LM1117MP-3.3

The LM1117MP-3.3 from Texas Instruments is an 800mA low-dropout positive linear regulator providing a fixed 3.3V output with ±1% initial accuracy. The device uses a PNP pass transistor to achieve 1.2V maximum dropout voltage at 800mA load, enabling regulation from a 5V supply with 0.5V headroom. The 75dB ripple rejection at 120Hz makes it effective as a post-regulator following a switching supply, cleaning up residual switching ripple. Over-current protection limits the output current to prevent damage during fault conditions, and thermal shutdown disables the output if the junction temperature exceeds approximately 165°C. The SOT-223 package features a large thermal tab (connected to VOUT) that provides a low thermal resistance path to the PCB copper, enabling effective heat dissipation. The MP suffix denotes the SOT-223 package with tape-and-reel packaging. The -3.3 suffix specifies the 3.3V fixed output voltage.

The LM1117MP-3.3 uses a PNP pass transistor between VIN and VOUT, controlled by an error amplifier that compares the output voltage against a zener-trimmed bandgap reference. When VOUT drops below 3.3V, the error amplifier drives the PNP base harder, increasing collector current (output current) flowing to the load. When VOUT rises above 3.3V, the error amplifier reduces base drive, limiting output current. The PNP pass transistor is key to the LDO operation: unlike NPN regulators where VOUT = VIN – VBE – VCESAT (minimum dropout ≈ 1.5V), the PNP configuration allows VOUT to approach VIN much more closely, limited only by VCE(sat) of the PNP. At 800mA, the worst-case dropout is 1.2V, meaning VIN must be at least 4.5V to maintain regulation. The 5mA quiescent current is the PNP base drive current that flows to ground rather than to the load; this increases with load current because higher load requires more base drive. The output capacitor (minimum 10µF) provides frequency compensation for the feedback loop; without it, the regulator may oscillate. The capacitor’s ESR must be between 0.3Ω and 22Ω for stability—modern low-ESR ceramics may require a small series resistor to maintain stability.

LM1117MP-3.3 uses the SOT-223-4 (MP) surface-mount package with Pin 1 = GND/Adjust, Pin 2 = VOUT (tab), Pin 3 = VIN, Pin 4 = Tab (connected to VOUT). The tab underneath the package must be soldered to a PCB copper pour for both electrical connection to VOUT and thermal dissipation. The MP package designation indicates SOT-223 from TI/National. Output current up to 800mA with 1.1V dropout at full load. A minimum 10µF tantalum output capacitor is required for stability. For fixed 3.3V output, the ADJ pin (Pin 1) is internally connected to a resistive divider — do not connect external components to Pin 1 on the fixed-voltage version.

• 5V to 3.3V: VIN=5V, VOUT=3.3V, 1.7V headroom; at 500mA load, Pdiss = (5-3.3)×0.5 = 850mW — needs PCB copper heat area
• Post-SMPS: 75dB PSRR attenuates 100mV switcher ripple to 18µV; adds clean analog-grade 3.3V
• 3.3V MCU Rail: 800mA supports MCU + peripherals; thermal tab solders to copper pour for heat dissipation
• Battery to Logic: 4.2V Li-ion to 3.3V; 0.9V dropout works until battery discharges to ~4.5V
• Multi-Rail System: LM1117-3.3 + LM1117-5.0 from 12V bus; thermal management critical at high VI-VO

LM1117MP-3.3 is an 800mA low-dropout regulator in SOT-223-4 with 1.1V dropout at 800mA and 3.3V fixed output. The TLV1117LV33DCYR is the modern TI replacement with lower 300mV dropout and ceramic capacitor stability. The AMS1117-3.3 is the most popular budget alternative with identical SOT-223-3 pinout. For anyCAP stability, MCP1826T guarantees stability with any capacitor type.