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AMS1117-3.3: A Classic 1A Low Dropout Linear Regulator
10/15/2026 7:39:44 PM
AMS1117-3.3: A Classic 1A Low Dropout Linear Regulator
1. Product Overview
The AMS1117-3.3 is a series low dropout linear regulator (LDO) from Advanced Monolithic Systems. It provides a fixed 3.3V output voltage and can deliver up to 1A of output current. With its low dropout voltage, high accuracy, and built-in protection features, it has become one of the most widely used 3.3V power solutions in embedded systems, microcontroller supplies, sensor modules, and communication interfaces. The AMS1117 family also offers fixed versions for 1.2V, 1.5V, 1.8V, 2.5V, and 5.0V, as well as an adjustable version. The 3.3V variant dominates the classic 5V-to-3.3V application space.
2. Package Options and Pinout
The AMS1117-3.3 is available in SOT-223, TO-252 (DPAK), and SOT-89 packages to suit different thermal and layout requirements. In the most common SOT-223 package, pin 1 is Vin (input supply), pin 2 is GND (ground), and pin 3 is Vout (3.3V output). The exposed thermal tab on the SOT-223 package is electrically connected to the GND pin. In PCB design, this tab must be soldered to the ground plane with sufficient copper area to enhance heat dissipation. For TO-252 packages, the three pins serve the same functions, and heat dissipation relies on the contact area between the pins and PCB copper pours.
3. Key Electrical Specifications
The AMS1117-3.3 offers output voltage accuracy of ±1% (some manufacturers specify ±2%). Maximum input voltage ranges from 15V to 18V depending on the vendor. At 1A full load, the typical dropout voltage is 1.3V, with a maximum of about 1.4V, meaning the input voltage must stay above approximately 4.7V for proper regulation. At reduced load currents such as 100mA, the dropout voltage drops to roughly 0.2V, allowing more flexibility in low-power designs. The quiescent current is typically 5mA, and the power supply rejection ratio (PSRR) is typically 75dB at 120Hz, effectively suppressing input ripple. Internal current limiting and thermal shutdown circuits protect the device under fault conditions.
4. Key Specifications Summary
| Parameter | Specification |
|---|---|
| Output Voltage | 3.3V (fixed) |
| Output Current | Up to 1A |
| Output Accuracy | ±1% (typical) |
| Maximum Input Voltage | 15V to 18V |
| Typical Dropout (@1A) | 1.3V |
| PSRR | 75dB @ 120Hz |
| Quiescent Current | 5mA (typical) |
| Protection Features | Current limit, thermal shutdown |
| Operating Junction Temperature | -40°C to 125°C |
| Package Options | SOT-223 / TO-252 / SOT-89 |
5. Thermal Design Considerations
An LDO essentially trades heat for voltage stability. Power dissipation is calculated as Pd = (Vin - Vout) × Iload, and junction temperature is Tj = Ta + Pd × θJA. For a TO-252 packaged AMS1117-3.3 with θJA of approximately 125°C/W, a 5V input at 1A output dissipates 1.7W, producing a temperature rise of 212.5°C - far exceeding the 125°C junction limit. Therefore, whenever the voltage drop exceeds 1V and the load current exceeds 300mA, a rigorous thermal calculation is mandatory. Optimization measures include: selecting larger packages such as TO-252 (θJA approximately 20% lower than SOT-223); laying out at least 2cm² of copper beneath the device; adding 6 to 12 thermal vias under the ground pad; and placing input and output capacitors close to the pins to minimize parasitic inductance. For large voltage differentials, pre-regulation or a DC-DC converter is recommended.
6. Frequently Asked Questions
Q1: Can the AMS1117-3.3 directly convert 5V input to 3.3V output?
A1: Yes. The 5V-to-3.3V conversion is its most typical application. The voltage drop is 1.7V, and at 1A load the chip dissipates 1.7W, requiring good thermal design. For load currents below 500mA, the SOT-223 package with adequate copper area is usually sufficient.
Q2: Why can't a 3.7V lithium battery reliably power the AMS1117-3.3?
A2: The AMS1117-3.3 requires an input voltage above approximately 4.7V to maintain regulation at 1A load, due to its maximum dropout voltage of about 1.3V to 1.4V. A 3.7V lithium battery operates between roughly 3.0V and 4.2V, which falls below the minimum input requirement even at full charge.
Q3: How should the output capacitor be selected?
A3: A 22µF electrolytic capacitor in parallel with a 0.1µF ceramic capacitor is recommended at the output. Avoid pure ceramic capacitors with excessively low ESR (below 0.1Ω), as this can cause oscillation. Tantalum capacitors or types with ESR in the 0.5 to 1Ω range provide more stable loop response. At the input, a 10µF electrolytic capacitor and a 0.1µF ceramic capacitor are recommended to filter high-frequency ripple.
Q4: Does the AMS1117-3.3 recover automatically after thermal shutdown?
A4: Yes. When the junction temperature exceeds approximately 165°C, the internal thermal shutdown circuit disables the output. Once the junction temperature falls back to a safe level, the device resumes normal operation automatically. However, repeated thermal shutdown cycles shorten device life, so thermal optimization should be addressed at the design stage.
7. Typical Applications
The AMS1117-3.3 is widely used in computer motherboard and graphics card power management, LCD monitor and LCD TV power sequencing, DVD decoder boards, ADSL modems, post-regulation for switching power supplies, battery chargers, and various embedded control systems. It is the default choice whenever a 5V or 12V rail needs to supply 3.3V to microcontrollers, sensors, and communication modules.
8. Conclusion
The AMS1117-3.3 remains a classic device in the 3.3V regulation space, thanks to its simple three-terminal structure, 1A output capability, ±1% accuracy, and built-in protection. Its primary design challenge lies in thermal management. Engineers must carefully calculate power dissipation based on the actual voltage drop and load current, select an appropriate package, and optimize PCB thermal layout to ensure long-term reliable operation.
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