LTST-S220KEKT SMD LED Datasheet - Side Looking - Red (AlInGaP) - 20mA - 50mcd - English Technical Document

1. Product Overview

The LTST-S220KEKT is a surface-mount device (SMD) light-emitting diode (LED) designed primarily for side-emitting illumination applications. Its core construction utilizes an Aluminum Indium Gallium Phosphide (AlInGaP) semiconductor chip, which is engineered to produce high-intensity red light. The primary design intent and key market for this component is integration as a backlight source for liquid crystal display (LCD) panels, where uniform edge-lighting is required.

The component is packaged in a standard EIA-compliant format, supplied on 8mm tape wound onto 7-inch diameter reels. This packaging is fully compatible with high-speed automated pick-and-place assembly equipment commonly used in modern electronics manufacturing. The LED also features compatibility with standard infrared (IR) reflow, vapor phase reflow, and wave soldering processes, making it suitable for volume production.

1.1 Core Advantages

• Specialized Optics: The side-looking lens design is optimized to direct light laterally, which is ideal for channeling light into light guides used in LCD backlight units (BLUs).
• High Brightness: The use of AlInGaP technology provides high luminous intensity from a small chip area.
• Manufacturing Readiness: Tape-and-reel packaging and reflow process compatibility enable efficient, automated assembly, reducing production time and cost.
• Reliability: The device is rated for operation across a wide temperature range from -55°C to +85°C, supporting applications in various environments.

2. In-Depth Technical Parameter Analysis

All specifications are defined at an ambient temperature (Ta) of 25°C unless otherwise stated. Understanding these parameters is critical for reliable circuit design and ensuring long-term performance.

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 for reliable operation.

• Power Dissipation (Pd): 75 mW. This is the maximum allowable power loss within the device.
• Continuous Forward Current (IF): 30 mA. The DC current that can be applied continuously.
• Peak Forward Current: 80 mA. Permissible only under pulsed conditions (1/10 duty cycle, 0.1ms pulse width).
• Derating Factor: 0.4 mA/°C. For every degree Celsius above 25°C, the maximum allowable continuous forward current must be reduced by this amount.
• Reverse Voltage (VR): 5 V. Exceeding this voltage in reverse bias can cause junction breakdown.
• Operating & Storage Temperature Range: -55°C to +85°C.

2.2 Electrical & Optical Characteristics

These are the typical performance parameters under normal operating conditions.

• Luminous Intensity (Iv): 30.0 mcd (Min), 50.0 mcd (Typ) at a forward current (IF) of 20 mA. Intensity is measured using a sensor filtered to match the CIE photopic eye-response curve.
• Viewing Angle (2θ½): 130 degrees (Typ). This wide viewing angle is characteristic of the side-looking design, indicating light is emitted over a broad lateral plane.
• Peak Emission Wavelength (λPeak): 632 nm (Typ). The wavelength at which the spectral output is strongest.
• Dominant Wavelength (λd): 624 nm (Typ). This is the single wavelength perceived by the human eye, derived from the CIE chromaticity coordinates, defining the red color point.
• Spectral Line Half-Width (Δλ): 20 nm (Typ). The bandwidth of the emitted spectrum at half the peak intensity, indicating color purity.
• Forward Voltage (VF): 2.0 V (Min), 2.4 V (Typ) at IF=20mA. This parameter is crucial for calculating series resistor values and power supply design.
• Reverse Current (IR): 100 µA (Max) at VR=5V.
• Capacitance (C): 40 pF (Typ) at VF=0V, f=1MHz. Relevant for high-frequency switching applications.

3. Performance Curve Analysis

While specific graphical data is not provided in the text extract, typical curves for such a device would be essential for design analysis. Engineers would expect to review the following relationships, which are standard for LED characterization:

3.1 Current vs. Voltage (I-V) Curve

This curve shows the exponential relationship between forward voltage and current. The knee voltage (where current begins to rise sharply) for AlInGaP LEDs is typically around 1.8-2.0V. The curve is essential for determining the dynamic resistance of the LED and for designing appropriate current-limiting circuitry.

3.2 Luminous Intensity vs. Forward Current

This plot typically shows a near-linear relationship between forward current and light output within the recommended operating range. It helps designers choose the drive current to achieve a desired brightness level while staying within thermal limits.

