LTW-326DSKS-5A SMD LED Datasheet - Side Looking - White & Yellow - 20mA - English Technical Document

1. Product Overview

The LTW-326DSKS-5A is a dual-chip, side-looking Surface Mount Device (SMD) LED specifically engineered for LCD backlighting applications. This component integrates two distinct semiconductor technologies within a single EIA-standard package: an ultra-bright InGaN (Indium Gallium Nitride) chip for white light emission and an AlInGaP (Aluminum Indium Gallium Phosphide) chip for yellow light emission. Its primary design purpose is to provide efficient, reliable, and compact edge-lighting for liquid crystal displays, where space constraints and uniform light distribution are critical. The side-emitting lens profile is optimized to direct light laterally across the light guide plate, a fundamental requirement for achieving even backlight illumination. The device is supplied on 8mm tape mounted on 7-inch diameter reels, making it fully compatible with high-speed automated pick-and-place assembly equipment used in modern electronics manufacturing.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

The absolute maximum ratings define the stress limits beyond which permanent damage to the device may occur. For the white InGaN chip, the maximum continuous DC forward current is specified at 20mA, with a peak forward current of 100mA permissible under pulsed conditions (1/10 duty cycle, 0.1ms pulse width). The yellow AlInGaP chip shares the same 20mA DC current limit but has a lower peak current rating of 80mA. The maximum power dissipation is 72mW for the white chip and 48mW for the yellow chip, calculated at an ambient temperature (Ta) of 25°C. These ratings are crucial for thermal management in the final application. The device is rated for an operating temperature range of -20°C to +80°C and a storage temperature range of -40°C to +85°C. A key specification for assembly is the infrared reflow soldering condition, which is rated for a peak temperature of 260°C for a duration of 10 seconds, aligning with common lead-free soldering profiles.

2.2 Electrical and Optical Characteristics

The electrical and optical characteristics are measured under standard test conditions at Ta=25°C and a forward current (IF) of 5mA. For the white LED, the luminous intensity (Iv) ranges from a minimum of 28.0 mcd to a maximum of 112.0 mcd. The yellow LED has a lower Iv range, from 7.1 mcd to 71.0 mcd. The typical viewing angle (2θ1/2) for both colors is 130 degrees, providing a wide emission pattern suitable for backlight diffusion. The forward voltage (VF) is typically 2.55V for white (max 3.15V) and 2.0V for yellow (max 2.4V). The reverse current (IR) is limited to a maximum of 10 µA at a reverse voltage (VR) of 5V; it is critical to note that the device is not designed for operation under reverse bias. The yellow LED's optical characteristics are further defined by a typical peak emission wavelength (λP) of 591 nm, a dominant wavelength (λd) of 590 nm, and a spectral half-width (Δλ) of 15 nm. The chromaticity coordinates are typically x=0.3, y=0.3 on the CIE 1931 diagram for the specified test conditions.

3. Binning System Explanation

The product utilizes a comprehensive binning system to categorize LEDs based on key performance parameters, ensuring consistency within a production batch. This is essential for applications requiring uniform color and brightness.

3.1 Forward Voltage (VF) Binning for White LED

White LEDs are sorted into three VF bins (A, B, C) based on their forward voltage at IF=5mA. Bin A covers 2.55V to 2.75V, Bin B covers 2.75V to 2.95V, and Bin C covers 2.95V to 3.15V. A tolerance of ±0.1V is applied to each bin.

3.2 Luminous Intensity (Iv) Binning

Separate Iv binning tables exist for white and yellow LEDs. For white: Bin N (28.0-45.0 mcd), Bin P (45.0-71.0 mcd), Bin Q (71.0-112.0 mcd). For yellow: Bin K (7.10-11.2 mcd), Bin L (11.2-18.0 mcd), Bin M (18.0-28.0 mcd), Bin N (28.0-45.0 mcd), Bin P (45.0-71.0 mcd). A tolerance of ±15% is applied to each intensity bin.

3.3 Hue (Chromaticity) Binning

The hue binning, applicable to the relevant LED color, uses the CIE 1931 chromaticity coordinates. Six bins are defined (S1 through S6), each specifying a quadrilateral area on the (x, y) coordinate chart. The coordinates for each corner of these quadrilaterals are precisely listed in the datasheet. A tolerance of ±0.01 is applied to each hue bin coordinate.

