Infrared LED Emitter LTE-3271T-A Datasheet - 940nm Wavelength - High Current & Low Vf - Water Clear Package - English Technical Document

1. Product Overview

The LTE-3271T-A is a high-performance infrared (IR) light-emitting diode (LED) designed for applications requiring robust optical output and reliable operation under demanding electrical conditions. Its core design philosophy centers on delivering high radiant power while maintaining a relatively low forward voltage, making it efficient for systems where power consumption is a concern. The device is packaged in a water-clear resin, which minimizes absorption of the emitted infrared light, thereby maximizing external radiant efficiency. It is engineered to support both continuous and pulsed driving modes, offering flexibility for various sensing, communication, and illumination applications in the near-infrared spectrum.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. Operation under or at these limits is not guaranteed.

• Power Dissipation (P_D): 150 mW. This is the maximum allowable power loss within the device, primarily as heat, calculated as the product of forward current and forward voltage.
• Peak Forward Current (I_FP): 2 A. This exceptionally high current rating is permissible only under specific pulsed conditions: a pulse width of 10 microseconds and a pulse repetition rate not exceeding 300 pulses per second (pps). This enables very high instantaneous optical output for short-range distance measurement or high-speed data transmission.
• Continuous Forward Current (I_F): 100 mA. The maximum DC current that can be applied continuously without exceeding the power dissipation or thermal limits.
• Reverse Voltage (V_R): 5 V. Exceeding this voltage in the reverse bias direction can cause junction breakdown.
• Operating & Storage Temperature: The device is rated for an ambient operating temperature (T_A) range of -40°C to +85°C and can be stored in environments from -55°C to +100°C.
• Lead Soldering Temperature: 320°C for 3 seconds, measured at a distance of 4.0mm from the package body. This guideline is critical for preventing thermal damage during PCB assembly.

2.2 Electrical & Optical Characteristics

These parameters are specified at an ambient temperature (T_A) of 25°C and define the typical performance of the device.

• Radiant Intensity (I_E): A key optical output metric. At a forward current (I_F) of 100 mA, the typical radiant intensity is 30 mW/sr. At the lower test current of 20 mA, it ranges from 6 mW/sr (Min) to 10.5 mW/sr (Typ). Radiant intensity describes the optical power emitted per unit solid angle.
• Aperture Radiant Incidence (E_e): 0.80 to 1.4 mW/cm² at I_F=20mA. This parameter, sometimes called irradiance, is useful for calculating the optical power density incident on a surface at a specific distance from the emitter.
• Peak Emission Wavelength (λ_P): 940 nm. This is the nominal wavelength at which the optical output power is maximum. It falls within the near-infrared (NIR) spectrum, which is invisible to the human eye but detectable by silicon photodiodes and many CMOS/CCD sensors.
• Spectral Line Half-Width (Δλ): 50 nm (Typ). This indicates the spectral bandwidth where the radiant intensity is at least half of its peak value. A value of 50 nm is characteristic of standard GaAlAs infrared LED material.
• Forward Voltage (V_F): This is a critical electrical parameter that varies with current.
  • At I_F = 50 mA: V_F (Typ) = 1.25V, (Max) = 1.6V.
  • At I_F = 250 mA: V_F (Typ) = 1.65V, (Max) = 2.1V.
  • At I_F = 450 mA: V_F (Typ) = 2.0V, (Max) = 2.4V.
  • At I_F = 1 A: V_F (Typ) = 2.4V, (Max) = 3.0V. The datasheet highlights "low forward voltage" as a feature, which is evident from these values, especially at medium currents, contributing to higher electrical-to-optical efficiency.
• Reverse Current (I_R): 100 µA (Max) at a reverse voltage (V_R) of 5V. This is the leakage current when the device is reverse-biased.
• Viewing Angle (2θ_1/2): 50° (Typ). This is the full angle at which the radiant intensity drops to half of its value at 0° (on-axis). A 50° angle provides a wide radiation pattern, useful for area illumination or sensing where alignment is less critical.

3. Performance Curve Analysis

The datasheet provides several characteristic graphs essential for circuit design and understanding performance under non-standard conditions.

3.1 Spectral Distribution (Fig. 1)

The curve shows the relative radiant intensity plotted against wavelength. It confirms the peak wavelength at approximately 940 nm with a broad spectral half-width. The shape is typical for an infrared LED, with the output tapering off on both sides of the peak. Designers of optical systems must consider this spectrum to ensure compatibility with the spectral sensitivity of the intended detector (e.g., a phototransistor or a silicon photodiode with a filter).

3.2 Forward Current vs. Ambient Temperature (Fig. 2)

This graph illustrates the derating of the maximum allowable continuous forward current as the ambient temperature increases. At 25°C, the full 100 mA is permissible. As temperature rises, the maximum current must be reduced linearly to prevent exceeding the 150 mW power dissipation limit and to manage junction temperature. This is a crucial graph for ensuring long-term reliability in high-temperature environments.

