LTP-2157AKR LED Display Datasheet - 2.0-inch (50.8mm) Matrix Height - AlInGaP Super Red - 2.6V Forward Voltage - 70mW Power Dissipation - English Technical Document

1. Product Overview

The LTP-2157AKR is a single-plane, 5x7 dot matrix alphanumeric LED display module. Its primary function is to display characters, symbols, or simple graphics in applications requiring a compact, low-power, and highly reliable visual output. The core component of this display is the utilization of AlInGaP (Aluminum Indium Gallium Phosphide) semiconductor material for the LED chips, which are engineered to emit light in the Super Red wavelength spectrum. The device features a gray faceplate with white dot coloration, providing a high-contrast visual appearance for the illuminated elements.

The display is categorized based on its luminous intensity, allowing for selection consistency in brightness across multiple units. It is designed with a standard ASCII and EBCDIC character code compatibility, making it suitable for integration into a wide range of digital systems for status indication, simple messaging, or data readout. A key mechanical feature is its stackable horizontal design, enabling the creation of multi-character displays by aligning multiple units side-by-side.

2. Technical Parameters Deep Objective Interpretation

2.1 Photometric and Optical Characteristics

The primary optical performance is defined under specific test conditions at an ambient temperature (Ta) of 25°C. The Average Luminous Intensity (Iv) is specified with a minimum of 1650 µcd, a typical value of 3500 µcd, and no maximum limit stated in the provided data. This measurement is taken under a pulsed drive condition of Ip=32mA with a 1/16 duty cycle. This pulsed operation is standard for multiplexed displays to achieve perceived brightness while managing power and heat.

The Peak Emission Wavelength (λp) is typically 639 nm, placing the output firmly in the red region of the visible spectrum. The Dominant Wavelength (λd) is specified as 631 nm. The difference between peak and dominant wavelength, along with the Spectral Line Half-Width (Δλ) of 20 nm, describes the color purity and the spread of the emitted light's wavelengths. A narrower half-width indicates a more monochromatic (pure color) output. The luminous intensity matching ratio between dots is specified as a maximum of 2:1, ensuring reasonable uniformity in brightness across the display matrix.

2.2 Electrical Characteristics

The electrical parameters define the operating limits and conditions for the device. The Forward Voltage (VF) per LED dot is between 2.0V and 2.8V depending on the drive current. At a standard test current of IF=20mA, VF is 2.0V (min), 2.6V (typ). At a higher pulsed current of IF=80mA, it increases to 2.3V (min), 2.8V (typ). The Reverse Current (IR) is a maximum of 100 µA when a reverse bias of VR=5V is applied, indicating the leakage characteristic of the LED junction.

2.3 Absolute Maximum Ratings and Thermal Considerations

These ratings define the stress limits beyond which permanent damage may occur. The Average Power Dissipation per Dot must not exceed 70 mW. The Peak Forward Current per Dot is limited to 90 mA, while the Average Forward Current per Dot has a base rating of 15 mA at 25°C. This average current rating derates linearly by 0.2 mA/°C as the ambient temperature rises above 25°C. This derating is crucial for thermal management, ensuring the junction temperature of the LED does not exceed safe limits during operation. The maximum Reverse Voltage per Dot is 5V. The device is rated for an Operating Temperature Range of -35°C to +85°C and the same range for storage.

3. Binning System Explanation

The datasheet indicates that the device is categorized for luminous intensity. This implies a binning or sorting process where manufactured units are tested and grouped based on their measured light output under standard conditions. This ensures that designers can select displays with consistent brightness levels, which is critical for applications where multiple displays are used together to avoid noticeable variations in intensity. The provided specification lists a minimum and typical intensity, defining the lower bound and expected performance for a given bin.

4. Performance Curve Analysis

The datasheet references Typical Electrical/Optical Characteristic Curves. While the specific graphs are not detailed in the text, such curves typically included in full datasheets would illustrate relationships such as forward voltage vs. forward current (V-I curve), luminous intensity vs. forward current, luminous intensity vs. ambient temperature, and spectral distribution. These curves are essential for designers to understand the non-linear behavior of LEDs. For instance, the V-I curve shows the exponential relationship, critical for designing current-limiting circuitry. The temperature curve would show how light output decreases as the junction temperature increases, informing heat sink requirements.

5. Mechanical and Package Information

The device comes in a specific package with defined dimensions (all in millimeters). The drawing included in the datasheet provides the critical physical outlines, mounting hole positions, and overall size. The Pin Connection table is vital for interfacing. The display uses a 14-pin configuration with a mix of anode rows and cathode columns for matrix addressing. Important notes specify internal connections: Pin 4 (Anode Column 3) and Pin 11 (Cathode Column 3) are connected internally, as are Pin 5 (Cathode Row 4) and Pin 12 (Anode Row 4). This internal wiring is part of the matrix layout and must be accounted for in the driving circuit design. The polarity is clearly defined by the anode/cathode designation for each pin.

6. Soldering and Assembly Guidelines

The absolute maximum ratings include a critical soldering parameter: the device can withstand a solder temperature of maximum 260°C for a maximum of 3 seconds, measured at 1.6mm (1/16 inch) below the seating plane. This defines the reflow soldering profile constraints. Exceeding this time-temperature combination can damage the internal wire bonds, the LED chip, or the plastic package. Proper handling to avoid electrostatic discharge (ESD) is also implied for semiconductor devices, though not explicitly stated here. Storage should be within the specified temperature range of -35°C to +85°C in a dry environment.

