LTP-1557AJD LED Display Datasheet - 1.2 inch (30.42mm) Height - AlInGaP Red - 5x7 Dot Matrix - English Technical Document

1. Product Overview

The LTP-1557AJD is a single-character alphanumeric display module designed for applications requiring clear, reliable character output. Its core function is to visually represent ASCII or EBCDIC coded characters through a grid of individually addressable light-emitting diodes (LEDs). The primary target markets include industrial control panels, instrumentation, point-of-sale terminals, communication equipment, and any embedded system requiring a simple, robust user interface for status or data display. Its solid-state construction offers significant advantages in reliability and longevity over older display technologies like vacuum fluorescent or incandescent displays.

The module's core advantage lies in its use of AlInGaP (Aluminum Indium Gallium Phosphide) LED technology. This semiconductor material system is known for producing high-efficiency red and amber light. Compared to older GaAsP (Gallium Arsenide Phosphide) LEDs, AlInGaP LEDs offer superior luminous efficiency, meaning brighter output for the same electrical input power, and better performance at elevated temperatures. The device features a gray faceplate with white dots, which enhances contrast and readability under various lighting conditions.

2. Technical Parameter Deep Dive

2.1 Photometric & Optical Characteristics

The optical performance is defined under standard test conditions at an ambient temperature (Ta) of 25°C. The key parameter is Average Luminous Intensity (Iv), which has a typical value of 2500 microcandelas (µcd) and a minimum of 1020 µcd. This measurement is taken with a pulsed drive current (Ip) of 32mA at a 1/16 duty cycle. The use of pulsed driving is common in multiplexed displays to achieve higher peak brightness while maintaining a safe average power dissipation per LED dot.

The color characteristics are defined by wavelength. The device has a Peak Emission Wavelength (λp) of 656 nm, which places it in the red region of the visible spectrum. The Dominant Wavelength (λd) is specified as 640 nm. It is important to note the difference: peak wavelength is the point of maximum spectral power, while dominant wavelength is the single-wavelength perception of color by the human eye. The Spectral Line Half-Width (Δλ) is 22 nm, indicating the spectral purity or bandwidth of the emitted light; a narrower half-width indicates a more saturated, pure color. A Luminous Intensity Matching Ratio (IV-m) of 2:1 maximum is specified, meaning the brightness variation between the brightest and dimmest dot in the array should not exceed this ratio, ensuring uniform appearance.

2.2 Electrical Characteristics

The electrical parameters define the operating limits and conditions for the device. The Forward Voltage (VF) for any single LED dot is between 2.1V (min) and 2.6V (max) at a test current (IF) of 20mA. This forward voltage is characteristic of AlInGaP technology and is crucial for designing the current-limiting circuitry. The Reverse Current (IR) is specified at a maximum of 100 µA when a reverse bias voltage (VR) of 5V is applied, indicating the diode's leakage characteristics in the off-state.

3. Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage may occur. They are not for continuous operation. Key limits include: Average Power Dissipation per Dot (33 mW), Peak Forward Current per Dot (90 mA), and Average Forward Current per Dot (13 mA at 25°C, derating linearly at 0.17 mA/°C above 25°C). The 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. The soldering temperature must not exceed 260°C for more than 3 seconds at a point 1.6mm below the seating plane of the package.

4. Binning System Explanation

The datasheet indicates the device is categorized for luminous intensity. This means units are tested and sorted (binned) based on their measured light output. This allows designers to select parts from a specific intensity bin to ensure consistent brightness across multiple displays in a product, avoiding noticeable variations. While not explicitly detailed in this document, common binning parameters for such LEDs can also include forward voltage (Vf) and dominant wavelength (λd) to ensure electrical and color consistency.

5. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves. While not displayed in the provided text, such curves typically include: Forward Current vs. Forward Voltage (I-V Curve): This shows the nonlinear relationship between current and voltage, essential for designing driver circuits. Luminous Intensity vs. Forward Current (I-L Curve): This shows how light output increases with current, usually in a roughly linear region before efficiency drops at very high currents. Luminous Intensity vs. Ambient Temperature: This curve demonstrates the thermal derating of light output, which is critical for applications operating in high-temperature environments. AlInGaP LEDs generally maintain performance better at high temperatures than older technologies.

6. Mechanical & Package Information

The device is a 1.2-inch (30.42 mm) matrix height display. The package dimensions are provided in a drawing with all measurements in millimeters. Tolerances are ±0.25 mm unless otherwise specified. The mechanical drawing is essential for PCB footprint design and ensuring proper fit within an enclosure. The package features a specific pinout for the 5x7 matrix, with connections for 7 row anodes and 5 column cathodes (or vice-versa, depending on internal circuit configuration).

6.1 Pin Connection & Internal Circuit

The pin connection table lists 14 pins. The internal circuit diagram shows a common-cathode or common-anode configuration for the 5x7 matrix. The specific pin assignment (e.g., Pin 1: Anode Row 5, Pin 3: Cathode Column 2) is provided. This configuration allows the display to be multiplexed. By sequentially energizing one row (or column) at a time and providing the appropriate column (or row) data, all 35 dots can be controlled with only 12 I/O lines (7+5), significantly reducing the microcontroller pin count required compared to direct driving each LED.

7. Soldering & Assembly Guidelines

The key assembly specification is the soldering profile. The absolute maximum rating states the package can withstand a maximum solder temperature of 260°C for a maximum of 3 seconds, measured 1.6mm (1/16 inch) below the seating plane. This is a typical specification for wave soldering or hand soldering. For reflow soldering, a standard lead-free profile with a peak temperature not exceeding 260°C should be used. It is critical to avoid excessive thermal stress to prevent damage to the LED chips, wire bonds, or the plastic package. Proper ESD (Electrostatic Discharge) handling procedures should always be followed during assembly.

