LTE-4208 Infrared Emitter Datasheet - 940nm Wavelength - T-1 3/4 Package - 5V Reverse Voltage - 100mW Power Dissipation - English Technical Document

1. Product Overview

The LTE-4208 series is a high-radiant-power infrared emitting diode designed for applications requiring reliable and efficient IR emission. Operating at a peak wavelength of 940nm, this device is housed in a standard T-1 3/4 package with a water-clear lens, making it suitable for various sensing and detection systems.

1.1 Core Features and Target Market

The primary advantages of the LTE-4208 include its high radiant intensity, clear lens for unobstructed emission, and its spectral matching with corresponding phototransistors like the LTR-3208 series, which is crucial for optimized receiver performance. It is a lead-free and RoHS compliant product. Its main target applications are in smoke detection systems and general-purpose infrared emitter circuits where precise, pulsed IR signals are required.

2. Technical Parameters: In-Depth Objective Interpretation

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. They are not for continuous operation.

• Power Dissipation (Pd): 100 mW. This is the maximum allowable power loss within the device, primarily as heat, calculated from forward voltage and current.
• Peak Forward Current (I_FP): 3 A under pulsed conditions (300 pps, 10µs pulse width). This high current capability enables very bright, short-duration pulses for long-range sensing.
• Continuous Forward Current (I_F): 50 mA. The maximum DC current for reliable continuous operation.
• Reverse Voltage (V_R): 5 V. Exceeding this voltage in reverse bias can cause breakdown.
• Operating & Storage Temperature: -40°C to +85°C and -55°C to +100°C, respectively, indicating a robust component for industrial environments.
• Lead Soldering Temperature: 260°C for 5 seconds at 1.6mm from the body, defining the thermal profile for hand or wave soldering.

2.2 Electrical & Optical Characteristics

These are the typical performance parameters measured at 25°C.

• Radiant Intensity (I_E): The core optical output parameter, measured in mW/sr (milliwatts per steradian). It is specified across multiple BIN codes (A through D4) with minimum and typical values, allowing for selection based on required output power. For example, BIN A offers 3.31-7.22 mW/sr, while BIN D4 offers 15.72 mW/sr (Min).
• Peak Emission Wavelength (λ_Peak): 940 nm. This is in the near-infrared spectrum, invisible to the human eye, and is a common wavelength for remote controls and sensors as it avoids ambient light interference.
• Spectral Line Half-Width (Δλ): 50 nm. This defines the bandwidth of the emitted light; a narrower bandwidth would indicate a more monochromatic source.
• Forward Voltage (V_F): 1.6 V (Typ.) at I_F=20mA. This is the voltage drop across the diode when conducting, important for driver circuit design.
• Reverse Current (I_R): 100 µA (Max.) at V_R=5V. A leakage parameter; the datasheet explicitly notes this condition is for test only and the device is not for reverse operation.
• Viewing Angle (2θ_1/2): 20 degrees. This narrow beam angle concentrates the radiant intensity into a directed beam, ideal for targeted sensing applications.

3. Binning System Explanation

The LTE-4208 employs a radiant intensity binning system. Components are tested and sorted into different performance groups (BINs) based on their measured radiant output at a standard test current of 20mA. This allows designers to select parts with guaranteed minimum optical output for their application, ensuring consistency in system performance, especially when multiple emitters are used. The bins range from A (lowest output) to D4 (highest output). Designers must specify the required BIN code when ordering to guarantee the optical power level.

4. Performance Curve Analysis

The datasheet provides several key graphs for design analysis.

4.1 Spectral Distribution (Fig.1)

This curve shows the relative radiant intensity as a function of wavelength, centered around the 940nm peak with the defined 50nm half-width. It confirms the emission is within the intended IR band.

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

This derating curve shows how the maximum allowable continuous forward current decreases as the ambient temperature increases. At 85°C, the maximum current is significantly lower than at 25°C, crucial for thermal management in the design.

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

The standard I-V characteristic curve of a diode. It shows the exponential relationship, with the typical V_F of 1.6V at 20mA marked. This curve is essential for designing the current-limiting resistor in series with the LED.

4.4 Relative Radiant Intensity vs. Ambient Temperature (Fig.4) & vs. Forward Current (Fig.5)

Fig.4 illustrates the temperature dependence of optical output, typically showing a decrease in efficiency as temperature rises. Fig.5 shows the sub-linear relationship between drive current and light output; doubling the current does not double the output, a common trait in LEDs.

4.5 Radiation Diagram (Fig.6)

A polar plot visually representing the 20-degree viewing angle. The intensity is normalized, showing the beam's concentration.

5. Mechanical and Packaging Information

The device uses a through-hole T-1 3/4 (5mm) package. The outline drawing specifies key dimensions including lead diameter, lens diameter, and overall height. Critical notes include: all dimensions in mm, a tolerance of ±0.25mm, a maximum resin protrusion under the flange of 1.0mm, and that lead spacing is measured at the lead emergence point from the package. The polarity is typically indicated by a longer anode lead or a flat spot on the package flange.

6. Soldering and Assembly Guidelines

6.1 Storage

Components should be stored at <30°C and <70% RH. After opening the moisture-sensitive bag, they must be used within 3 months in a controlled environment (<25°C, <60% RH) to prevent lead oxidation which affects solderability.

