Adjustable Voltage Regulator LM317

The LM317 is a three-terminal adjustable positive regulator that can be set to any output voltage from 1.25V to 37V using just two external resistors. Audio amplifier builders use it to power op-amp stages at any voltage a project demands, without stocking a shelf full of fixed regulators.

Core Concept

The LM317 regulates by holding a fixed reference voltage of 1.25V between its OUTPUT pin and its ADJUST (ADJ) pin. This 1.25V appears across resistor R1, which is connected from OUT to ADJ. A steady current flows through R1, and this same current (plus a small adjustment current Iadj ≈ 50 µA) flows through R2, which goes from ADJ to ground. The output voltage is therefore set by the ratio R2/R1.

The LM317 operates from input voltages of 3V to 40V and can supply up to 1.5A of output current. It needs at least 3V more than its output to regulate (its dropout voltage). Unlike the LM7805, you can dial in any output from 1.25V to 37V with no modification to the IC itself, just by changing R1 and R2.

The datasheet recommends R1 = 240 Ω as the standard value. A 0.1 µF ceramic capacitor at the output improves noise. A 10 µF electrolytic in parallel with R2 reduces output ripple. A protection diode from output to input prevents the output capacitor from discharging back through the IC during a fast input transient.

Key Equations

Output voltage: Vout = Vref * (1 + R2/R1) + Iadj * R2

Simplified (Iadj negligible): Vout ≈ 1.25 * (1 + R2/R1)

where Vref = 1.25V (internal reference), R1 is typically 240 Ω, and Iadj ≈ 50 µA (usually neglected).

Solving for R2 given a target Vout: R2 = R1 * ((Vout / 1.25) - 1)

Minimum input voltage: Vin_min = Vout + 3V (dropout)

Given:
  LM317 circuit
  R1   = 240 Ω
  Vout = 9V required
  Find: R2

Why this formula:
  LM317 holds 1.25V across R1, so Vout depends on R2/R1 ratio.

Formula:
  Vout = 1.25 * (1 + R2/R1)
  R2   = R1 * ((Vout / 1.25) - 1)

Substitution:
  R2 = 240 * ((9 / 1.25) - 1)
     = 240 * (7.2 - 1)
     = 240 * 6.2

Calculation:
  R2 = 1488 Ω
  Use nearest standard value: 1.5 kΩ

  Verify with R2 = 1.5 kΩ:
  Vout = 1.25 * (1 + 1500/240)
       = 1.25 * (1 + 6.25)
       = 1.25 * 7.25
       = 9.0625 V

Final Answer:
  R2 = 1.5 kΩ (standard value) gives Vout = 9.06 V

Exam Tip: GATE problems frequently give you Vout and R1, then ask for R2. Always use Vout = 1.25 * (1 + R2/R1) with Vref = 1.25V. Students often confuse the LM317 with the LM723 and use the wrong reference voltage. Also remember the minimum output is 1.25V (when R2 = 0), not zero volts. You cannot use the LM317 to regulate to ground.

Key Properties

• The LM317 holds exactly 1.25V (Vref) between its OUTPUT pin and its ADJ pin at all times.
• Output voltage range is 1.25V to 37V, set by R1 and R2. The minimum possible output is 1.25V when R2 = 0.
• Maximum output current is 1.5A in TO-220 package with adequate heatsinking; the LM317T (T suffix) is the TO-220 variant.
• Dropout voltage is approximately 3V, so Vin must be at least Vout + 3V for proper regulation.
• Adjust current Iadj is typically 50 µA and causes a slight error in Vout; this error is minimized by keeping R1 small (240 Ω recommended).
• The LM337 is the negative voltage complement of the LM317, adjustable from -1.25V to -37V using the same resistor formula.
• Line regulation is typically 0.01% per volt of input change, and load regulation is typically 0.1% from 10 mA to 1.5A.

Quick Revision

• LM317 holds Vref = 1.25V between OUT and ADJ pins.
• Output voltage: Vout = 1.25 * (1 + R2/R1).
• Standard R1 = 240 Ω; R2 sets the output level.
• Output range: 1.25V to 37V; current up to 1.5A.
• Dropout: Vin must be at least Vout + 3V.
• LM337 is the negative counterpart (adjustable negative output).
• Exam trap: Students use Vref = 1.2V instead of 1.25V, which leads to a wrong value of R2 in numerical problems. The correct reference voltage for LM317 is 1.25V.