4. Performance Curve Analysis

The datasheet references typical electrical and optical characteristic curves, which are essential for understanding device behavior under non-standard conditions. While the specific graphs are not reproduced in the provided text, they typically include the relationship between forward current (IF) and forward voltage (VF), which is non-linear and crucial for driver circuit design. Another standard curve shows luminous intensity (Iv) versus forward current (IF), illustrating how output scales with drive current and highlighting efficiency roll-off at higher currents. The relationship between luminous intensity and ambient temperature is also critical, as LED output generally decreases with increasing junction temperature. For the yellow LED, a spectral distribution graph would typically show the relative intensity versus wavelength, centered around the 590-591 nm peak, with the 15 nm half-width defining the color purity.

5. Mechanical and Packaging Information

5.1 Device Dimensions and Pinout

The LED conforms to an EIA standard package outline. The side-looking lens is a key mechanical feature. The pin assignment is clearly defined: Pin C2 is for the green/white InGaN chip, and Pin C1 is for the yellow AlInGaP chip. All dimensions in the package drawing are in millimeters, with a standard tolerance of ±0.10 mm unless otherwise specified. This precise dimensional data is necessary for creating accurate PCB footprints and ensuring proper fit within the assembly.

5.2 Suggested Solder Pad Design and Polarity

The datasheet provides suggested solder pad dimensions to ensure a reliable solder joint and proper alignment during reflow. It also indicates the suggested soldering direction relative to the tape reel orientation, which can help optimize the placement process. Correct polarity identification during placement is vital, as reverse installation will prevent the LED from illuminating.

6. Soldering and Assembly Guidelines

6.1 Reflow Soldering Process

The device is fully compatible with infrared (IR) reflow soldering processes. The absolute maximum condition is 260°C for 10 seconds. A suggested reflow profile is implied, which typically includes a preheat zone, a thermal soak zone, a reflow zone with a controlled peak temperature and time above liquidus (TAL), and a controlled cooling zone. Adhering to a profile that does not exceed the 260°C/10s limit is critical to prevent damage to the LED's epoxy lens and internal wire bonds.

6.2 Cleaning and Handling

Cleaning must be performed with care. Only specified chemicals should be used. The datasheet recommends immersion in ethyl alcohol or isopropyl alcohol at normal temperature for less than one minute if cleaning is necessary. Unspecified chemicals may damage the package material. A critical handling note emphasizes protection against Electrostatic Discharge (ESD). Although not always considered highly ESD-sensitive like some ICs, LEDs can be damaged by static electricity and surges. It is recommended to use a wrist strap or anti-static gloves, and ensure all equipment is properly grounded.

6.3 Storage Conditions

Storage conditions differ based on whether the moisture-sensitive package is sealed or opened. When the original sealed bag (with desiccant) is intact, LEDs should be stored at ≤30°C and ≤90% Relative Humidity (RH) and used within one year. Once the moisture-proof bag is opened, the storage environment must not exceed 30°C or 60% RH. It is strongly recommended that devices removed from their original packaging be subjected to IR reflow within one week. For longer-term storage outside the original bag, they should be kept in a sealed container with desiccant or in a nitrogen-purged desiccator to prevent moisture absorption, which can cause \"popcorning\" during reflow.

7. Packaging and Ordering Information

The product is supplied in a tape-and-reel format compatible with automated assembly. The tape width is 8mm. The reels have a 7-inch diameter and typically contain 3000 pieces per reel. For order quantities that are not multiples of 3000, a minimum packing quantity of 500 pieces is specified for remainders. The packaging conforms to ANSI/EIA 481 specifications. Key quality notes for the reel include: empty component pockets are sealed with cover tape, and the maximum number of consecutive missing components (lamps) on the reel is two.

8. Application Notes and Design Considerations

8.1 Target Application Scenarios

The primary and designed application for the LTW-326DSKS-5A is as an edge-light source for LCD backlight units (BLUs) in consumer and industrial electronics. This includes monitors, televisions, laptop displays, instrument panels, and signage. The side-looking lens is specifically engineered to couple light efficiently into the edge of a light guide plate (LGP), which then distributes the light uniformly across the display area using micro-structures or diffuser patterns.

8.2 Circuit Design Considerations

Designers must implement appropriate current-limiting mechanisms, as LEDs are current-driven devices. A simple series resistor is common for low-current applications, but constant-current drivers are recommended for better stability and longevity, especially when brightness uniformity is critical. The circuit must respect the absolute maximum ratings for forward current, reverse voltage, and power dissipation. Thermal management is also important; while the package itself dissipates heat, ensuring adequate PCB copper area or thermal vias can help maintain a lower junction temperature, preserving light output and device lifetime.

8.3 Optical Design Considerations

The 130-degree viewing angle must be considered in the optical design of the light guide and diffuser system. The distance from the LED emitting surface to the light guide plate edge, as well as the use of reflective tape around the LED, can significantly impact coupling efficiency and hotspot formation. The use of a dual-color LED (white and yellow) in this package suggests applications where color mixing or specific color temperature tuning might be required, controlled by driving the two chips independently.

