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Nov 19 2020

LM317 Voltage Regulator: Principle, Application [FAQ]

Description

This blog briefly introduces LM317 in two parts. The first part is the principle of the LM317 three-terminal adjustable voltage regulator; the second part is three application circuits with a three-terminal voltage regulator integrated circuit LM317.

LM317

Catalog

I Description

II LM317 Overview

III LM317 Power Supply

IV LM317 Voltage Source

V Programming LM317 Regulator Circuit

VI Conclusion

Component Datasheet

FAQ

II LM317 Overview

LM317 is an adjustable three-terminal regulator. The world's major IC manufacturers have similar products to choose from. Therefore, LM317 is a widely used series of integrated voltage regulators, and can be used as a voltage regulator circuit in a DC regulated power supply. LM317 performs well in the following aspects:

  • Regulating accuracy
  • Ripple rejection ratio
  • The output voltage
  • Overcurrent and overheat protection
  • Regarding the protection of the safe working area of the regulator.

Therefore, LM317 has a very wide range of applications.

The output voltage of LM317 is 1.2V-37V, which guarantees an output current of 1.5A and good voltage stability. Its 3 pins (as shown in Figure 1) are

  • Adjust Terminal
  • Output Terminal
  • Input Terminal

It is worth mentioning that LM317 has no ground terminal. LM317 uses a floating circuit structure with a reference voltage of 1.25V.

lm317 pinout

Figure 1. LM317 Pinout

The voltage difference between LM317 output terminal and adjustment terminal is constant at 1.25V. Refer to Figure 2 to know that through the external series resistor divider, a stable current is set and remains unchanged. In this way, the output voltage can be stabilized, and its value is:

The output terminal is connected with an external resistor R1 and the adjustment terminal is connected with an external resistor R2. The resistance of R2 can be adjusted from the above formula to change the output voltage Vo and make it stable. In actual use of LM317, R1 should be as close to pin 2 as possible. In this way, a more stable and accurate output voltage can be obtained.

lm317 structure

Figure 2. LM317 Structure

Next, we will introduce you to 3 kinds of LM317 adjustable voltage stabilizer application circuit. These three circuits are:

  • Tracking pre-adjusted regulated power supply;
  • The automatic balance voltage source of the output current is expanded through the parallel regulator;
  • Programmable output voltage regulator circuit.

III LM317 Power Supply

A circuit composed of two stages (as shown in Figure 3) can get a very stable output voltage. Connect the adjustment terminal of the first stage to the output terminal of the second stage through 2R. In this way, the input and output potential difference of the second stage can be limited. The difference is calculated as:

To change the output voltage value, R4 can be adjusted, and the output voltage of the first LM317 (that is, the input voltage of the second LM317) also changes accordingly. After the first stage pre-adjustment, the second stage working conditions are more stable. The final output voltage stability accuracy can reach 1PPM and DC voltage less than 1μV.

lm317 circuit

Figure 3. LM317 Circuit

IV LM317 Voltage Source

The maximum stable output current of LM317 is 1.5A. The method of expanding the output current can be used to increase the output current. Generally, it is difficult to achieve synchronous balance due to the difference in device performance in the parallel connection of multiple regulators. As a result, the output current is unstable, and the external power tube to amplify the output current will reduce the working accuracy of the regulator. Therefore, LM317 can use a unique parallel method to expand the output current while keeping the accuracy index of its regulator unchanged.

Two LM317s (Figure 4) can be used to form the adjustable voltage source of 3A output. The output current of the two LM317 regulators is balanced by the integrated operational amplifier, and the output voltage is adjusted uniformly using R4 and R5.

lm317 circuit

Figure 4. LM317 Circuit

In the figure, two small 100mΩ resistors are used to realize current sampling.

When the voltage regulator U1 increases the output current, the current on R1 increases, and its voltage drop increases. Subsequently, the signal at the non-inverting input terminal of the integrated operational amplifier changes, causing the output terminal voltage to decrease.

In this way, the output voltage of the regulator U1 decreases, and the output current decreases, offsetting the original increase.

If the regulator U2 increases the output current, the current on R3 also increases, and its voltage drop increases. Subsequently, the signal at the inverting input terminal of the integrated operational amplifier changes, and the voltage at the output terminal of the integrated operational amplifier increases. This makes the output voltage of the regulator U1 increase and the output current increases.

Finally, the output current of the two regulators is automatically balanced.

V Programming LM317 Regulator Circuit

lm317 circuit

 

Figure 5. LM317 Circuit

As shown in Figure 5, a fixed resistor R is connected between the output terminal and the adjustment terminal of the LM317. By adding a programmable resistor network, a voltage stabilizing circuit with programmed output can be made. According to actual needs, the transistor and resistance R can be selected by oneself, and the circuit can be used in intelligent instruments and control.

VI Conclusion

It can be seen that this chip can complete circuits with different functions through different matching methods during use. So LM317 is a practical chip with powerful functions and simple structure.


Component Datasheet

LM317 Datasheet


FAQ

  • What is lm317 used for?

The LM317 serves a wide variety of applications including local, on card regulation. This device can also be used to make a programmable output regulator, or by connecting a fixed resistor between the adjustment and output, the LM317 can be used as a precision current regulator.

  • What is the maximum input voltage of lm317?

The LM317 is an adjustable voltage linear regulator that can output 1.25 – 37V at up to 1.5A current with an input voltage range of 3 – 40V.

  • What is the difference between lm317 and lm317t?

Member. There is no functional difference as they are one in the same. The T at the end just indicates that it's in a TO-220 package. They usually tag on extra things after the part name to reference things like package, temp range, etc.

  • Is lm317 a transistor?

The LM317 is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 32 V. ... By using a heat-sinked pass transistor such as a 2N3055 (Q1) we can produce several amps of current far above the 1.5 amps of the LM317.

  • How does an lm317 work?

The circuit consists of a low-side resistor and high-side resistor connected in series forming a resistive voltage divider which is a passive linear circuit used to produce an output voltage which is a fraction of its input voltage. 

  • What is IC lm317?

The LM317 device is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 37 V. It requires only two external resistors to set the output voltage. The device features a typical line regulation of 0.01% and typical load regulation of 0.1%.

  • How do I know if my lm317 is working?

Testing lm317t.
If you look to the ic, the legs towards you, the right one is the input pin. you must see a difference of minimum 1.2V between the two pins, otherwise the IC is faulty. furthermore, the first test is to see if you have input voltage !

  • What is the working priciple of lm317?

LM 317 works on a very simple principle. It is a variable voltage regulator i.e. supports different output voltage levels for a constant applied input voltage supply.

  • How to make a simple voltage regulator circuit using LM317?

 

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