Infrared Emitter LTE-4208M Datasheet - 940nm Wavelength - T-1 3/4 Package (5mm) - 1.6V Forward Voltage - 100mW Power Dissipation - English Technical Document

1. Product Overview

The LTE-4208M is a high-performance infrared (IR) emitting diode designed for applications requiring reliable and efficient non-visible light emission. Its core function is to convert electrical energy into infrared radiation at a peak wavelength of 940 nanometers (nm). This wavelength is ideal for applications where visible light interference must be minimized, as it is largely invisible to the human eye while being highly detectable by silicon-based photodetectors like phototransistors and photodiodes.

The device is housed in a standard T-1 3/4 (approximately 5mm diameter) package with a water-clear lens. This miniature plastic package offers a cost-effective solution while providing mechanical robustness. A key design feature is its spectral and mechanical matching with corresponding phototransistor series (e.g., LTR-3208), which simplifies optical system design by ensuring optimal alignment and signal coupling between the emitter and detector pairs.

1.1 Core Advantages and Target Market

The primary advantages of the LTE-4208M include its high radiant intensity output, consistent performance through a rigorous binning process, and its compact, low-cost form factor. It is pre-selected into specific radiant intensity ranges (bins), allowing designers to choose a component that precisely meets their system's sensitivity requirements without needing external calibration or trimming circuits. This predictability enhances manufacturing yield and system reliability.

The target market for this component is primarily industrial and consumer electronics requiring proximity sensing, object detection, or optical encoding. Its most prominent application is in smoke detectors, where an IR beam is used to detect smoke particles by measuring the scattering or attenuation of light. Other potential applications include touchless switches, data transmission over short distances (e.g., remote control systems), industrial automation sensors, and object counters.

2. In-Depth Technical Parameter Analysis

Understanding the electrical and optical parameters is crucial for reliable circuit design and ensuring the LED operates within its safe operating area (SOA).

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. Operation at or near these limits is not recommended for extended periods.

• Power Dissipation (Pd): 100 mW. This is the maximum amount of power the device can dissipate as heat at an ambient temperature (TA) of 25°C. Exceeding this limit risks thermal runaway and failure.
• Peak Forward Current (I_FP): 3 A. This is the maximum allowable instantaneous current under pulsed conditions (300 pulses per second, 10μs pulse width). It is significantly higher than the continuous current rating, highlighting the device's capability for short, high-intensity bursts.
• Continuous Forward Current (I_F): 50 mA. This is the maximum DC current that can be applied continuously without exceeding the power dissipation rating, assuming typical forward voltage.
• Reverse Voltage (V_R): 5 V. The device has a very low tolerance for reverse bias. Applying a voltage greater than 5V in reverse can cause immediate breakdown. The datasheet explicitly notes the device is not designed for reverse operation.
• Operating & Storage Temperature: -40°C to +85°C and -55°C to +100°C, respectively. These ranges define the environmental conditions for reliable operation and non-operational storage.
• Lead Soldering Temperature: 260°C for 5 seconds at a distance of 4.0mm from the package body. This is critical for wave or reflow soldering processes to prevent damage to the internal semiconductor die or the plastic package.

2.2 Electrical & Optical Characteristics

These parameters are measured under standard test conditions (TA=25°C, I_F=20mA unless noted) and define the typical performance of the device.

