum capacitor of at least 10 μF to improve stability and transient response. The regulator can also be used to generate a negative output voltage.
The LT3080 is a low-dropout (up to 350 mV) linear regulator with output voltage adjustable between 1.2 V and 36 V, maximum current equal to 1.1 A, and accuracy up to 1%.
An important feature of the LT3080 is that it can be parallelized in such a way as to obtain a greater output current and improve thermal management.
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