7. Packaging and Ordering Information

The part number is clearly identified as LTP-2157AKR. The naming convention likely follows an internal coding system where "LTP" may denote the product family (LED dot matrix), "2157" may relate to the size (2.0-inch, 5x7) and perhaps color, and "AKR" could indicate specific binning, packaging, or revision details. The datasheet itself is referenced by Spec No.: DS30-2001-251. Standard packaging for such displays is often in anti-static tubes or trays to protect the pins and prevent ESD damage during shipping and handling.

8. Application Suggestions

8.1 Typical Application Scenarios

This display is suited for applications requiring a simple, rugged, and low-power character readout. Typical uses include: industrial control panel status indicators, test and measurement equipment displays, medical device interfaces, consumer appliances (e.g., older microwave ovens, stereo systems), and embedded system project interfaces. Its stackability allows for creating multi-digit displays for counters or timers.

8.2 Design Considerations

1. Drive Circuitry: A microcontroller with sufficient I/O pins or a dedicated display driver IC (like a MAX7219) is required to multiplex the 5x7 matrix. The circuit must provide current limiting, typically via resistors in series with each column or row line.
2. Current Limits: Design must adhere to the absolute maximum ratings for average and peak current. Using the 1/16 duty cycle multiplexing helps keep average power within limits while allowing higher pulsed currents for brightness.
3. Thermal Management: Ensure adequate ventilation if operating at high ambient temperatures, considering the current derating factor of 0.2 mA/°C.
4. Software: Character font data for the 5x7 grid must be stored in the controlling system's memory and output according to the multiplexing timing and the specific pin mapping of the LTP-2157AKR.

9. Technical Comparison

Compared to older technologies like standard GaAsP (Gallium Arsenide Phosphide) red LEDs, the AlInGaP technology used in this display offers significantly higher luminous efficiency, resulting in brighter output for the same drive current. It also typically provides better temperature stability and longer operational lifetime. Compared to modern surface-mount 7-segment or matrix displays, this through-hole package is larger and requires more manual assembly but can be more robust in high-vibration environments and easier to prototype with. Its 2.0-inch character height is relatively large, offering excellent visibility from a distance compared to smaller SMD displays.

10. Frequently Asked Questions (Based on Technical Parameters)

Q: Can I drive this display with a constant DC current on all dots?
A: No. The display is designed for multiplexed (scanned) operation. Applying constant DC to all dots would exceed the average power dissipation rating per dot and likely cause overheating and failure.

Q: What value current-limiting resistor should I use?
A: The resistor value depends on your drive voltage and desired current. For example, to achieve a pulsed current of 20mA per dot with a 5V supply and a typical Vf of 2.6V, you would calculate R = (5V - 2.6V) / 0.02A = 120 ohms. Use the maximum Vf for a safer design.

Q: The pins for row 4 and column 3 are internally connected. How does this affect my design?
A: This internal connection is part of the matrix wiring. You must follow the pin connection table precisely. Your driving software/hardware must activate the correct pair of anode and cathode pins to light a specific dot, respecting these internal links. It does not mean you can ignore one of the connected pins; the matrix addressing logic depends on the full set.

11. Practical Use Case

Case: Building a 4-Digit Scoreboard Timer. Four LTP-2157AKR displays are aligned horizontally. A microcontroller (e.g., an Arduino or PIC) with 20+ I/O pins is used. The controller's firmware manages the multiplexing: it cycles through activating one cathode column (or a set, depending on internal wiring) at a time while sending the anode row data for all four displays corresponding to the digits to be shown. Current-limiting resistors are placed on the common cathode lines. The software includes a lookup table for numerals 0-9 and perhaps a colon for time separation. The timer counts down or up, updating the multiplexing data accordingly. The large 2-inch characters make the scoreboard easily readable from several meters away.

12. Principle Introduction

The device operates on the principle of a matrix-addressable LED array. Individual LEDs are arranged at the intersections of 7 anode rows and 5 cathode columns (or vice-versa, as per the pinout). To illuminate a specific dot, its corresponding anode line is driven high (provided with a positive voltage through a current limit), and its corresponding cathode line is driven low (sinked to ground). By rapidly scanning through the columns (or rows) and updating the row (or column) data synchronously, the persistence of vision creates the illusion of a stable image. The AlInGaP LED chips themselves work on the principle of electroluminescence in a direct bandgap semiconductor, where electron-hole recombination releases energy in the form of photons (light) at a wavelength determined by the material's bandgap energy.

13. Development Trends

While through-hole dot matrix displays like the LTP-2157AKR are mature technology, the underlying LED technology continues to evolve. Trends in display technology relevant to its function include: 1) A shift towards surface-mount device (SMD) packages for automated assembly and smaller footprints. 2) Adoption of even more efficient materials like InGaN for different colors and higher brightness. 3) Integration of the driver IC and sometimes even a microcontroller directly into the display module, creating "intelligent" displays that communicate via serial interfaces (I2C, SPI) rather than requiring direct matrix scanning from the host. 4) The rise of organic LED (OLED) and flexible displays for more complex graphics. However, for simple, high-brightness, rugged, and cost-effective character display needs in industrial or legacy systems, discrete LED matrix modules remain a viable and reliable solution.