3.3 Temperature Dependence

Key parameters like forward voltage and luminous intensity vary with junction temperature. VF typically decreases with increasing temperature (negative temperature coefficient), while luminous intensity generally decreases. Understanding these shifts is vital for designs operating over a wide temperature range or at high power levels.

3.4 Spectral Distribution

A graph of relative intensity versus wavelength would show a peak around 632nm with a typical half-width of 20nm, confirming the monochromatic red output of the AlInGaP chip.

4. Mechanical & Packaging Information

4.1 Device Dimensions

The LED conforms to a standard EIA package outline. Critical dimensions include the body length, width, height, and the placement of the cathode identifier (typically a notch or a green mark on the tape). The exact millimeter dimensions and tolerances (±0.1mm) are provided in the package drawing within the datasheet.

4.2 Polarity Identification

Correct orientation is mandatory. The cathode is usually marked on the device body or indicated by a specific feature in the tape pocket. Misorientation will prevent the LED from illuminating and applying reverse bias can damage it.

4.3 Suggested Soldering Pad Layout

A recommended footprint for the PCB lands is provided to ensure proper solder joint formation, mechanical stability, and thermal relief during reflow. Adhering to this layout minimizes tombstoning and other assembly defects.

4.4 Tape and Reel Specifications

The component is supplied in embossed carrier tape with a protective cover tape. Key specifications include: 8mm tape width, 7-inch reel diameter, and 4000 pieces per reel. The packaging follows ANSI/EIA 481-1-A-1994 standards. A maximum of two consecutive missing components (empty pockets) is allowed per reel.

5. Soldering & Assembly Guidelines

5.1 Reflow Soldering Conditions

The LED is rated for common soldering processes. The datasheet specifies maximum exposure conditions to prevent thermal damage to the plastic package and the wire bonds:

• Infrared (IR) / Wave Soldering: 260°C peak temperature for a maximum of 5 seconds.
• Vapor Phase Soldering: 215°C for a maximum of 3 minutes.

A detailed reflow profile (preheat, soak, reflow, cooling) with time and temperature constraints is typically suggested to ensure reliable solder joints without degrading the LED.

5.2 Cleaning

Post-solder cleaning requires caution. Only specified chemicals should be used. The datasheet explicitly recommends:

• Immersion in ethyl alcohol or isopropyl alcohol at normal room temperature.
• Immersion time should be less than one minute.
• Unspecified chemical liquids must be avoided as they may damage the LED's epoxy lens or package.

5.3 Storage & Handling

Devices should be stored in their original, sealed moisture-barrier bags with desiccant in a controlled environment (within the -55°C to +85°C range). Exposure to excessive humidity before soldering can lead to \"popcorning\" during reflow. Standard ESD (electrostatic discharge) precautions should be observed during handling.

6. Application Notes & Design Considerations

6.1 Primary Application: LCD Backlighting

The side-looking design is ideal for edge-lit backlight units. Multiple LEDs are placed along one or more edges of a light guide plate (LGP). The light from the LEDs is injected into the edge of the LGP, where it propagates via total internal reflection and is extracted upwards towards the LCD panel by printed or molded surface features, creating a uniform area light source.

6.2 Drive Circuit Design

LEDs are current-driven devices. A series current-limiting resistor is the simplest drive method. The resistor value (R) is calculated using Ohm's Law: R = (Vcc - VF) / IF, where Vcc is the supply voltage, VF is the LED forward voltage (use typical or max value for reliability), and IF is the desired forward current (e.g., 20mA). For constant brightness across multiple LEDs or varying temperatures, a constant-current driver circuit is recommended.

6.3 Thermal Management

Although power dissipation is low (75mW max), effective thermal management is crucial for longevity and stable light output. The PCB acts as a heat sink. Ensure adequate copper area connected to the LED's thermal pads (if any) or the solder lands to conduct heat away from the junction. Adhere to the current derating curve above 25°C ambient.

6.4 Optical Integration

For backlight applications, the precise mechanical alignment and distance between the LED emitting surface and the light guide plate edge are critical to maximize coupling efficiency and minimize optical losses. The wide 130-degree viewing angle aids in this coupling.