3.3 Forward Current vs. Forward Voltage (Fig. 3)

This is the current-voltage (I-V) characteristic curve. It shows the exponential relationship typical of a diode. The curve is essential for designing the current-limiting driver circuit. The slope of the curve in the operating region helps determine the dynamic resistance of the LED. The graph visually confirms the low V_F characteristic across a wide current range.

3.4 Relative Radiant Intensity vs. Forward Current (Fig. 4)

This plot shows how the optical output (normalized to its value at 20 mA) increases with forward current. The relationship is generally linear at lower currents but may show signs of saturation or reduced efficiency at very high currents due to increased thermal effects and internal quantum efficiency droop. This curve helps designers choose an operating point that balances output power with efficiency and device stress.

3.5 Relative Radiant Intensity vs. Ambient Temperature (Fig. 5)

This graph depicts the temperature dependence of the optical output. Typically, the radiant intensity of an LED decreases as the junction temperature increases. This curve quantifies that drop, showing the normalized output power relative to its value at 20 mA across a temperature range from -20°C to 80°C. This information is vital for applications requiring stable optical output over varying environmental conditions.

3.6 Radiation Diagram (Fig. 6)

This polar plot provides a detailed visualization of the spatial emission pattern. The concentric circles represent relative radiant intensity levels (e.g., 1.0, 0.9, 0.7). The plot confirms the wide viewing angle, showing how intensity distributes across different angles from 0° to 90°. This diagram is indispensable for optical design, allowing engineers to model the illumination profile on a target surface.

4. Mechanical & Packaging Information

4.1 Package Dimensions

The device uses a standard LED package format with a flange for mechanical stability and heat dissipation. Key dimensional notes from the datasheet include:

• All dimensions are provided in millimeters, with tolerances typically ±0.25mm unless otherwise specified.
• A small protrusion of resin under the flange is allowed, with a maximum height of 1.5mm.
• Lead spacing is measured at the point where the leads exit the package body, which is critical for PCB footprint design.
• The leads are solder-plated to ensure good solderability.

The water-clear package material is specifically chosen for infrared emitters because it has minimal absorption in the 940 nm region, unlike colored epoxy packages used for visible LEDs which would block the IR light.

5. Soldering & Assembly Guidelines

To ensure device integrity during PCB assembly, the following guidelines must be observed:

• Hand Soldering: If hand soldering is necessary, it should be performed quickly, applying heat to the lead and not the package body.
• Wave Soldering: Standard wave soldering profiles can be used, but the total exposure time to solder heat should be minimized.
• Reflow Soldering: The device can withstand a lead temperature of 320°C for a maximum of 3 seconds, as specified. Standard infrared or convection reflow profiles with a peak temperature below this limit are suitable. The 4.0mm distance specification ensures the thermal mass of the lead protects the sensitive semiconductor junction inside the package.
• Cleaning: After soldering, standard PCB cleaning processes can be used, but compatibility with the clear resin should be verified.
• Storage: Devices should be stored in their original moisture-barrier bags in an environment within the specified storage temperature range (-55°C to +100°C) and at low humidity to prevent lead oxidation.

6. Application Suggestions

6.1 Typical Application Scenarios

• Infrared Illumination: For security cameras, night vision systems, and machine vision lighting where invisible illumination is required.
• Proximity & Presence Sensing: In automatic faucets, soap dispensers, hand dryers, and touchless switches. The wide viewing angle is beneficial here.
• Optical Switches & Encoders: For detecting position, rotation, or movement by interrupting or reflecting the IR beam.
• Short-Range Data Communication: In IrDA-compatible devices or simple serial data links (e.g., remote controls, inter-device communication). The high pulse current capability supports modulated data transmission.
• Industrial Sensing: Object counting, level detection, and break-beam sensors.

6.2 Design Considerations

• Current Driving: An LED is a current-driven device. Always use a series current-limiting resistor or a constant-current driver circuit. The resistor value is calculated using R = (V_supply - V_F) / I_F, using the maximum V_F from the datasheet to ensure the current does not exceed the desired value.
• Thermal Management: For continuous operation at high currents (e.g., near 100 mA), consider the power dissipation (P_D = V_F * I_F). Ensure adequate PCB copper area or heatsinking to keep the junction temperature within safe limits, especially in high ambient temperatures.
• Pulsed Operation: To achieve very high peak optical power, use the pulsed mode specification (2A, 10µs, 300pps). This requires a driver circuit capable of delivering high-current pulses, such as a MOSFET switched by a pulse generator.
• Optical Design: Consider the radiation pattern (Fig. 6) when designing lenses, reflectors, or apertures to shape the beam for the specific application. The water-clear lens is hemispherical, affecting the initial divergence.
• Detector Matching: Pair the emitter with a photodetector (photodiode, phototransistor) that has peak sensitivity around 940 nm. Using an IR filter on the detector can help reject ambient visible light.