9. Technical Comparison and Differentiation

The key differentiating feature of this component is its side-looking lens geometry combined with a dual-chip (white/yellow) configuration in a standard SMD footprint. Compared to top-emitting LEDs, side-emitters are inherently better suited for edge-lit backlight applications as they direct light into the plane of the light guide rather than perpendicular to it, reducing optical losses. The integration of two colors allows for design flexibility not available in single-color side-emitting packages. The use of InGaN for white and AlInGaP for yellow represents standard, reliable semiconductor technologies for these respective colors, offering good efficiency and stability.

10. Frequently Asked Questions (FAQ)

Q: Can I drive the white and yellow chips simultaneously at their maximum DC current of 20mA each?
A: Yes, but you must consider the total power dissipation. The white chip dissipates up to 72mW and the yellow up to 48mW, totaling 120mW. The thermal design of the PCB must manage this combined heat load.

Q: What is the purpose of the binning codes?
A: Binning ensures electrical and optical consistency. For a uniform backlight, you would typically specify LEDs from the same Iv and Hue bins to avoid visible brightness or color variations across the display.

Q: The datasheet mentions a \"peak forward current\" rating. Can I use this for PWM dimming?
A: Yes, the peak current rating (100mA for white, 80mA for yellow under 1/10 duty cycle, 0.1ms pulse) allows for brief over-driving, which can be used in certain PWM dimming schemes to achieve a wider dynamic range. However, the average current over time must still respect the DC forward current rating, and the driver circuit must be carefully designed to deliver clean, fast current pulses.

Q: How critical is the 1-week reflow deadline after opening the moisture barrier bag?
A: It is a strong recommendation to prevent moisture-induced defects. If the deadline is exceeded, the LEDs should be baked according to the appropriate moisture sensitivity level (MSL) profile before reflow to remove absorbed moisture.

11. Practical Application Example

A typical use case is in a 7-inch industrial touchscreen display. The design calls for an edge-lit backlight with high uniformity and a specific color temperature. The engineer selects the LTW-326DSKS-5A LED. They design a PCB with 12 LEDs placed along the bottom edge of the display cavity. The solder pad layout follows the datasheet's suggested dimensions. A constant-current driver IC is selected to provide a stable 5mA to each LED string. To achieve the desired 4500K white point, the designer decides to drive only the white InGaN chips. They specify all LEDs from Hue bin S3 and Luminous Intensity bin P to ensure color and brightness consistency. During assembly, the tape-and-reel packaging is used with an automated pick-and-place machine. The board undergoes a lead-free reflow process with a peak temperature carefully controlled below 260°C. After assembly, the light guide plate and optical films are assembled on top, resulting in a bright, uniform backlight for the LCD.

12. Technology Principle Introduction

The device operates on the principle of electroluminescence in semiconductor materials. When a forward voltage is applied across the p-n junction of the LED chip, electrons and holes are injected into the active region where they recombine. This recombination releases energy in the form of photons (light). The wavelength (color) of the emitted light is determined by the bandgap energy of the semiconductor material. The InGaN chip has a wider bandgap, engineered to emit blue light. This blue light then excites a phosphor coating inside the package, which down-converts some of the blue light to longer wavelengths (yellow, red), resulting in the perception of white light—a method known as phosphor-converted white. The AlInGaP chip has a narrower bandgap, directly emitting light in the yellow/amber region of the spectrum without the need for phosphor conversion. The side-looking lens is made of molded epoxy or silicone that shapes the light output pattern.

13. Industry Trends and Developments

The trend in backlighting for LCDs, particularly in consumer electronics, has been toward miniaturization and higher efficiency. This drives the development of LEDs with higher luminous efficacy (more lumens per watt), allowing for fewer LEDs or lower drive currents to achieve the same brightness, saving energy and reducing heat. There is also a trend toward better color gamut coverage, often using LEDs with narrower emission spectra or combining multiple primary colors (RGB). While this specific product uses a white+yellow combination, other solutions might use blue LED + red phosphor or multiple monochromatic chips. For very thin displays, the precise optical coupling of the side-emitting LED to increasingly thinner light guide plates remains a key engineering challenge. Furthermore, the rise of direct-lit Mini-LED backlights, which use arrays of very small top-emitting LEDs behind the panel, represents an alternative technological path for high-dynamic-range (HDR) displays, though edge-lit solutions like the one this LED enables remain dominant for cost-sensitive and space-constrained applications.