Design: Set Target Output Voltage
V_out = 1.25 × (1 + R2/R1)
Analysis: Find V_out from R1 and R2
📊 Common LM317 Output Voltages (R1 = 240 Ω)
| V_out | R2 (exact) | R2 (E24 nearest) | Actual V_out |
|---|---|---|---|
| 1.5V | 48 Ω | 47 Ω | 1.495V |
| 1.8V | 105.6 Ω | 100 Ω | 1.771V |
| 2.5V | 240 Ω | 240 Ω | 2.5V |
| 3.3V | 393.6 Ω | 390 Ω | 3.281V |
| 5.0V | 720 Ω | 720 Ω | 5.0V |
| 6.0V | 960 Ω | 1kΩ | 6.042V |
| 9.0V | 1.488 kΩ | 1.5 kΩ | 9.063V |
| 12.0V | 2.064 kΩ | 2.0 kΩ | 11.667V |
| 15.0V | 2.640 kΩ | 2.7 kΩ | 15.25V |
📐 LM317 Design Guide
Output Voltage
Vout = 1.25×(1 + R2/R1)
Vref = 1.25V
Solve for R2
R2 = R1×(Vout/1.25 − 1)
Min Dropout
Vin ≥ Vout + 3V
(LM317 needs ~2–3V headroom)
Power Dissipation
Pd = (Vin − Vout) × I_load
Use heatsink if Pd > 1W
❓ FAQ
Why is R1 = 240 Ω recommended?
TI's datasheet recommends R1 = 120–240 Ω. Using 240 Ω keeps the quiescent current through R1 low (about 5mA) while still dominating the small adjust pin current (Iadj ≈ 50–100µA), minimizing output voltage error. Lower R1 values slightly improve regulation but waste more power.
What is the LM317's output range?
The LM317 adjusts from 1.25V (minimum, R2=0) up to 37V. Maximum input voltage is 40V. For 5V output you need at least 8V input; for 12V output you need at least 15V. The output current is up to 1.5A with adequate heatsinking.
Do I need a heatsink?
Calculate power dissipation: Pd = (Vin − Vout) × Iload. Above about 1W, a heatsink is needed. For Pd > 5W, you need a substantial heatsink or consider a switching regulator. Without a heatsink, thermal shutdown occurs around 1–2W depending on the package.
What capacitors should I add?
Add a 0.1µF ceramic capacitor on the input (IN to GND) for stability, and a 1µF–10µF electrolytic on the output (OUT to GND) to improve transient response. If using a large input filter capacitor, also add a 10µF electrolytic across R2 to improve ripple rejection.