LM317 IC: Pinout, Specifications, Circuits and Applications

This article explains the LM317 integrated circuit which is commonly used as a linear voltage regulator in DC circuits. It is widely available as a TO-220 package which is called LM317T.  Electronic circuits that use linear voltage regulators can maintain a constant voltage regardless of changes in the input voltage or load. This is crucial since a lot of electronic devices, like sensors and microcontrollers, depend on a steady voltage to operate.

LM317 IC Pinout

Given below is the pinout of LM317 TO-220 package or the LM317T:

Pin 1- ADJUST: This pin is used to modify the regulator’s output voltage with the help of external resistors.

Pin 2- OUTPUT: The regulator’s output terminal. This terminal will source current such that its voltage remains 1.25V more than the voltage at the ADJUST terminal.

Pin 3- INPUT: The regulator’s input terminal.

The SOT-223 package of LM317 has 4 pins, where both pins 2 and 4 act as output.

Electrical Characteristics of LM317 IC

Here are some of the specifications of LM317 as mentioned in the datasheet

LM317 Voltage Parameters

  • Output voltage ranges from 1.25V to 37V according to the datasheet.
  • Max input voltage of LM317 is not specified in the datasheet. Instead, an input-to-output differential voltage range is given. This difference can be a maximum of up to 40V. For example, if we desire 15V at the output, then the voltage at the input terminal of LM317 can be 55V (i.e.15V+40V).
  • LM317 requires a minimum dropout voltage of 3V. Dropout voltage is the voltage difference between the input and output terminals for reliable voltage regulation.

LM317 Current Parameters

  • The maximum output current is 1.5 A if the input voltage ranges between 4.25V to 40V.
  • The IC requires a minimum load current of around 3.5 mA to maintain regulation.

Basic LM317 Circuits

LM317 IC can be combined with external components for a wide range of applications. Given below are some of its most common uses.

LM317T as a Linear Voltage Regulator

The circuit diagram below illustrates how we can use IC LM317 as a voltage regulator using just two transistors and two capacitors.

In the LM317T voltage regulator circuit above, resistors R1 and R2 determine the output voltage of the regulator. C1 can be in the range of 0.1uF to 1uF to filter noise on the input terminal. The capacitor also improves the transient response of the circuit when high current spikes occur at the output. C2 ensures the stability of the output voltage.

The formula for output voltage(Vo) is given by:

$V_{O}=1.25V (1 + \frac{R_{2}}{R_{1}})$

LM317 Current Limiter

A single resistor or a potentiometer connected between the ADJUST and OUTPUT terminal can help LM317 to act as a precision current regulator. Below is the schematic of a current limiter using LM317.

The ground wire is common to both the input and output. It does not connect to the LM317 when it is used as a current limiter. The current(ILIMIT) at the output is given by the formula:

$I_{LIMIT}=\frac{1.25}{R_{1}}$

For example, if we want to limit the output current to 1 A, we can use a potentiometer to set the value of R1 to 1.25 ohm.

LM317T Battery Charger Circuit

The above two circuits can be combined to make a simple battery charger using LM317 IC. Battery chargers often require both current limitation and voltage regulation. The schematic below can be used as a simple 12V lead acid battery charger.

The voltage at the output can be set using the resistors R1 and R2, and Rs sets the maximum current that LM317 can source to the battery.

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Advantages & Disadvantages Of LM317 IC

Advantages:

  • One of the main advantages of LM317 IC is its small form factor and minimal external component requirement.
  • Another advantage of LM317 over a switched-mode power supply is its low noise. This makes it ideal for use in audio projects where low noise is desired.

Disadvantages:

  • Since it is a linear voltage regulator, it can generate a lot of heat. Heat sinks are required for high-current applications. Also when the difference between the input and output voltage is high, a heat sink of adequate size must be used.
  • Its upper voltage input is limited to only about 40V.
  • Output current is limited to 1.5A.

Alternatives to LM317 IC

  • LM78XX series of three-terminal positive voltage regulators can be used when a constant voltage is desired. For example, LM7805 can be used for 5V output, LM7809 for 9V output, and so on.
  • For high-current applications, LM338 can be used to output up to 5A current.
  • If a low dropout voltage is required, LM2940 can be used which can have a dropout voltage of 0.5V at 1A output current.
  • For negative voltage regulation, LM337-N IC can be used.
  • For high-voltage applications, LM317HV can be used which can regulate input voltages up to 60V.

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