LTL-E7939Q3K Infrared LED Datasheet - Through Hole Package - 850nm Wavelength - 20mW/sr Radiant Intensity - 1.6V Forward Voltage - English Technical Document

1. Product Overview

The LTL-E7939Q3K is a high-performance infrared (IR) light-emitting diode (LED) designed for through-hole mounting on printed circuit boards (PCBs) or panels. It is engineered for applications requiring reliable, high-speed optical signaling or illumination in the near-infrared spectrum. The device utilizes an AlGaAs (Aluminum Gallium Arsenide) semiconductor material, which is optimized for emission at 850 nanometers, a common wavelength for IR communication, sensing, and night-vision illumination systems.

Its core advantages include a combination of high radiant intensity, compatibility with integrated circuits due to low current requirements, and a robust through-hole package suitable for a variety of assembly processes. The product is compliant with RoHS directives, indicating it is manufactured without the use of hazardous substances like lead (Pb). The primary target markets include industrial automation, security systems (e.g., CCTV night vision), optical encoders, remote controls, and proximity sensors where dependable infrared light sources are critical.

2. Technical Parameters Deep Objective Interpretation

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 and should be avoided in reliable designs.

• Power Dissipation (Pd): 120 mW maximum. This is the total power (Vf * If) the package can dissipate as heat without exceeding its maximum junction temperature. Exceeding this limit risks thermal runaway and failure.
• Peak Forward Current (I_FP): 1 A under pulsed conditions (300 pulses per second, 10 μs pulse width). This rating is significantly higher than the DC rating, allowing for brief, high-intensity pulses useful in data transmission.
• DC Forward Current (I_F): 60 mA continuous. This is the maximum steady-state current for reliable long-term operation.
• Reverse Voltage (V_R): 5 V. Applying a reverse bias voltage greater than this can cause breakdown and catastrophic failure of the LED's PN junction.
• Operating & Storage Temperature: -30°C to +85°C and -40°C to +100°C, respectively. These define the environmental limits for operation and non-operational storage.
• Lead Soldering Temperature: 260°C for 5 seconds, measured 2.0mm from the LED body. This guides hand-soldering processes to prevent thermal damage to the epoxy lens and internal die bonds.

2.2 Electrical / Optical Characteristics

These are the typical and guaranteed performance parameters measured under standard test conditions (Ta=25°C).

• Radiant Intensity (I_e): Minimum 20.0 mW/sr at I_F = 20mA. Radiant intensity measures the optical power emitted per unit solid angle (steradian). This is a key parameter for determining the effective range and signal strength in IR systems. The datasheet notes a ±15% tolerance should be applied to the guaranteed value.
• Viewing Angle (2θ_1/2): Typical 30 degrees. This is the full angle at which the radiant intensity drops to half of its peak (on-axis) value. A 30° angle indicates a moderately focused beam, suitable for directed applications.
• Peak Wavelength (λ_P): Typical 850 nm. This is the wavelength at which the LED emits the most optical power. 850nm is in the near-infrared range, invisible to the human eye but detectable by silicon photodiodes and many camera sensors.
• Spectral Line Half-Width (Δλ): Typical 40 nm. This specifies the wavelength bandwidth where the emission intensity is at least half of the peak intensity. A 40nm width is common for IR LEDs.
• Forward Voltage (V_F): Typical 1.3V, Maximum 1.6V at I_F = 20mA. This is the voltage drop across the LED when conducting current. It is crucial for designing the current-limiting circuitry.
• Reverse Current (I_R): Maximum 10 μA at V_R = 5V. This is the small leakage current that flows when the LED is reverse-biased within its safe limit.

3. Binning System Explanation

The datasheet indicates the use of a classification or binning system for Radiant Intensity (I_e). The note states: \"Ie classification code is marked on each packing bag.\" This implies that manufactured LEDs are tested and sorted (binned) based on their measured radiant intensity. The part number LTL-E7939Q3K specifies a minimum radiant intensity (18~21.5 mW/sr Min, as indicated in the part number breakdown table), but individual units within a shipment may fall into specific sub-ranges (bins). Designers should be aware that the actual intensity of a specific LED may vary within the guaranteed minimum and the bin range. The datasheet does not detail explicit bins for wavelength (λ_P) or forward voltage (V_F), listing only typical and maximum/minimum values.

4. Performance Curve Analysis

The datasheet references several typical characteristic curves, which provide deeper insight into device behavior under varying conditions.