7. Technical Comparison & Differentiation

Compared to other LED technologies for red emission:

• vs. Traditional GaAsP: AlInGaP offers significantly higher luminous efficiency and better temperature stability, resulting in brighter and more consistent red light.
• vs. AllnGaP Top-View LEDs: The key differentiator is the beam pattern. This side-looking variant emits light parallel to the PCB plane, whereas standard LEDs emit perpendicularly. This makes it unsuitable for direct indication but optimal for edge-lighting.
• vs. White LEDs for Backlighting: Monochromatic red LEDs like this are often used in multi-color (RGB) backlighting systems to create a wide color gamut, or in monochrome displays requiring specific red illumination.

8. Frequently Asked Questions (FAQ)

8.1 Can I drive this LED directly from a 5V or 3.3V logic output?

No. You must use a series resistor or constant-current driver to limit the current to the specified maximum (30mA continuous). Connecting it directly to a voltage source will cause excessive current flow, potentially destroying the LED.

8.2 What is the difference between peak wavelength and dominant wavelength?

Peak wavelength (λPeak) is the physical wavelength where the spectral power is highest. Dominant wavelength (λd) is a perceptual metric derived from color science (CIE diagram) that represents the single wavelength the human eye would perceive as matching the LED's color. For monochromatic LEDs, they are often close but not identical.

8.3 How many LEDs can I connect in series?

The number depends on your supply voltage (Vcc) and the forward voltage (VF) of each LED. The sum of the VF of all LEDs in the string must be less than Vcc, with enough headroom for the current-limiting element (resistor or regulator). For example, with a 12V supply and VF=2.4V, you could theoretically connect 4 LEDs in series (4 * 2.4V = 9.6V), leaving 2.4V for the current-limiting resistor.

8.4 Is this LED suitable for automotive applications?

The operating temperature range (-55°C to +85°C) covers many automotive requirements. However, true automotive-grade components typically require additional qualification for vibration, humidity, and extended lifetime under harsh conditions. This datasheet does not specify AEC-Q101 or similar automotive qualifications, so it may not be suitable for safety-critical or exterior automotive lighting without further verification.

9. Practical Design Case Study

Scenario: Designing a simple status indicator for a portable device that requires side-illumination of a small acrylic light pipe.

Implementation: The LTST-S220KEKT is an excellent choice. It is placed on the main PCB with its emitting surface aligned to the edge of the acrylic light pipe. A series resistor is calculated for a 3.3V system: R = (3.3V - 2.4V) / 0.020A = 45 Ohms. A standard 47 Ohm resistor is selected, resulting in a forward current of approximately 19.1mA, well within limits. The wide viewing angle ensures efficient coupling into the light pipe, providing a bright, even red glow at the indicator's exit point on the device casing.

10. Technology Principle Introduction

The LTST-S220KEKT is based on Aluminum Indium Gallium Phosphide (AlInGaP) semiconductor technology. When a forward voltage is applied across the p-n junction, electrons and holes are injected into the active region where they recombine. In AlInGaP, this recombination event primarily releases energy in the form of photons (light) in the red to yellow-orange spectrum, depending on the exact alloy composition. The side-looking package incorporates a molded epoxy lens that is shaped to refract and direct the emitted light sideways, parallel to the mounting plane, rather than upwards. This is achieved through specific lens curvature and the positioning of the semiconductor chip within the package.

11. Industry Trends & Developments

The market for side-emitting LEDs continues to evolve. Key trends include:

• Increased Efficiency: Ongoing material science improvements aim to boost lumens per watt (efficacy) for AlInGaP and other color LEDs, reducing power consumption in backlight units.
• Miniaturization: There is a constant drive for smaller package sizes (e.g., 0603, 0402 metric) to enable thinner displays and more compact devices.
• Integrated Solutions: Trends move towards multi-LED modules or \"light bars\" that combine multiple colors (RGB) or white LEDs with drivers and optics in a single pre-assembled unit, simplifying design and assembly for backlighting.
• Alternative Technologies: For white backlighting, blue LEDs with phosphor conversion remain dominant. However, for color displays, direct-emission red, green, and blue (RGB) LEDs or mini/micro-LED arrays are gaining traction for their superior color gamut and local dimming capabilities in high-end displays.