LTP-2557KS 5x7 Dot Matrix LED Display Datasheet - 2-inch Height - AlInGaP Yellow - English Technical Document

1. Product Overview

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

1.1 Core Advantages and Target Market

This device offers several key advantages for system designers. Its primary benefit is the low power requirement, making it suitable for battery-operated or energy-conscious applications. The solid-state reliability of LED technology ensures long operational life and resistance to shock and vibration compared to filament-based displays. The wide viewing angle and single-plane construction provide consistent visibility from various positions. It is stackable horizontally, allowing for the creation of multi-character displays. Finally, being a lead-free package compliant with RoHS directives makes it suitable for modern electronic manufacturing with environmental regulations in mind. The target market includes industrial control panels, instrumentation, test equipment, point-of-sale terminals, and other embedded systems where durable, low-power character display is needed.

2. Technical Specifications Deep Dive

This section provides a detailed, objective analysis of the device's key performance parameters as defined in the datasheet.

2.1 Photometric and Optical Characteristics

The display utilizes Aluminium Indium Gallium Phosphide (AlInGaP) semiconductor material for its yellow LED chips. This material system is known for high efficiency and good color purity in the amber/yellow/red spectrum. The typical peak emission wavelength (λp) is 588 nm, with a dominant wavelength (λd) of 587 nm, placing it firmly in the yellow region. The spectral line half-width (Δλ) is 15 nm, indicating a relatively narrow spectral bandwidth which contributes to color purity.

The key brightness parameter is the Average Luminous Intensity (Iv). Under the specified test condition of a 32 mA peak current and a 1/16 duty cycle, the intensity ranges from a minimum of 800 μcd to a maximum of 3600 μcd, with a typical value provided. The datasheet also specifies a Luminous Intensity Matching Ratio of 2:1 maximum for dots within a similar light area, which is a measure of brightness uniformity across the display matrix.

2.2 Electrical Parameters

The electrical characteristics define the operating limits and conditions for the device. The Absolute Maximum Ratings set the boundaries for safe operation: 70 mW average power dissipation per dot, 60 mA peak forward current per dot, and an average forward current per dot of 25 mA at 25°C, derating linearly by 0.33 mA/°C as temperature increases. The maximum reverse voltage per dot is 5 V.

Under typical operating conditions, the forward voltage (Vf) for any single LED dot ranges from 2.05V to 2.6V when driven at 20 mA. The reverse current (Ir) is specified at a maximum of 100 μA when 5V is applied in reverse bias. The operating and storage temperature range is broad, from -35°C to +105°C.

2.3 Thermal Characteristics and Soldering

The derating curve for average forward current is a critical thermal parameter, indicating that the maximum permissible continuous current decreases as ambient temperature rises above 25°C. For assembly, the datasheet specifies soldering conditions: the device can be subjected to 260°C for 3 seconds, measured 1/16 inch (approximately 1.59 mm) below the seating plane of the package. This is a standard reflow soldering profile guideline.

3. Binning System Explanation

The datasheet indicates that the devices are categorized for luminous intensity. This implies a binning process where units are sorted and labeled based on their measured light output (Iv) under standard test conditions. Designers can select bins to ensure consistent brightness across multiple displays in a system or to meet specific brightness requirements for an application. The provided intensity range (800-3600 μcd) defines the possible bins available.

4. Performance Curve Analysis

While the datasheet references typical characteristic curves, specific plots are not detailed in the provided text. Typically, such curves for an LED display would include:

• Forward Current (If) vs. Forward Voltage (Vf) Curve: Shows the exponential relationship, crucial for designing current-limiting circuitry.
• Luminous Intensity (Iv) vs. Forward Current (If) Curve: Demonstrates how light output increases with current, up to the maximum rating.
• Luminous Intensity (Iv) vs. Ambient Temperature (Ta) Curve: Illustrates the decrease in light output as junction temperature rises, important for thermal management.
• Spectral Distribution Curve: A graph plotting relative intensity against wavelength, showing the peak at ~588 nm and the 15 nm half-width.

These curves are essential for predicting performance under non-standard conditions and for robust circuit design.

5. Mechanical and Package Information

The LTP-2557KS is a through-hole package. The matrix height is 2 inches (50.8 mm). The package has a gray face and white dot color for optimal contrast when the LEDs are off. The detailed dimensioned drawing shows a 14-pin dual-in-line package. All dimensions are in millimeters with a standard tolerance of ±0.25 mm unless otherwise noted. The pin tip shift tolerance is specified as ±0.4 mm, which is important for PCB hole placement design.

6. Pin Connection and Internal Circuit

The device uses a 5x7 array with X-Y select architecture. The internal circuit diagram and pin connection table reveal a multiplexed design. Pins are assigned to specific anode rows (1-7) and cathode columns (1-5). This multiplexing reduces the number of required driver pins from 35 (for individual control) to 12 (7 rows + 5 columns), simplifying the interface circuitry. The pinout is as follows: Pin 1: Anode Row 5, Pin 2: Anode Row 7, Pin 3: Cathode Column 2, Pin 4: Cathode Column 3, Pin 5: Anode Row 4, Pin 6: Cathode Column 5, Pin 7: Anode Row 6, Pin 8: Anode Row 3, Pin 9: Anode Row 1, Pin 10: Cathode Column 4, Pin 11: Cathode Column 3 (Note: Column 3 appears twice, likely a datasheet typo; one should be Column 1 or another column), Pin 12: Anode Row 4 (duplicate of Pin 5, likely a typo), Pin 13: Cathode Column 1, Pin 14: Anode Row 2. Correct interpretation of this table is crucial for proper PCB layout and driver software.

