TI LDO Selection Guide: LM1117IDTX-3.3 vs LM1117MPX-1.8 vs LM317AMDTX

LM1117IDTX-3.3 vs LM1117MPX-1.8 vs LM317AMDTX: Full Comparison

When to Use Each

Choose LM1117IDTX-3.3 when:

• You need a standard 3.3V rail from 5V or higher
• Load current is 400–800mA — TO-252 handles the heat
• Industrial temperature range (−40°C) is required
• Simplicity matters: two capacitors and you're done

Choose LM1117MPX-1.8 when:

• You need a 1.8V rail for FPGA core, DDR2, or low-voltage MCU
• Load current is under 500mA (SOT-223 thermal limit)
• Board space is tight — SOT-223 is half the TO-252 footprint
• Input voltage is 3.3V or 5V (low headroom = low dissipation)

Choose LM317AMDTX when:

• You need a non-standard voltage (2.8V, 4.2V, 9V, 13.6V, etc.)
• Output current exceeds 800mA (up to 1.5A)
• Input voltage is above 15V (up to 40V — industrial, automotive)
• You need a constant-current source (LED driver, battery charger)
• Prototyping: one part covers any voltage with a resistor change

Thermal Headroom: Same Task, Different Packages

At 5V-3.3V/500mA, the LM1117IDTX runs comfortably at 71°C junction. The SOT-223 LM1117MPX is at the edge at 124°C. The LM317 in TO-252 would exceed its rating — its 103°C/W θ_JA and 2.25V dropout make it unsuitable for this load without a heatsink.

Pinout: The Trap

The pinouts look similar — both TO-252, both 3-pin plus tab. But pin 1 is GND on the 1117 and ADJ on the LM317. Swapping one into the other's footprint shorts the adjustment/reference circuit and outputs 1.25V (LM317 in 1117 board) or nothing useful (1117 in LM317 board).

Frequently Asked Questions

Q1: Can I use an LM317 with R2=0 to get 3.3V and replace an LM1117-3.3?

A: No — the LM317's minimum output is 1.25V, not 0V. And the pinouts differ. An LM317 outputs 1.25V with R2=0. For 3.3V you'd need R1=240Ω, R2=394Ω. But the pinout mismatch means you can't drop an LM317 into an LM1117 footprint. If you need adjustable in an LM1117 footprint, use the LM1117-ADJ version.

Q2: Which one for a 5V USB to 3.3V ESP32 supply?

A: LM1117IDTX-3.3. 5V in, 3.3V out, 500mA peak — the TO-252 runs at ~70°C junction with no extra copper. The LM1117MPX-3.3 (SOT-223 version) would work too at 500mA, running ~95°C. The LM317 is overkill: you'd need two resistors and get worse dropout.

Q3: Which one for a 12V to 5V, 1A supply?

A: LM317AMDTX — but check the thermals. 12V−5V=7V headroom, 1A = 7W dissipation. At 103°C/W, the TO-252 LM317 would hit 721°C rise — obviously impossible. Use the LM317T (TO-220) with a heatsink, or better, a switching regulator. The LM1117 maxes out at 15V input but only 800mA, and the TO-252 version still can't dissipate 7W.

Q4: Why does the LM317 need more external components?

A: It's a floating regulator with no fixed ground reference. The LM317 regulates the voltage between OUT and ADJ to 1.25V. The output voltage is determined by the R1/R2 divider referenced to ground. Protection diodes are recommended because the ADJ pin can't handle large capacitor discharge currents. The fixed LM1117 has the divider built into the silicon — you only need input and output capacitors.

Q5: Can I use two LM1117s from different voltage variants on the same board?

A: Yes — and this is common. A typical design uses an LM1117-3.3 for the I/O rail and an LM1117-1.8 for the core rail, both from the same 5V input. Same external capacitor requirements, same layout rules. This is simpler than using one adjustable regulator with two different resistor pairs.

Related Products

• LM1117IDTX-3.3/NOPB — Fixed 3.3V, TO-252
• LM1117MPX-1.8/NOPB — Fixed 1.8V, SOT-223
• LM317AMDTX/NOPB — Adjustable 1.5A, TO-252
• LM317T — Adjustable 1.5A, TO-220 for heatsinking
• LM1117-ADJ — Adjustable LDO, same pinout as fixed 1117