LM317 Adjustable Regulator Resistors
LM317 / LM1117-ADJ design: solve R2 for the target output, check actual output, minimum load, dropout, and junction temperature, with R1 / R2 dissipation.
Key points
LM317 Adjustable Regulator Resistors: LM317 / LM1117-ADJ design: solve R2 for the target output, check actual output, minimum load, dropout, and junction temperature, with R1 / R2 dissipation. Formula: Vout = Vref·(1 + R2/R1) + Iadj·R2 ｜ P = (Vin − Vout)·Iload ｜ Tj = Ta + P·θJA. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. LM317 usually uses R1 = 240 Ω to guarantee minimum load current. Increasing R2 raises the output, but ADJ current and drift add error. Insufficient input-output dropout loses regulation; at high current / high differential the IC dissipates a lot, so verify Tj with θJA and the heatsink.
Inputs
Vout = Vref·(1 + R2/R1) + Iadj·R2 ｜ P = (Vin − Vout)·Iload ｜ Tj = Ta + P·θJACalculation model (updates with parameters)
| Ideal R2 | 713.2 Ω | R2 = (Vout − Vref)/(Vref/R1 + Iadj) |
| Recommended R2 (E96) | 715 Ω | and R1 = 240 Ω 配对 |
| actual output voltage | 5.01 V | target 5 V, error 0.19% |
| Suggested R1 maximum | 250 Ω | R1 ≤ Vref/Imin（Imin = 5 mA） |
| Divider / quiescent current | 5.258 mA | 越大dissipation越高、load调整率越好 |
| R1 / R2 dissipation | 6.51 mW / 19.77 mW | 按 1/4W 或 1/8W Resistance校核 |
| Regulator dissipation | 3.496 W | (Vin − Vout)·Iload + Iadj·Vin |
| Estimated junction temperature Tj | 200 °C | Ta 25 °C + P·θJA 50 °C/W |
| Dropout margin | 6.99 V | needs ≥ 2 V |
Engineering notes
- LM317 usually uses R1 = 240 Ω to guarantee minimum load current. Increasing R2 raises the output, but ADJ current and drift add error.
- Insufficient input-output dropout loses regulation; at high current / high differential the IC dissipates a lot, so verify Tj with θJA and the heatsink.
- Add 1–10 µF at ADJ and at least 0.1 µF + 10 µF at input and output; watch ESR stability with tantalum / ceramic capacitors.
FAQ
- What is LM317 Adjustable Regulator Resistors for?
- LM317 / LM1117-ADJ design: solve R2 for the target output, check actual output, minimum load, dropout, and junction temperature, with R1 / R2 dissipation.
- What is the formula for LM317 Adjustable Regulator Resistors?
- Core formula: Vout = Vref·(1 + R2/R1) + Iadj·R2 ｜ P = (Vin − Vout)·Iload ｜ Tj = Ta + P·θJA. The tool returns results instantly using this formula and the selected parameters.
- What engineering notes should I keep in mind when using LM317 Adjustable Regulator Resistors?
- LM317 usually uses R1 = 240 Ω to guarantee minimum load current. Increasing R2 raises the output, but ADJ current and drift add error. Insufficient input-output dropout loses regulation; at high current / high differential the IC dissipates a lot, so verify Tj with θJA and the heatsink. Add 1–10 µF at ADJ and at least 0.1 µF + 10 µF at input and output; watch ESR stability with tantalum / ceramic capacitors.