6.2 Cleaning

Only alcohol-based solvents like isopropyl alcohol (IPA) are recommended.

6.3 Lead Forming

Bends must be made at least 3mm from the base of the lens. The base cannot be used as a fulcrum. Forming must be done at room temperature and before soldering.

6.4 Soldering

Two methods are specified with strict limits to prevent thermal damage:
Lead Soldering: Max 350°C for 3 seconds, with the solder point no closer than 1.6mm from the lens base.
Wave Soldering: Pre-heat to max 100°C for 60s, solder wave at max 260°C for 5s, with the dipping point no lower than 1.6mm from the base.
Critical Warning: The lens must never be immersed in solder. IR reflow is NOT suitable for this through-hole package. Excessive heat or time can deform the lens or destroy the LED.

7. Application Suggestions

7.1 Typical Application Scenarios

• Smoke Detectors: Used in photoelectric smoke detectors where the emitted IR beam is scattered by smoke particles onto a receiver.
• Proximity & Object Sensing: Paired with a matched phototransistor (e.g., LTR-3208) in interruptive or reflective sensing configurations.
• Industrial Automation: For object counting, position sensing, and edge detection.

7.2 Design Considerations and Drive Method

LEDs are current-driven devices. To ensure uniform brightness when driving multiple LEDs in parallel, it is mandatory to use a individual current-limiting resistor in series with each LED (Circuit Model A). Using a single resistor for a parallel array (Circuit Model B) is not recommended due to variances in the forward voltage (V_F) of individual LEDs, which leads to uneven current distribution and thus uneven brightness. The resistor value is calculated using R = (V_supply - V_F) / I_F.

7.3 ESD (Electrostatic Discharge) Protection

Infrared LEDs are sensitive to ESD. Mandatory precautions include: using grounded wrist straps and workstations, employing ionizers to neutralize static on plastic lenses, and ensuring all personnel handling the devices are ESD-trained. A detailed checklist for static-safe areas is provided in the datasheet.

8. Technical Comparison and Differentiation

The LTE-4208's key differentiators are its high pulsed current capability (3A), which enables very high instantaneous radiant power for long-range or noise-immune pulsed operation, and its specific matching to the LTR-3208 phototransistor series. The narrow 20-degree viewing angle provides higher intensity on-axis compared to wider-angle emitters, making it more suitable for directed beam applications. The clear binning structure allows for predictable optical performance.

9. Frequently Asked Questions Based on Technical Parameters

Q: Can I drive this LED directly from a 5V microcontroller pin?
A: No. You must use a series current-limiting resistor. For example, with a 5V supply, a V_F of 1.6V, and a desired I_F of 20mA, the resistor would be (5V - 1.6V) / 0.02A = 170 Ohms (use a standard 180 Ohm resistor).

Q: What is the purpose of the BIN code?
A: It guarantees a minimum radiant intensity. For a critical application like a smoke detector where signal strength is vital, specifying a higher BIN (e.g., D2) ensures a stronger IR beam compared to a lower BIN (e.g., A).

Q: Why is the viewing angle so narrow?
A: A narrow beam concentrates the optical power into a smaller solid angle, increasing the intensity along the central axis. This improves signal-to-noise ratio in directed sensing applications and allows for longer sensing distances.

Q: Can I use this for continuous wave (CW) operation at its peak current?
A: No. The 3A rating is for pulsed operation only (10µs pulses). The maximum continuous current is 50mA. Exceeding the continuous rating will overheat and damage the device.

10. Practical Design and Usage Case

Case: Designing a Slot-Type Object Counter.
An LTE-4208 IR emitter is placed on one side of a slot, and an LTR-3208 phototransistor is placed directly opposite. When no object is in the slot, the IR beam hits the receiver, generating a high signal. When an object passes through, it interrupts the beam, causing the receiver signal to drop. The high pulsed current capability of the LTE-4208 allows the designer to pulse the LED at a high current (e.g., 1A) for very short durations. This creates a very bright flash that can overcome ambient IR noise, increasing system reliability. The designer selects BIN C LEDs to ensure sufficient beam strength across the gap. Individual 10-Ohm resistors are used in series with each LED in a multi-sensor array to ensure consistent current. The assembly follows the soldering guidelines to prevent thermal damage during PCB population.

11. Principle Introduction

An Infrared Emitting Diode (IRED) is a semiconductor p-n junction diode that emits incoherent infrared light when forward biased. Electrons recombine with holes within the device, releasing energy in the form of photons. The wavelength of these photons is determined by the bandgap energy of the semiconductor material used (e.g., Gallium Arsenide variants for 940nm). The T-1 3/4 package houses the semiconductor chip, provides mechanical protection, and incorporates an epoxy lens that shapes the emitted light beam (in this case, to a 20-degree pattern).

12. Development Trends

The field of infrared emitters continues to evolve towards higher efficiency (more radiant power per electrical watt), higher speed for data communication applications, and increased integration. Trends include the development of surface-mount device (SMD) packages for automated assembly, multi-chip arrays for higher power output, and devices with even narrower spectral widths for specific gas sensing applications. There is also a drive towards lower operating voltages to be compatible with modern low-voltage digital circuits. The fundamental principle of electroluminescence in a semiconductor junction remains constant, but material science and packaging technology are the key drivers of advancement.