• Spectrum Curve: Illustrates the relative radiant power as a function of wavelength, centering around the 850nm peak with the defined 40nm half-width.
• Forward Voltage vs. Forward Current (I-V Curve): Shows the non-linear relationship between voltage and current. The curve will have a threshold voltage (around 1.1-1.2V for AlGaAs) after which current increases rapidly with small increases in voltage, highlighting why current control (not voltage control) is essential.
• Relative Radiant Power vs. Forward DC Current: Demonstrates how optical output power increases with drive current, typically in a near-linear relationship within the operating range before efficiency drops at very high currents due to thermal effects.
• Relative Radiant Power vs. Peak Current (Pulsed): Similar to the DC curve but for pulsed operation, showing achievable peak output at currents up to the 1A maximum.
• Relative Radiant Power vs. Temperature: A critical curve showing the decrease in optical output as the junction temperature rises. This thermal derating factor must be accounted for in designs where ambient temperature is high or thermal management is poor.
• Directivity Pattern: A polar plot showing the angular distribution of emitted light, visually defining the 30° viewing angle.

5. Mechanical and Packaging Information

5.1 Package Dimensions

The LED is housed in a standard through-hole, T-1 3/4 (5mm) round package. Key dimensions from the drawing include:

• Lens Diameter: Approximately 5.0mm.
• Package Height: Approximately 8.7mm from the bottom of the leads to the top of the lens.
• Lead Diameter: 0.56mm nominal.
• Lead Spacing: 2.54mm (0.1\") standard, measured where leads emerge from the package body.
• Flange/Base: A flange aids in panel mounting and provides a mechanical stop during insertion. Protruded resin under the flange is a maximum of 1.0mm.

5.2 Polarity Identification

The cathode is identified in the dimension drawing. For a standard LED, the cathode is typically the shorter lead and/or the lead adjacent to a flat spot on the package flange. The provided drawing should be consulted for the exact identification mark.

6. Soldering and Assembly Guidelines

Proper handling is crucial to prevent damage.

• Lead Forming: Must be done before soldering at room temperature. Bends should be made at least 3mm from the base of the LED lens. The leadframe base should not be used as a fulcrum.
• PCB Assembly: Use minimum clinch force to avoid mechanical stress on the leads.
• Soldering:
  • Maintain a minimum 2mm clearance from the lens base to the solder point.
  • Avoid immersing the lens in solder.
  • Do not stress the leads during soldering while the LED is hot.
  • Hand Soldering: Iron temperature ≤ 350°C, time ≤ 3 seconds (one time only).
  • Wave Soldering: Pre-heat ≤ 100°C for ≤ 60 sec, solder wave ≤ 260°C, contact time ≤ 5 sec.
  • IR reflow soldering is NOT suitable for this through-hole package.
• Cleaning: Use alcohol-based solvents like isopropyl alcohol if necessary.
• Storage: Out of original packaging, use within 3 months. For longer storage, use a sealed container with desiccant or a nitrogen ambient. Storage should not exceed 30°C and 70% relative humidity.

7. Packaging and Ordering Information

• Unit Packaging: 1000 pieces per anti-static packing bag.
• Inner Carton: 6 packing bags (6,000 pieces total).
• Outer Carton: 8 inner cartons (48,000 pieces total).
• Part Number: LTL-E7939Q3K. The breakdown suggests: LTL (Lamp), E79 (series/code), 39 (likely related to viewing angle or intensity bin), Q3K (specific variant code). The lens color is \"Water Clear\" (transparent).

8. Application Suggestions

8.1 Typical Application Scenarios

• Infrared Illumination: For CCTV cameras in low-light or nighttime security applications.
• Optical Switching & Encoding: In slot-type or reflective optical sensors for position sensing, motor speed control, and rotary encoders.
• Data Transmission: In infrared data association (IrDA) compliant devices or simple short-range serial data links, leveraging its high-speed capability.
• Proximity and Object Detection: In conjunction with a photodetector to sense the presence or absence of an object.