8. Application Suggestions

8.1 Typical Application Scenarios

This display is ideal for applications requiring a single character or digit, or multiple units can be stacked horizontally to form multi-character displays. Common uses include: digital panel meters (voltage, current, temperature), simple status indicators on industrial machinery (showing error codes, mode numbers), basic readouts on consumer appliances, and prototyping or educational kits for learning about multiplexed LED driving.

8.2 Design Considerations

Driver Circuit Design: A multiplexing driver circuit is required. This typically involves a microcontroller with sufficient I/O pins or a dedicated LED driver IC (like a MAX7219 or similar). The circuit must include current-limiting resistors for each column or row line to set the forward current to a safe value, typically between 10-20mA per segment based on the desired brightness and power dissipation limits. Power Supply: The forward voltage of ~2.4V must be considered. A 3.3V or 5V supply is common, with appropriate voltage dropped across the current-limiting resistor. Refresh Rate: The multiplexing scan rate must be high enough (typically >60 Hz) to avoid visible flicker. Viewing Angle: The datasheet mentions a wide viewing angle, which is beneficial for applications where the display may be viewed from off-axis positions.

9. Technical Comparison & Differentiation

The primary differentiator of the LTP-1557AJD is its use of AlInGaP LED technology. Compared to displays using older GaAsP or standard red GaP LEDs, AlInGaP offers: Higher Luminous Efficacy: More light output per unit of electrical power, leading to lower power consumption for the same brightness or higher brightness for the same power. Better High-Temperature Performance: AlInGaP LEDs experience less efficiency droop at elevated junction temperatures, making them more suitable for industrial environments. Superior Color Saturation: The spectral characteristics often result in a deeper, more visually distinct red color.

10. Frequently Asked Questions (Based on Technical Parameters)

Q: Why is the luminous intensity tested with a pulsed current (1/16 duty cycle) instead of DC?
A: This reflects the intended multiplexed operation. Testing under pulsed conditions simulates real-world use and allows specification of a higher, more relevant peak brightness that the user will perceive.

Q: Can I drive this display with a constant DC current on each dot?
A: Technically yes, but it is highly inefficient. It would require 35 individual current-limited drivers. Multiplexing is the standard and intended method, drastically reducing component count and power consumption in the driver circuitry.

Q: What is the purpose of the intensity matching ratio (2:1)?
A: It guarantees visual uniformity. Without binning, some dots might be noticeably brighter or dimmer than others, creating an uneven, unprofessional appearance in the formed characters.

Q: How do I interpret the derating factor for average forward current (0.17 mA/°C)?
A: This means for every degree Celsius the ambient temperature rises above 25°C, the maximum safe continuous current per dot must be reduced by 0.17 mA. For example, at 50°C (25°C above), the max current would be 13 mA - (25 * 0.17 mA) = 8.75 mA per dot.

11. Practical Design Case Study

Consider designing a single-digit temperature display for an incubator using the LTP-1557AJD. A microcontroller (e.g., an ATmega328P) reads a temperature sensor. Seven of its I/O pins are configured as outputs to drive the row anodes (through small NPN transistors or a ULN2003 darlington array for higher current capability). Five other I/O pins drive the column cathodes directly or through transistors. The firmware scans through the seven rows rapidly. For each row, it outputs a 5-bit pattern on the column pins corresponding to the segments of the digit (0-9) that need to be lit on that specific row to form the desired number. Current-limiting resistors are placed on the column lines. The scan routine runs in a timer interrupt to ensure a consistent, flicker-free refresh rate of about 100 Hz. The AlInGaP technology ensures the display remains clearly readable even if the incubator's internal ambient temperature rises.

12. Operating Principle Introduction

The LTP-1557AJD operates on the principle of a multiplexed 5x7 dot matrix. Internally, the 35 LEDs are arranged in a grid with their anodes connected by rows and cathodes connected by columns (or vice-versa in a common-anode configuration). To illuminate a specific dot, a voltage is applied to its corresponding row line (making it high for a common-cathode type), while its corresponding column line is pulled low (sinking current). To display a pattern or character, the controller rapidly cycles (scans) through each row. When a particular row is activated, the controller sets the appropriate column lines to create the pattern for that row. The human eye's persistence of vision blends these rapidly changing row images into a stable, complete character. This method reduces the number of required control lines from 35 (one per LED) to just 12 (rows + columns).

13. Technology Trends

While discrete 5x7 dot matrix displays like the LTP-1557AJD remain relevant for specific, cost-sensitive, or simple applications, broader display technology trends have moved towards integrated solutions. Integrated Controller Displays: Modern character LCDs (Liquid Crystal Displays) and OLED (Organic LED) modules often include a built-in controller chip that handles character generation and refresh, communicating via simple serial (I2C, SPI) or parallel interfaces, greatly simplifying software development. Higher Resolution & Graphics: For more complex information, small graphic OLED or TFT-LCD modules are now common, offering pixel-addressable graphics. Surface-Mount Technology (SMT): Newer LED indicators and displays predominantly use SMT packages (e.g., 0805, 0603 LEDs arranged in a matrix) for automated assembly, whereas through-hole packages like this one are more typical for prototyping or manual assembly. The underlying AlInGaP and InGaN (for blue/green/white) LED chip technology continues to advance, offering ever-higher efficiency and reliability.