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Power supply design scheme of monolithic or integrated linear regulator

Power supply design scheme of monolithic or integrated linear regulator

Monolithic or integrated linear regulators allow you to create a stable power supply with only a few components, saving a lot of time and space. In addition, many devices on the market today integrate advanced functions such as short-circuit, over-voltage and over-current protection. Since its introduction, linear regulators have occupied the power supply field for a long time, at least until the advent of switching technology. Because of their simplicity, they are still used in a variety of applications, especially low-power applications. The main limitation of linear regulators is efficiency: the lower the efficiency, the greater the difference between the output voltage and the input voltage.

Author: powerelectronicsnews

Monolithic or integrated linear regulators allow you to create a stable power supply with only a few components, saving a lot of time and space. In addition, many devices on the market today integrate advanced functions such as short-circuit, over-voltage and over-current protection. Since its introduction, linear regulators have occupied the power supply field for a long time, at least until the advent of switching technology. Because of their simplicity, they are still used in a variety of applications, especially low-power applications. The main limitation of linear regulators is efficiency: the lower the efficiency, the greater the difference between the output voltage and the input voltage.

Figure 1 shows the efficiency trend as a function of the VOUT/VIN ratio: when the ratio tends to 1, the best results can be obtained. If the difference between the output voltage and the input voltage is not negligible, then appropriate heat must also be provided. Manage solutions because the excess energy is dissipated in the form of heat.

Power supply design scheme of monolithic or integrated linear regulator
Figure 1: Efficiency and VOUT/VIN ratio

A very important parameter of each linear regulator is the dropout voltage, which is the difference between the input voltage and the output voltage. For example, when the output voltage of the regulator is VOUT = 5 V, and the dropout voltage VDROPOUT = 1.5 V, the input voltage must never drop below 6.5 V. The main features and application circuits involved some of the most popular linear regulators available on the market are as follows.

78xx and 79xx series

The 78xx series monolithic linear regulators (for positive voltage) and the 79xx series (for negative voltage) are one of the longest-lived series on the market. These regulators have a wide range of output voltage VOUT and up to 40 V input voltage VIN, can provide up to 1 A of output current, depending on the model, in plastic or metal packaging. The regulator includes internal circuits for thermal protection and short-circuit protection.

Depending on the model, the differential voltage varies from 1.7V to 2.5V. Figure 2 shows a typical application circuit for a 78xx series regulator-in this case 7812. It should be noted that capacitors C1 and C2 will not be very small (a few millivolts) due to the ripple output of the regulator, and therefore require particularly high capacitance values. Although these two capacitors are not strictly necessary, they can provide stability to the circuit and improve transient response.

Power supply design scheme of monolithic or integrated linear regulator
Figure 2: Application circuit of 78xx regulator

Figure 3 shows a similar circuit, this time based on a 79xx negative voltage regulator (note the inversion of the diode bridge).

Power supply design scheme of monolithic or integrated linear regulator
Figure 3: Application circuit of 79xx regulator

By using a center-tapped transformer, two circuits can be easily combined to create a dual power supply (for example, 12 V and C12 V).

LM1117

We decided to choose this regulator instead of LM317 (very popular in the past few years), because compared with the latter, the LM1117 has a lower voltage drop (1.2V at a maximum output current of 800 mA) and a larger voltage drop . Suitable for generating 3.3V output voltage (widely used by today’s microcontrollers). LM1117 provides both fixed output voltage (1.8 V, 2.5 V, 3.3 V and 5 V) versions, as well as a variable output voltage version, which can be adjusted by a pair of external resistors. Figure 4 shows the application circuit of a regulator with a fixed voltage output, and in Figure 5, the corresponding circuit of a regulator with a variable output voltage can be seen.

Power supply design scheme of monolithic or integrated linear regulator
Figure 4: Application circuit of LM1117 fixed output version

Power supply design scheme of monolithic or integrated linear regulator
Figure 5: Application circuit of LM1117 variable output version

The LM1117 has integrated functions for current limiting and thermal shutdown, as well as an internal Zener diode to ensure an output voltage accuracy of ±1%. Although not absolutely necessary, it is recommended to insert a tantalum capacitor of at least 10 F at the output to improve stability and transient response. The circuit in Figure 6 shows a regulator used to generate a negative output voltage (C5 V).

Power supply design scheme of monolithic or integrated linear regulator
Figure 6: LM1117 used to generate negative voltage

LT3080

LT3080 is a low-dropout (up to 350mV) linear regulator, its output voltage is adjustable from 1.2V to 36V, the maximum current is equal to 1.1A, and the accuracy is as high as 1%. Figure 7 shows the application circuit of the voltage regulator: the external resistor RSET allows you to select the output voltage value.

Power supply design scheme of monolithic or integrated linear regulator
Figure 7: Monolithic linear regulator

An important feature of the LT3080 is that it can be parallelized to obtain greater output current and improve thermal management. An example is shown in Figure 8.

Power supply design scheme of monolithic or integrated linear regulator
Figure 8: Multiple LT3080s connected in parallel

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