LM1117S-3.3

The LM1117S-3.3 is one of the most widely used low-dropout (LDO) voltage regulators in the electronics industry, providing a fixed 3.3 V output at up to 1 A from a higher input voltage. It is the de facto standard 3.3 V regulator for embedded systems, IoT devices, Arduino shields, and virtually any circuit that needs a 3.3 V rail from a 5 V or higher supply.

The LM1117 family was originally introduced by National Semiconductor (now part of TI) as an improved replacement for the older LM317 adjustable regulator in fixed-voltage applications. The key improvement is the lower dropout voltage: 1.2 V typical at 1 A for the LM1117 versus 2-3 V for the LM317. This means the LM1117S-3.3 can regulate 3.3 V from an input as low as 4.5 V, while the LM317 would need at least 5.3 V.

The LM1117S-3.3 is a commodity part, widely second-sourced by dozens of manufacturers. The most common sources include HTC Korea TAEJIN Technology, Texas Instruments (TLV1117), ON Semiconductor (NCP1117), Diodes Incorporated (AZ1117), Advanced Monolithic Systems (AMS1117), and many others. While the basic specifications are similar across manufacturers, the exact dropout voltage, output accuracy, and stability requirements can vary slightly. Always consult the specific manufacturer’s datasheet.

The 3.3 V output voltage is by far the most popular variant, driven by the ubiquitous use of 3.3 V logic in modern digital ICs, microcontrollers, and wireless modules. The LM1117 family also offers fixed output voltages of 1.2 V, 1.5 V, 1.8 V, 2.5 V, and 5.0 V, plus an adjustable version (LM1117S-ADJ) that uses an external resistor divider to set any output voltage from 1.25 V to 13.8 V.

The SOT-223-3 package is the most common package for the LM1117. It is a surface-mount package with three pins (GND/ADJ, VOUT, VIN) plus a large tab that is electrically connected to pin 2 (VOUT) and serves as a heat sink. The tab must be soldered to a copper pad on the PCB for proper heat dissipation. The thermal resistance from junction to ambient is approximately 130 degrees C/W with minimal PCB copper, but can be reduced to 50-80 degrees C/W with a large copper area under the tab.

The LM1117 requires an output capacitor of at least 10 uF for stability. Unlike newer LDO regulators that are stable with ceramic capacitors, the LM1117 was designed for tantalum or aluminum electrolytic capacitors with higher ESR. Using low-ESR ceramic capacitors can cause instability (oscillation) unless a series resistor is added to simulate the ESR. Many modern designs use a 10 uF tantalum or a 22 uF ceramic with a 1 Ohm series resistor.

At $0.09 per unit in volume, the LM1117S-3.3 is one of the most cost-effective ways to generate a 3.3 V rail. However, designers should be aware of its limitations: relatively high quiescent current (5 mA), high dropout voltage (1.2 V), and limited PSRR (75 dB). For battery-powered or noise-sensitive applications, newer LDO regulators such as the TLV1117LV33, AP2112, or RT9013 offer lower Iq, lower dropout, and better noise performance at a slightly higher cost.

The LM1117S-3.3 operates as a series-pass linear voltage regulator using a PNP pass transistor to achieve low dropout voltage.

Series-Pass Topology: The LM1117 uses a PNP transistor as the series-pass element between the input (VIN) and output (VOUT). Unlike NPN-based regulators (like the LM317) where the base drive must be higher than the output voltage (requiring VCC headroom), the PNP transistor is driven from the output side, so the minimum input-output differential is determined only by the transistor’s VCE(sat) voltage, which is approximately 1.2 V at 1 A. This is the fundamental reason the LM1117 can regulate with less dropout than NPN-based regulators.

Voltage Reference and Error Amplifier: An internal bandgap voltage reference generates a stable reference voltage. For the fixed 3.3 V version, an internal resistor divider scales the output voltage down to the reference voltage level. The error amplifier compares the divided output voltage with the reference voltage and drives the base of the PNP pass transistor to maintain the output at 3.3 V. Any change in output voltage (due to load current, input voltage, or temperature) is corrected by the feedback loop adjusting the pass transistor drive.

Feedback Loop Stability: The LM1117 is a feedback control system that requires a minimum output capacitance (10 uF) for stability. The output capacitor creates a dominant pole in the feedback loop’s frequency response. Without sufficient capacitance (or with a capacitor that has too low ESR), the loop can oscillate. The LM1117 was designed when tantalum capacitors (with ESR of 0.5-5 Ohms) were standard, so it relies on a certain amount of ESR for phase margin. When using modern low-ESR ceramic capacitors, a small series resistance (0.5-2 Ohms) must be added to ensure stability.

Current Limiting: The LM1117 includes an internal current-limiting circuit that protects the pass transistor from damage due to output short circuits. When the output current exceeds approximately 1.2-1.6 A (depending on manufacturer), the current limiter reduces the pass transistor drive, preventing further current increase. The current limit is temperature-dependent and increases at lower temperatures.

Thermal Shutdown: A thermal shutdown circuit monitors the junction temperature. When the junction temperature exceeds approximately 150-165 degrees C (depending on manufacturer), the thermal shutdown circuit turns off the pass transistor, reducing the output current to near zero. This prevents damage from excessive power dissipation (e.g., output short circuit with high input voltage). The thermal shutdown has approximately 10-20 degrees C of hysteresis to prevent oscillation near the threshold. When the junction temperature drops below the threshold, the regulator resumes normal operation.

Dropout Voltage: The dropout voltage is the minimum input-output voltage differential required for regulation. For the LM1117S-3.3, the dropout voltage is 1.2 V typical at 1 A. At lower output currents, the dropout voltage decreases: approximately 1.1 V at 800 mA, 0.9 V at 500 mA, and 0.3 V at 100 mA. This is because the PNP transistor VCE(sat) decreases with decreasing collector current. The minimum input voltage for a 3.3 V output is therefore 3.3 + 1.2 = 4.5 V at full load.

Quiescent Current: The ground pin current (quiescent current) is approximately 5 mA typical and varies with load current. At light loads, the Iq is primarily the bias current for the internal reference and error amplifier. At full load, additional current flows through the base drive of the PNP pass transistor, increasing the total Iq to approximately 10-15 mA. This Iq is significantly higher than modern low-Iq LDOs (which can be under 1 uA at light load), making the LM1117 less suitable for battery-powered always-on applications.