• Radiant Intensity (I_E): This is the core optical output parameter, measured in milliwatts per steradian (mW/sr). It indicates the optical power emitted per unit solid angle. The device is sorted into bins (A through G) with minimum and typical values ranging from 3.6/13.2 mW/sr (Bin A) to 28.8 mW/sr (Bin G). This binning allows for selection based on required signal strength.
• Peak Emission Wavelength (λ_Peak): 940 nm. This is the wavelength at which the emitted optical power is maximum. It falls within the near-infrared spectrum.
• Spectral Line Half-Width (Δλ): 50 nm. This parameter, also known as Full Width at Half Maximum (FWHM), defines the spectral bandwidth. A 50nm width means the emitted light covers wavelengths from approximately 915nm to 965nm at half the peak intensity.
• Forward Voltage (V_F): 1.2V (Min), 1.6V (Typ). This is the voltage drop across the diode when conducting 20mA. It is essential for calculating the series resistor value in a driving circuit: R = (V_supply - V_F) / I_F.
• Reverse Current (I_R): 100 μA (Max) at V_R=5V. This is the small leakage current that flows when the diode is reverse-biased at its maximum rating.
• Viewing Angle (2θ_1/2): 20 degrees. This is the full angle at which the radiant intensity drops to half of its maximum value (on-axis). A 20° angle indicates a relatively narrow, focused beam, which is beneficial for directed sensing applications.

3. Binning System Explanation

The LTE-4208M employs a single, critical binning parameter: Radiant Intensity. Devices are tested and sorted into groups (Bins A through G) based on their measured output at the standard test current of 20mA. This system provides several benefits:

1. Design Consistency: Engineers can select a specific bin to ensure consistent optical signal levels across all units in a production run, improving product uniformity.
2. Performance Matching: When used with a matched photodetector, selecting emitter bins allows for tighter control over the overall sensitivity and dynamic range of the optical sensor system.
3. Cost Optimization: Applications with less stringent sensitivity requirements can potentially use lower-binned (e.g., Bin A, B) parts, which may be more cost-effective.

The datasheet does not indicate binning for forward voltage or wavelength for this model, suggesting tight process control on those parameters or that they are not critical differentiators for its target applications.

4. Performance Curve Analysis

The typical characteristic curves provide visual insight into how the device behaves under varying conditions, which is vital for robust system design beyond the nominal 25°C point.

4.1 Spectral Distribution (Fig.1)

The curve shows a Gaussian-like distribution centered at 940nm with a FWHM of approximately 50nm. This confirms the monochromatic nature of the LED's output, which is crucial for filtering out ambient light interference in sensing applications. The curve's shape is typical for an AlGaAs-based IR LED.

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

This derating curve is essential for thermal management. It shows the maximum allowable continuous forward current decreasing as ambient temperature increases. At 85°C (the maximum operating temperature), the allowable current is significantly less than the 50mA rating at 25°C. Designers must use this graph to ensure the operating current does not exceed the curve at the system's maximum expected ambient temperature.

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

This is the standard I-V curve for a diode. It shows the exponential relationship between current and voltage. The curve allows designers to estimate the V_F at currents other than the 20mA test condition, which is important for power supply design and efficiency calculations.

4.4 Relative Radiant Intensity vs. Ambient Temperature (Fig.4)

This graph illustrates the temperature dependence of optical output. The relative radiant intensity decreases as temperature increases. For example, at 85°C, the output may be only about 60-70% of its value at 25°C. This negative temperature coefficient must be accounted for in systems designed to operate over a wide temperature range to avoid signal loss at high temperatures.

4.5 Relative Radiant Intensity vs. Forward Current (Fig.5)

This curve shows that optical output is roughly proportional to forward current in the typical operating range (e.g., up to 50mA). However, the relationship is not perfectly linear, and efficiency (radiant intensity per mA) may decrease slightly at very high currents due to increased thermal effects and other non-idealities within the semiconductor.

4.6 Radiation Diagram (Fig.6)

This polar plot visually defines the viewing angle. The normalized intensity is plotted against the angle from the central axis (0°). The plot confirms the 20° half-angle, showing a rapid drop in intensity beyond approximately ±10° from the center. This pattern is characteristic of a LED with a simple dome lens, providing a focused beam suitable for directed applications.

5. Mechanical & Packaging Information

5.1 Outline Dimensions

The device conforms to the standard T-1 3/4 through-hole package dimensions. Key measurements include a body diameter of approximately 5mm, a typical lead spacing of 2.54mm (0.1") where leads emerge from the package, and an overall length. A maximum resin protrusion of 1.0mm under the flange is noted. The leads are typically made of tinned copper alloy. The package features a clear, colorless epoxy lens.