8.2 Design Considerations

• Drive Circuit: LEDs are current-driven devices. To ensure uniform brightness, especially when connecting multiple LEDs in parallel, a current-limiting resistor should be placed in series with EACH LED (Circuit Model A). Driving multiple LEDs in parallel directly from a voltage source with a single resistor (Circuit Model B) is discouraged due to variations in individual LED forward voltage (Vf), which cause uneven current distribution and brightness.
• Thermal Management: While the through-hole package dissipates heat through its leads, attention should be paid to the PCB layout and ambient conditions to prevent the junction temperature from exceeding limits, which reduces output and lifespan.
• ESD Protection: The LED is susceptible to electrostatic discharge. Handling procedures should include the use of grounded wrist straps, anti-static mats, and ionizers. ESD damage can manifest as high reverse leakage, low forward voltage, or failure to emit light at low currents.

9. Technical Comparison and Differentiation

Compared to standard visible LEDs or lower-power IR LEDs, the LTL-E7939Q3K offers a balanced combination of high radiant intensity (20 mW/sr min) and a moderate, focused viewing angle (30°). This makes it more suitable for longer-range or higher-signal-strength applications than wide-angle, low-power devices. Its AlGaAs construction is typical for 850nm emission, offering good efficiency. The key differentiator in its class is the explicit specification for high-speed operation, making it a candidate for pulsed applications beyond simple illumination.

10. Frequently Asked Questions (Based on Technical Parameters)

Q: Can I drive this LED directly from a 3.3V or 5V microcontroller pin?

A: No. You must use a series current-limiting resistor. For example, with a 5V supply, Vf=1.3V, and desired I_F=20mA, the resistor value would be R = (5V - 1.3V) / 0.02A = 185Ω. A 180Ω or 220Ω resistor would be appropriate. Driving it directly would likely destroy the LED due to excessive current.

Q: Why is the pulsed current rating (1A) so much higher than the DC rating (60mA)?

A> During a very short pulse, the heat generated in the semiconductor junction does not have time to spread to the package and surrounding environment. Therefore, the junction temperature does not rise as dramatically, allowing a much higher instantaneous current without causing thermal damage. The duty cycle (300pps * 10μs = 0.3%) is very low, keeping average power well within limits.

Q: The lens is \"Water Clear\". Why does it emit invisible infrared light?

A: The clear epoxy lens is transparent to both visible and infrared wavelengths. The invisibility of the light is a property of the semiconductor material (AlGaAs), which emits photons at 850nm—a wavelength outside the sensitivity range of the human eye. The clear lens is often preferred in covert applications or where a visible red glow (common with 660nm LEDs) is undesirable.

11. Practical Design and Usage Case

Case: Designing a Simple Object Counter using a Break-Beam Sensor.

Two of these IR LEDs can be used with two matching phototransistors to create a two-channel break-beam sensor for counting objects on a conveyor belt. Each LED is driven by a constant current source set to 20mA using a transistor circuit or a dedicated LED driver IC to ensure stable output intensity regardless of supply voltage fluctuations. The LEDs are positioned on one side of the conveyor, and the phototransistors on the opposite side. When an object breaks the beam, the phototransistor's output changes state. The 30° viewing angle of the LED allows for some misalignment tolerance while providing a sufficiently collimated beam to minimize cross-talk between the two closely spaced channels. The high radiant intensity ensures a strong signal reaches the detector, providing good signal-to-noise ratio even in environments with some ambient IR light.

12. Principle Introduction

An LED is a semiconductor diode. When a forward voltage is applied across its P-N junction, electrons from the N-type material recombine with holes from the P-type material. This recombination process releases energy in the form of photons (light). The specific wavelength (color) of the emitted light is determined by the energy bandgap of the semiconductor material. For the LTL-E7939Q3K, the AlGaAs alloy has a bandgap corresponding to photon energies of approximately 1.46 electron volts, which translates to light with a wavelength near 850 nanometers, in the infrared region. The epoxy lens serves to protect the semiconductor die, shape the emission pattern, and enhance light extraction from the chip.

13. Development Trends

The field of infrared LEDs continues to evolve. Trends include the development of devices with higher wall-plug efficiency (more light output per electrical watt input), which reduces power consumption and heat generation. There is also ongoing work to increase modulation speeds for faster data communication applications, such as in Li-Fi (Light Fidelity) or advanced optical sensors. Packaging innovations aim to provide better thermal management, allowing for higher drive currents and greater optical power from smaller form factors. Furthermore, the integration of LEDs with drivers and control circuitry into smart modules is a growing trend, simplifying system design for end-users. The fundamental principle of electroluminescence in semiconductors remains unchanged, but material science and packaging technology drive continuous performance improvements.