LM1117IMPX-3.3 - 3.3V Fixed LDO Regulator by Texas Instruments

What Is LM1117IMPX-3.3 and How Does It Work?

It's a linear regulator: it takes an unregulated DC input and outputs a stable 3.3V by dissipating the difference as heat. The dropout voltage - the minimum difference between input and output needed to maintain regulation - is 1.2V at 800 mA. That's why you need at least 4.5V in to get a clean 3.3V out at full load. At lighter loads, dropout drops below 1V, so a 5V rail works comfortably.

The "IMPX" suffix tells you two things. "I" = industrial temperature range (-40°C to +125°C). "MP" = SOT-223 package. The /NOPB suffix means lead-free / RoHS-compliant - the current production version.

The LM1117 is essentially the same silicon as the original National Semiconductor LM1117 (National got acquired by TI in 2011). The design is proven, well-characterized, and documented in a datasheet that's been through more revisions than most engineers. That's not a bad thing: you know exactly how it behaves.

Position in the product line: The LM1117 sits between the 500 mA AMS1117 (cheaper but lower current) and the 1A LM1085 (higher current but larger package). The LM1117IMPX-3.3 specifically is the modern orderable version of the classic LM1117-3.3.

What Are the Specifications of LM1117IMPX-3.3?

Key Numbers That Matter

• Dropout = 1.2V @ 800 mA: If your input rail is 5V, you lose 1.2V across the regulator. The 0.5V headroom is fine - but if your "5V" rail actually sits at 4.8V under load, you're dangerously close to the dropout limit. Check under worst-case conditions.
• PSRR = 75 dB @ 120 Hz: Excellent at line frequency. Kills 120 Hz ripple from full-wave rectifiers cleanly. At 10 kHz it drops to about 30 dB - typical for a simple LDO topology. For switching-regulator ripple rejection, you'd want a higher-PSRR part like the TPS7A49.
• Iq = 5 mA: Not battery-friendly. If every μA counts, look at the TPS7A05 (Iq = 1 μA). But if you've already got a 50 mA system load, 5 mA quiescent is irrelevant.
• Capacitor: 10 μF tantalum: The LM1117 needs ESR in the 0.3–22 ohm range. Ceramics are way too low (<10 mOhm) and can cause oscillation. If you want ceramic, add a 1–2 ohm series resistor or pick a newer LDO designed for it.

What Is LM1117IMPX-3.3 Used For?

MCU Power Supply

The most common application: 5V to 3.3V conversion for microcontrollers. At 800 mA, you can power an STM32F4 or similar MCU plus peripherals. The ±1% accuracy keeps the core voltage well within spec.

Post-Regulation for Switching Converters

Feed it from a buck converter. The switcher handles efficiency, the LDO cleans up the noise. A 5V buck → LM1117-3.3 chain is a classic two-stage power architecture.

Audio Circuits

Clean 3.3V rails for DACs, op-amps, and audio codecs. The 40 μVrms output noise is good enough for 16-bit audio. For 24-bit designs, you'd want a dedicated audio LDO like the TPS7A47.

Sensor Power

Precision analog sensors need clean power for accurate readings. The LM1117's low noise and good line/load regulation keep measurement drift in check.

Industrial Control Modules

The -40°C to +125°C range covers industrial environments. Combine with the LM2596 for a 24V → 5V → 3.3V power chain that handles noisy industrial rails.

LM1117IMPX-3.3 vs AMS1117-3.3 vs TLV1117-33

Pick LM1117IMPX-3.3 when: You need a known-quantity part with decades of field data. Safe choice for industrial products where long-term reliability matters more than BOM cost.

Pick AMS1117-3.3 when: Cost is the main driver. Pin-compatible, functionally similar. Just watch for counterfeits - they're everywhere in the AMS1117 market.

Pick TLV1117-33 when: You want a newer TI design that works with ceramic caps (no stability worries) and draws lower quiescent. Starting a new design today? This is probably the better technical choice.

Frequently Asked Questions About LM1117IMPX-3.3

Q1: Is LM1117IMPX-3.3 the same as LM1117-3.3?

A: Yes, same die. The IMPX suffix specifies SOT-223 package and industrial temperature range. LM1117IMPX-3.3/NOPB is the current lead-free orderable version.

Q2: Can I use LM1117IMPX-3.3 to replace AMS1117-3.3?

A: In most cases yes. Both are SOT-223 with the same pinout (Input/GND/Output, tab=GND). AMS1117 is rated 1A vs LM1117's 800 mA.

Q3: What is the minimum input voltage for 3.3V output?

A: 4.5V at 800 mA load. At lighter loads, dropout drops below 1V so you can go as low as 4.3V.

Q4: Why use an LDO instead of a buck converter for 5V to 3.3V?

A: Below 200 mA, an LDO is simpler, cheaper, quieter, and takes less space. Efficiency (3.3/5 = 66%) is fine for most apps. Above 300 mA, the heat adds up.

Q5: Does LM1117IMPX-3.3 need a heatsink?

A: At 500 mA, dissipation is 0.85W, giving a 116°C rise above ambient - too hot without heatsinking. Use copper pours or a clip-on heatsink. At 100 mA it's fine without.

Q6: Can I use ceramic capacitors?

A: Not directly. The LM1117 needs output capacitor ESR between 0.3–22 ohms. Ceramics are too low. Use tantalum/electrolytic, or add a 1–2 ohm series resistor.

Q7: What happens if I exceed 800 mA?

A: Current limit kicks in at ~1.2A typical. The output voltage drops but the device doesn't die immediately. If overcurrent persists, thermal shutdown takes over at ~160°C.

Q8: What reverse voltage protection does it have?

A: None. If input goes below ground or output exceeds input, the LM1117 can be damaged. Standard practice: add a reverse protection diode (1N4148 or BAT54).

Pricing & Availability

Contact ICMASS for current pricing. We carry original TI LDOs with full traceability. Pin-compatible alternatives (AMS1117-3.3, TLV1117-33) also available for multi-sourcing.