5.2 Polarity Identification

For through-hole packages like the T-1 3/4, polarity is usually indicated by the length of the leads (the longer lead is typically the anode, or positive side) and/or a flat spot on the plastic flange near the cathode (negative) lead. The datasheet drawing should be consulted for the specific marker used on this component.

6. Soldering & Assembly Guidelines

Adherence to soldering specifications is critical to prevent thermal shock and latent failures.

• Hand Soldering: Use a temperature-controlled iron. Limit soldering time per lead to 3-5 seconds at a temperature not exceeding 350°C. Apply heat to the lead, not the package body.
• Wave/Reflow Soldering: The specified condition is 260°C for 5 seconds, measured 4.0mm from the package body. This implies the component can withstand typical infrared or convection reflow profiles, but the thermal mass of the leads must be considered to ensure the package itself does not overheat.
• Cleaning: If cleaning is required after soldering, use solvents compatible with the epoxy package material. Avoid ultrasonic cleaning unless verified to be safe for the component.
• Storage: Store in a dry, anti-static environment within the specified temperature range (-55°C to +100°C). Moisture-sensitive devices should be kept in sealed bags with desiccant if they are not baked prior to use.

7. Application Suggestions & Design Considerations

7.1 Typical Application: Smoke Detector

In a photoelectric smoke detector, the LTE-4208M is placed in a chamber such that its beam does not directly hit the paired phototransistor under clear-air conditions. When smoke particles enter the chamber, they scatter the IR light, causing some of it to be deflected onto the phototransistor. The resulting increase in detector current triggers the alarm. For this application:

• Choose a radiant intensity bin that provides sufficient signal for reliable smoke detection while minimizing power consumption.
• Drive the LED with a pulsed current (e.g., a short, high pulse like 100mA for 10μs) rather than DC to increase peak signal for better signal-to-noise ratio and reduce average power consumption, extending battery life.
• Consider the temperature derating of both radiant intensity and maximum current, as detectors may be installed in attics or other environments with wide temperature swings.

7.2 General Design Considerations

• Current Limiting: Always use a series resistor or constant current driver to limit the forward current. Never connect the LED directly to a voltage source.
• Reverse Voltage Protection: In circuits where reverse voltage transients are possible (e.g., inductive loads, hot-plugging), consider adding a protection diode in parallel with the LED (cathode to anode) to clamp any reverse voltage below 0.7V.
• Heat Sinking: For continuous operation near the maximum current rating, consider the PCB layout. Providing ample copper area around the leads helps dissipate heat.
• Optical Design: The narrow 20° viewing angle simplifies optical design for collimation but requires careful mechanical alignment with the receiver. For wider coverage, diffusers or lenses may be needed.

8. Technical Comparison & Differentiation

Compared to generic, unbinned IR LEDs, the LTE-4208M's key differentiator is its guaranteed radiant intensity bins, offering predictable performance. Compared to surface-mount device (SMD) IR LEDs, the T-1 3/4 through-hole package offers higher possible power dissipation due to its larger thermal mass and longer leads, potentially allowing for higher continuous or pulsed drive currents. Its clear package is advantageous over tinted or diffused packages when maximum forward light output and beam definition are required, though it offers no inherent shielding from visible light.

9. Frequently Asked Questions (Based on Technical Parameters)

Q: Can I drive this LED with 3A continuously since the peak rating is 3A?
A: No. The 3A rating is for very short pulses (10μs) under a specific duty cycle. The maximum continuous current is 50mA. Exceeding this will quickly destroy the device due to overheating.

Q: Why is the reverse voltage rating only 5V?
A: Infrared LEDs are optimized for forward conduction. The semiconductor structure is not designed to withstand high reverse bias. Always ensure the circuit prevents reverse voltage application.