7. Soldering and Assembly Guide

As per the Absolute Maximum Ratings, the recommended soldering condition is 260°C for 3 seconds, measured at a point 1.59mm below the package body. This aligns with typical lead-free reflow profiles. Care should be taken to avoid exceeding this temperature or time to prevent damage to the LED chips or plastic package. During handling, standard ESD (Electrostatic Discharge) precautions should be observed for semiconductor devices. Storage should be within the specified temperature range of -35°C to +105°C in a low-humidity environment.

8. Application Suggestions

8.1 Typical Application Scenarios

• Industrial Control Panels: Displaying setpoints, status codes, or error messages.
• Test and Measurement Equipment: Showing numerical readings or channel identifiers.
• Embedded Systems Prototyping: As a simple output for microcontrollers.
• Legacy System Upgrades: Replacing older incandescent or vacuum fluorescent displays.

8.2 Design Considerations

• Driver Circuitry: Requires a multiplexing driver circuit (e.g., using transistor arrays or dedicated LED driver ICs) capable of sourcing/sinking the peak currents (up to 60 mA per dot, but typically driven lower for multiplexing).
• Current Limiting: External resistors are necessary to set the forward current for each column or row, calculated based on the supply voltage and the LED forward voltage.
• Refresh Rate: The multiplexing scheme requires a sufficiently high scan rate (typically >100 Hz) to avoid visible flicker.
• Power Supply: Must be able to handle the peak current demands during multiplexing.
• Software: Requires a character font map (5x7 pixel) stored in memory and a routine to sequentially activate the correct rows and columns to form characters.

9. Technical Comparison and Differentiation

Compared to contemporary 5x7 dot matrix displays using different technologies, the AlInGaP yellow LED offers distinct advantages. Versus older red GaAsP LEDs, AlInGaP provides higher efficiency and brighter output. Compared to standard green or blue GaN LEDs, the yellow color offers excellent visibility in various ambient lighting conditions and is often chosen for caution or status indicators. The through-hole package differentiates it from surface-mount alternatives, making it suitable for prototyping, hobbyist projects, or applications where through-hole assembly is preferred for mechanical robustness.

10. Frequently Asked Questions (Based on Technical Parameters)

Q: What does \"1/16 duty cycle\" mean in the luminous intensity test condition?
A: It means each LED dot is powered on for only 1/16th of the total measurement cycle time. This is representative of a multiplexed driving scheme where only one row is active at any time in a 16-row system (or a time division of a 5x7 matrix). The specified intensity is an \"average\" value over the full cycle.

Q: Can I drive this display with a constant DC current without multiplexing?
A: Technically yes, but it is highly inefficient and not the intended use. Driving all 35 dots continuously at 20 mA would require a total current of 700 mA, exceeding practical limits and causing significant heat. Multiplexing is the standard and efficient method.

Q: The pin connection table has duplicates (Column 3, Row 4). Is this an error?
A> Most likely, this is a typographical error in this version of the datasheet. In a standard 5x7 matrix, there should be 7 unique anode row pins and 5 unique cathode column pins, totaling 12 unique signal pins plus possibly common power pins. The physical pinout diagram is the authoritative source. Always verify with the package drawing.

11. Design and Usage Case Example

Case: Microcontroller-Based Single-Digit Display. A designer uses an Arduino microcontroller to display numbers 0-9. The 7 anode rows are connected to the microcontroller via 7 current-limiting resistors (one per row). The 5 cathode columns are connected to 5 NPN transistors (or a transistor array IC like ULN2003) whose bases are driven by microcontroller pins. The software runs a loop that: 1) Sets one anode row pin HIGH (e.g., Row 1), 2) Sets the corresponding 5 cathode column pins LOW/HIGH according to the pixels needed for that row of the desired character, 3) Waits a short time (e.g., 2ms), 4) Turns off Row 1, and 5) Moves to Row 2, repeating the process. This scans through all 7 rows rapidly, creating a persistent image. The current for each lit LED is determined by the supply voltage (e.g., 5V), the LED Vf (~2.3V), and the series resistor value: R = (5V - 2.3V) / 0.020A = 135 Ohms (use 150 Ohms standard).

12. Operating Principle

The LTP-2557KS operates on the principle of a multiplexed LED matrix. The 35 individual LED dots are arranged in a grid of 7 horizontal rows (anodes) and 5 vertical columns (cathodes). An LED at the intersection of a row and column will light only when that specific row is set to a positive voltage (anode high) and that specific column is connected to ground (cathode low). By sequentially activating one row at a time and setting the appropriate columns for that row, and doing this fast enough (typically >60 Hz), the human eye perceives a stable, fully-formed character due to persistence of vision. This method drastically reduces the number of required control lines from 35 to 12.

13. Technology Trends and Context

While discrete 5x7 through-hole LED displays like the LTP-2557KS represent a mature technology, they are still relevant in specific niches requiring high reliability, wide viewing angles, and simplicity. The trend in general display technology has moved towards integrated modules with built-in controllers (e.g., HD44780-based LCDs), higher-density graphical displays (OLED, TFT LCD), and surface-mount devices for miniaturization. However, the fundamental advantage of LEDs—their brightness, longevity, and ruggedness—ensures continued use in industrial, outdoor, and high-visibility applications where other technologies may fail. The shift to AlInGaP from older materials like GaAsP reflects the ongoing improvement in LED efficiency and performance even within this established product category.