LM1117MPX-ADJ

The LM1117MPX-ADJ is the adjustable-output version of TI’s LM1117 low-dropout linear regulator in a SOT-223-4 surface-mount package with tape-and-reel packaging. The LM1117 is one of the most widely used LDO regulator families in the electronics industry, originally developed by National Semiconductor (now TI).

The adjustable version uses an internal 1.262 V bandgap reference trimmed to ±1% accuracy. The output voltage is set by two external resistors: VOUT = VREF × (1 + R1/R2) + IADJ × R1, where IADJ is typically 50 µA and can usually be neglected. This allows any output voltage from 1.25 V to 13.8 V with a single BOM change.

The 1.2 V typical dropout voltage at 800 mA (1.3 V max) means the input must be at least 1.3 V above the desired output for proper regulation. At lower currents, dropout scales proportionally: approximately 1.1 V at 100 mA and 1.25 V at 500 mA.

The SOT-223-4 package features a large tab (pins 2 and 4) connected to VOUT that serves as both the output connection and the primary thermal path. With RθJA of approximately 136°C/W on a standard FR4 board, the package can dissipate about 0.9 W at 50°C ambient.

TI also offers the LM1117I variant with -40°C to +125°C extended temperature range. For new designs, TI recommends the TLV1117 as a drop-in replacement with improved performance, or the TLV761 for fixed-voltage SOT-223 configurations with enhanced functionality.

Note: The LM1117MPX-ADJ (without /NOPB suffix) is listed as obsolete by some distributors. The LM1117MPX-ADJ/NOPB with Pb-free terminal finish remains active. Designers should specify the /NOPB variant.

**Bandgap Reference and Error Amplifier:** The LM1117 uses a Zener-trimmed bandgap reference that produces a stable 1.262 V reference voltage. This reference is compared against a fraction of the output voltage (fed back through the ADJ pin) by a high-gain error amplifier. The amplifier drives the series pass transistor to maintain regulation.

**Pass Transistor and Dropout:** The pass element is a PNP-driven NPN Darlington configuration. The minimum input-output differential (dropout voltage) is the saturation voltage of this Darlington, approximately 1.2 V at full load. Unlike CMOS LDOs where dropout equals IOUT × RDS(on), the bipolar Darlington has a relatively fixed saturation voltage that increases modestly with current.

**ADJ Pin and Resistor Divider:** In the adjustable configuration, the ADJ pin connects to the junction of two external resistors. The internal error amplifier drives the pass transistor to maintain VADJ = VREF (1.262 V). The ADJ pin sources a small bias current (IADJ ≈ 50 µA) that flows through R1, adding a small error term: VOUT = VREF × (1 + R1/R2) + IADJ × R1.

**Protection Circuits:** Current limiting restricts output current to approximately 1.2 A under fault conditions. Thermal shutdown disables the output when junction temperature exceeds approximately 165°C, with hysteresis to prevent oscillation.