LTP-2557KD LED Dot Matrix Display Datasheet - 2.0 Inch (50.8mm) Height - AlInGaP Hyper Red - 5x7 Array - English Technical Document

1. Product Overview

The LTP-2557KD is a single-digit, alphanumeric display module designed for applications requiring clear, bright character output. Its core function is to visually represent data, typically ASCII or EBCDIC coded characters, through a grid of individually addressable light-emitting diodes (LEDs).

The device is built around a 5x7 dot matrix configuration, which is the standard for representing alphanumeric characters with sufficient resolution for readability. The primary technological foundation of this display is the use of Aluminium Indium Gallium Phosphide (AlInGaP) semiconductor material for the LED chips, specifically in a Hyper Red color formulation. This material system is known for its high efficiency and brightness in the red-orange to red spectral region. The chips are fabricated on a non-transparent Gallium Arsenide (GaAs) substrate. Visually, the module features a gray faceplate with white-colored dots, which enhances contrast when the LEDs are off and diffuses the emitted light when they are illuminated.

1.1 Core Advantages & Target Market

The display offers several key advantages stemming from its design and technology. It features a relatively large 2.0-inch (50.80 mm) character height, promoting excellent visibility from a distance. The solid-state LED construction ensures high reliability, long operational life, and resistance to shock and vibration compared to legacy technologies like filament-based displays. Its design requires low power to operate, making it suitable for battery-powered or energy-conscious applications. The wide viewing angle provided by the single-plane design ensures the display remains legible from various positions. Furthermore, the modules are designed to be stackable horizontally, allowing for the creation of multi-character displays or message boards.

The primary target market for this component includes industrial control panels, instrumentation, test and measurement equipment, point-of-sale systems, and other embedded electronic devices where a simple, reliable, and bright numeric or alphanumeric readout is required. Its compatibility with standard character codes makes it easy to interface with microcontrollers and other digital systems.

2. Technical Parameters & Objective Interpretation

This section provides a detailed, objective analysis of the device's electrical, optical, and environmental specifications as defined in the datasheet. Understanding these parameters is critical for proper circuit design and ensuring reliable 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 in reliable design.

• Average Power Dissipation per Dot: 33 mW. This is the maximum continuous power each individual LED segment (dot) can handle without risk of overheating.
• Peak Forward Current per Dot: 90 mA. This is the maximum instantaneous current allowed, typically relevant for pulsed operation schemes common in multiplexed displays.
• Average Forward Current per Dot: 15 mA at 25°C. This current derates linearly at 0.2 mA/°C as the ambient temperature (Ta) increases above 25°C. For example, at 85°C, the maximum allowable average current would be approximately: 15 mA - [0.2 mA/°C * (85°C - 25°C)] = 3 mA.
• Reverse Voltage per Dot: 5 V. Exceeding this voltage in reverse bias can break down the LED junction.
• Operating & Storage Temperature Range: -35°C to +85°C. The device is rated to function and be stored within this wide temperature range.
• Solder Temperature: 260°C for 3 seconds, measured 1/16 inch (≈1.59 mm) below the seating plane. This defines the reflow soldering profile.

2.2 Electrical & Optical Characteristics (at Ta=25°C)

These are the typical performance parameters under specified test conditions, representing the expected behavior of the device.

• Average Luminous Intensity (I_V): 2100 (Min), 4600 (Typ) µcd. Test Condition: Peak current (I_p) = 32 mA with a 1/16 duty cycle. This multiplexing scheme is standard for driving matrix displays. The luminous intensity is categorized, meaning devices are binned according to measured output.
• Peak Emission Wavelength (λ_p): 650 nm (Typ). This is the wavelength at which the optical output power is greatest. Measured at I_F = 20 mA.
• Spectral Line Half-Width (Δλ): 20 nm (Typ). This indicates the spectral purity or bandwidth of the emitted light. A value of 20 nm is characteristic of AlInGaP LEDs. Measured at I_F = 20 mA.
• Dominant Wavelength (λ_d): 639 nm (Typ). This is the wavelength perceived by the human eye, which may differ slightly from the peak wavelength. Measured at I_F = 20 mA.
• Forward Voltage per Dot (V_F): 2.1 V (Min), 2.6 V (Typ). The voltage drop across an LED when conducting 20 mA. This is crucial for designing the current-limiting circuitry.
• Reverse Current per Dot (I_R): 100 µA (Max). The small leakage current when 5 V is applied in reverse bias.
• Luminous Intensity Matching Ratio (I_V-m): 2:1 (Max). This specifies the maximum allowable ratio between the brightest and dimmest dot within a single unit, ensuring uniform appearance.

Note on Measurement: Luminous intensity values are measured using a sensor and filter combination that approximates the CIE photopic luminosity function, which models the spectral sensitivity of the human eye under normal lighting conditions.

3. Binning System Explanation

The datasheet indicates that the devices are \"categorized for luminous intensity.\" This refers to a binning or sorting process.

• Luminous Intensity Binning: After manufacture, each display unit is tested, and its average luminous intensity is measured. Units are then sorted into different bins or categories based on their measured output (e.g., a \"standard brightness\" bin and a \"high brightness\" bin). This allows customers to select parts that meet specific brightness requirements and ensures consistency within a production run. The typical value of 4600 µcd represents the center of the distribution, while the minimum of 2100 µcd likely defines the lower limit of the standard bin.

