LM317 Adjustable Linear Regulator
The LM317 is a classic three-terminal adjustable positive linear voltage regulator. Instead of manufacturing a different IC for every output voltage, the desired output is set with two external resistors.
higher DC→LM317
ADJUSTABLE REGULATOR→chosen DC output
ADJUSTABLE REGULATOR→chosen DC output
INOUTADJ
The adjustment idea
The regulator maintains about 1.25 V between its OUT and ADJ terminals. A resistor divider translates that internal reference into a higher output voltage.
VOUT ≈ 1.25(1 + R2/R1)
That beginner's form ignores the small adjustment-pin current. The full datasheet equation includes it.
Easy example: R1 = 240 Ω and R2 = 720 Ω. Since 720 / 240 = 3, the approximate output is 1.25 × (1 + 3) = 5 V.
Why it became so useful
- Only two resistors are needed to select the basic output voltage.
- The same device can serve many different supply voltages.
- It includes current limiting and thermal protection.
- It can also be arranged as a current regulator.
Practical limits still apply
The LM317 is still a linear regulator. It needs input-to-output headroom and it dissipates approximately the voltage it drops multiplied by load current. A large voltage drop at high current can require a substantial heat sink.
TI's current LM317 documentation specifies an adjustable range beginning around 1.25 V and devices capable of more than 1.5 A under appropriate conditions. Those numbers are not permission to ignore thermal limits—the package must still get rid of the heat.
Manufacturer reference: TI LM317.