7. Technical Comparison & Differentiation

While the datasheet does not compare specific competitor parts, the LTE-3271T-A's key differentiating features can be inferred:

• High Current Capability: The combination of a 2A pulse rating and a 100mA continuous rating is notable for a standard LED package, offering high output flexibility.
• Low Forward Voltage: A V_F of around 1.25V at 50mA is relatively low for a high-power IR emitter, leading to better power efficiency and reduced heat generation compared to devices with higher V_F.
• Water-Clear Package: Unlike tinted packages that attenuate output, this maximizes external quantum efficiency for IR light.
• Wide Viewing Angle: The 50° half-angle provides broad coverage, which is an advantage for area illumination over narrower-beam alternatives.

8. Frequently Asked Questions (Based on Technical Parameters)

Q1: Can I drive this LED directly from a 5V microcontroller pin?
A: No. A microcontroller GPIO pin typically cannot source more than 20-50mA and has a fixed voltage near 5V or 3.3V. You must use a current-limiting resistor and likely a transistor (BJT or MOSFET) as a switch to drive the LED, especially at currents above 20mA.

Q2: What is the difference between Radiant Intensity (mW/sr) and Aperture Radiant Incidence (mW/cm²)?
A: Radiant Intensity is a measure of how much power the source emits per unit solid angle (steradian). It describes the directionality of the source. Aperture Radiant Incidence (or Irradiance) is the power per unit area incident on a surface at a specific distance. They are related through the inverse-square law (for a point source) and the viewing angle.

Q3: Why is the peak wavelength 940nm significant?
A: 940nm is a very common wavelength for IR systems because it is outside the visible spectrum (invisible), and silicon-based detectors (photodiodes, camera sensors) still have reasonably good sensitivity at this wavelength. It also avoids the 850nm wavelength, which has a faint red glow that can be visible in darkness.

Q4: How do I interpret the "Relative Radiant Intensity" graphs?
A: These graphs show how the light output changes relative to a reference condition (usually at I_F=20mA and T_A=25°C). They do not give absolute output values. To find the absolute output at a different current, you would multiply the relative factor from Fig. 4 by the absolute radiant intensity value given in the table for 20mA.

9. Practical Design Case Study

Scenario: Designing a Proximity Sensor for a Touchless Switch.

1. Goal: Detect a hand within 10cm of the sensor.
2. Design Choices:
   • Operate the LTE-3271T-A in continuous mode at I_F = 50mA for consistent illumination. From the datasheet, V_F ≈ 1.4V (typical).
   • Power supply is 5V. Series resistor R = (5V - 1.4V) / 0.05A = 72Ω. Use a standard 75Ω resistor.
   • Place a matched silicon phototransistor opposite the emitter, with a small gap between them (a "break-beam" configuration). When a hand interrupts the beam, the detector signal drops.
   • Alternatively, use a reflective configuration where both emitter and detector face the same direction. The wide 50° viewing angle of the LTE-3271T-A helps cover a larger detection area. The signal on the detector will increase when a hand reflects light back.
   • Use an operational amplifier circuit to amplify the small photocurrent from the detector and compare it to a threshold set by a potentiometer to account for ambient light variations.
   • Thermal consideration: Power dissipation P_D = 1.4V * 0.05A = 70mW, which is well below the 150mW maximum. No special heatsink is needed.

10. Technical Principle Introduction

Infrared LEDs like the LTE-3271T-A are semiconductor devices based on materials such as Gallium Aluminum Arsenide (GaAlAs). When a forward voltage is applied, electrons and holes recombine in the active region of the semiconductor junction. The energy released during this recombination is emitted as photons (light). The specific wavelength of 940 nm is determined by the bandgap energy of the semiconductor material, which is engineered during the crystal growth process. The water-clear epoxy package acts as a lens, shaping the emitted light's radiation pattern and providing environmental protection. The "low forward voltage" feature is achieved through optimized doping profiles and material quality, reducing the voltage drop across the junction for a given current, which directly improves electrical-to-optical conversion efficiency.

11. Industry Trends & Developments

The field of infrared optoelectronics continues to evolve. Trends relevant to devices like the LTE-3271T-A include:

• Increased Power Density: Ongoing research aims to pack more optical power into the same or smaller package sizes while managing heat dissipation, driven by demands for longer-range sensing and illumination.
• Improved Efficiency: Development of new semiconductor materials and structures (e.g., multi-quantum wells) seeks to increase the Wall-Plug Efficiency (WPE), which is the ratio of optical output power to electrical input power.
• Integration: There is a trend towards integrating the IR emitter with a driver IC or even with a photodetector in a single module, simplifying system design for end-users.
• Wavelength Specificity: While 940nm remains dominant, there is growing use of other IR wavelengths (e.g., 850nm, 1050nm) for specific applications like eye-safe LiDAR or compatibility with different sensor types.
• Packaging Innovations: Advances in packaging materials and lens design aim to provide more precise and customizable radiation patterns (e.g., batwing, side-emitting) for specialized applications.