4. Performance Curve Analysis

The datasheet references \"Typical Electrical/Optical Characteristic Curves.\" While the specific graphs are not provided in the text, standard curves for such devices would typically include:

• Forward Current vs. Forward Voltage (I_F-V_F Curve): Shows the exponential relationship, crucial for determining the required drive voltage for a given current.
• Luminous Intensity vs. Forward Current (I_V-I_F Curve): Displays how light output increases with current, typically in a near-linear relationship within the operating range before efficiency droop occurs at very high currents.
• Luminous Intensity vs. Ambient Temperature: Illustrates the decrease in light output as the junction temperature rises, a key consideration for high-temperature environments.
• Spectral Distribution: A plot of relative intensity versus wavelength, showing the peak at ~650 nm and the ~20 nm half-width.

5. Mechanical & Packaging Information

5.1 Package Dimensions

The physical outline drawing is referenced. Key details noted are that all dimensions are provided in millimeters, and standard tolerances are ±0.25 mm (±0.01 inch) unless a specific feature note states otherwise. The 2.0-inch (50.80 mm) dimension refers to the height of the character matrix itself.

5.2 Pin Connection & Internal Circuit

The device has a 14-pin configuration. The pinout table details the function of each pin, which are a mix of anode rows and cathode columns. There are 7 anode pins (Rows 1-7) and 5 cathode pins (Columns 1-5), corresponding to the 5x7 matrix. The internal circuit diagram shows the matrix arrangement: each LED dot is located at the intersection of a row (anode) line and a column (cathode) line. To illuminate a specific dot, its corresponding row pin must be driven high (or with a current source), and its corresponding column pin must be driven low (sinked to ground).

6. Soldering & Assembly Guidelines

The primary guidance provided is the absolute maximum rating for solder temperature: 260°C for 3 seconds, measured at a point 1/16 inch (1.59 mm) below the seating plane of the package. This defines a critical parameter for wave or reflow soldering processes. Exceeding this temperature or time can damage the internal die, wire bonds, or plastic package. Standard ESD (Electrostatic Discharge) precautions should be observed during handling. The wide storage temperature range (-35°C to +85°C) indicates no special low-temperature storage requirements are needed.

7. Application Suggestions

7.1 Typical Application Circuits

This display requires an external driver circuit. A common design uses a microcontroller with sufficient I/O pins or paired with external shift registers and driver ICs. The driving scheme is multiplexing: the controller rapidly cycles through activating one row (anode) at a time while providing the pattern data for the columns (cathodes) for that row. The 1/16 duty cycle mentioned in the test condition suggests a possible multiplexing scheme (e.g., 1/7 duty for rows plus possibly a sub-duty cycle). Proper current-limiting resistors are required on either the anode or cathode lines to set the forward current for each LED, calculated using the typical V_F (2.6V), the supply voltage, and the desired current (e.g., 10-15 mA for average brightness).

7.2 Design Considerations

• Current Limiting: Essential to prevent exceeding the average and peak current ratings.
• Multiplexing Frequency: Must be high enough to avoid visible flicker (typically >60 Hz refresh rate).
• Heat Dissipation: In high ambient temperatures or high-brightness applications, consider the derating of average forward current.
• Viewing Angle: The wide viewing angle is beneficial but ensure the display is mounted facing the intended viewer.
• Interfacing: The pinout must be correctly mapped to the driver circuit. The stackable feature requires mechanical design for alignment and electrical design for connecting multiple units in series (e.g., sharing column lines while having separate row enables).

8. Technical Comparison & Context

Compared to earlier technologies like vacuum fluorescent displays (VFDs) or smaller LED modules, the LTP-2557KD's use of AlInGaP Hyper Red technology offers advantages in efficiency, reliability (no filament to burn out), and potentially lower drive voltage than some high-voltage VFDs. Its 2.0-inch size is larger than common 0.56-inch or 1-inch modules, catering to applications needing longer viewing distances. Compared to modern graphic OLEDs or TFTs, it is a much simpler, cost-effective solution for fixed-format character display where full graphics are not required.

9. Frequently Asked Questions (Based on Parameters)

• Q: What driver current should I use? A: For reliable long-term operation, design for the Average Forward Current of 15 mA or less per dot at your expected maximum ambient temperature, applying the derating factor if necessary. The 32 mA test condition uses pulsed current with a low duty cycle.
• Q: Can I connect multiple dots directly in parallel? A: It is not recommended due to V_F variation between LEDs, which can cause uneven current sharing and brightness. Each dot/segment should ideally have its own current-limiting resistor in a multiplexed matrix drive.
• Q: How do I create a multi-digit display? A: Use the horizontal stackability feature. Mechanically align the modules. Electrically, you can connect the corresponding column (cathode) lines of all modules together and then drive each module's row (anode) lines independently to multiplex across all digits.
• Q: What is the difference between Peak and Dominant Wavelength? A: Peak wavelength is where the most optical power is emitted. Dominant wavelength is the single wavelength of monochromatic light that would appear to have the same color to the human eye. For this red LED, they are close (650 nm vs 639 nm).

10. Operating Principle

The fundamental principle is electroluminescence in a semiconductor p-n junction. When a forward voltage exceeding the diode's turn-on threshold (approximately the V_F) is applied, electrons and holes are injected into the active region of the AlInGaP semiconductor. These charge carriers recombine, releasing energy in the form of photons (light). The specific composition of the AlInGaP alloy determines the bandgap energy and thus the wavelength (color) of the emitted light, in this case, hyper red. The 5x7 matrix is formed by placing 35 of these individual LED chips in a grid pattern and connecting them via a common-anode-row and common-cathode-column wiring scheme, allowing individual control via matrix addressing.