Q: How do I choose the right bin (A through G)?
A: Select based on your system's required signal strength at the receiver. If your detector circuit has high gain and you need to minimize power, a lower bin (A, B) may suffice. For longer distances, weaker detectors, or systems requiring high signal-to-noise ratio, choose a higher bin (E, F, G). Testing with your specific optical path is recommended.

Q: The forward voltage is 1.6V typical. What resistor should I use with a 5V supply for 20mA?
A: R = (V_supply - V_F) / I_F = (5V - 1.6V) / 0.020A = 170 Ohms. Use the nearest standard value (e.g., 180 Ohms) and check the actual current: I_F = (5V - 1.6V) / 180 = ~18.9mA, which is acceptable.

10. Practical Design Case Study

Scenario: Designing a low-power, battery-operated object counter for an industrial conveyor belt. The system uses a through-beam sensor where the LTE-4208M faces an LTR-3208 phototransistor across the belt.

Design Steps:

1. Goal: Maximize battery life while ensuring reliable detection of all objects.
2. Drive Method: Use pulsed operation. Microcontroller generates a 100Hz, 10% duty cycle pulse (1ms ON, 9ms OFF).
3. Current Calculation: To stay within average power limits, choose a pulse current. With Pd=100mW and V_F~1.6V, average I_F can be ~62.5mA. For a 10% duty cycle, pulse I_F can be up to 625mA. A conservative pulse current of 100mA is selected for strong signal.
4. Component Selection: Choose LTE-4208M from Bin D or E for good signal strength. Select matching LTR-3208 phototransistor.
5. Circuit: Use a microcontroller GPIO pin to drive a transistor (e.g., NPN BJT or N-channel MOSFET) that switches the 100mA pulse through the LED. A series resistor sets the current: R = (3.3V_GPIO - V_CE(sat) - V_F) / I_F. The phototransistor output is connected to a comparator or microcontroller ADC.
6. Considerations: Account for ambient light by synchronizing detection with the LED pulse (synchronous detection). Consider temperature effects on output intensity.

This approach reduces average current consumption to roughly 10mA (100mA * 10%) instead of a continuous 20-50mA, significantly extending battery life while maintaining a strong, detectable pulse of light.

11. Operating Principle

The LTE-4208M is a semiconductor p-n junction diode fabricated from materials like Aluminum Gallium Arsenide (AlGaAs). When a forward voltage exceeding the material's bandgap energy is applied, electrons from the n-region and holes from the p-region are injected into the junction region. When these charge carriers recombine, they release energy. In a light-emitting diode (LED), this energy is released primarily as photons (light). The wavelength (color) of the emitted light is determined by the bandgap energy of the semiconductor material. For AlGaAs tuned to 940nm, the bandgap energy is approximately 1.32 electron volts (eV). The clear epoxy package acts as a lens, shaping the emission pattern and providing environmental protection.

12. Technology Trends

Infrared emitter technology continues to evolve. Trends relevant to devices like the LTE-4208M include:

• Increased Efficiency: Ongoing material science research aims to improve the wall-plug efficiency (optical power out / electrical power in) of IR LEDs, reducing heat generation and power consumption for the same optical output.
• Higher Speed Modulation: Development of LEDs capable of faster switching for applications in optical data communication (e.g., IrDA, Li-Fi) and high-speed sensing.
• Integration: Movement towards integrated optoelectronic assemblies that combine the emitter, detector, and sometimes driving circuitry in a single module, simplifying design and improving alignment and performance consistency.
• Alternative Wavelengths: Expansion into other near-IR wavelengths (e.g., 850nm, 880nm) for specific applications like eye-tracking (where 940nm is preferred as it's less visible) or compatibility with different silicon detector sensitivities.
• Packaging Miniaturization: While through-hole packages remain popular for high-power or high-reliability applications, there is a strong trend towards surface-mount technology (SMD) for automated assembly and space-constrained designs.

The LTE-4208M, with its proven T-1 3/4 package, high radiant output, and rigorous binning, represents a mature and reliable solution well-suited for its primary applications, particularly where through-hole mounting is preferred or required.