LTP-2857JD LED Display Datasheet - 2.0 Inch (50.8mm) Height - AlInGaP Red - 5x7 Dot Matrix - English Technical Document

1. Product Overview

The LTP-2857JD is a single-digit, alphanumeric display module built around a 5x7 dot matrix configuration. Its primary function is to generate visible characters and symbols, making it suitable for applications requiring clear, legible information presentation in a compact form factor. The core technology employs AlInGaP (Aluminum Indium Gallium Phosphide) semiconductor material for the light-emitting diodes, which is known for producing high-efficiency red light output.

The device features a gray faceplate with white dots, providing a high-contrast background for the illuminated red LEDs, which enhances readability. A key design aspect is its stackability, allowing multiple units to be placed side-by-side horizontally to form multi-character displays without significant gaps, facilitating the creation of words or longer numerical strings.

2. Technical Specifications Deep Dive

2.1 Optical Characteristics

The optical performance is central to the display's functionality. The device uses AlInGaP LED chips grown on a non-transparent GaAs substrate. The typical average luminous intensity (Iv) per dot ranges from 1300 to 3000 microcandelas (µcd) when driven under specific test conditions: a peak current (Ip) of 32mA with a 1/16 duty cycle. This measurement uses a filter that approximates the CIE photopic eye-response curve, ensuring the value correlates with human visual perception.

The color characteristics are defined by specific wavelengths. The peak emission wavelength (λp) is typically 656 nanometers (nm), while the dominant wavelength (λd) is 640 nm, defining the perceived red color. The spectral line half-width (Δλ) is 22 nm, indicating the spectral purity or the narrowness of the emitted light band.

2.2 Electrical Characteristics

The electrical parameters define the operating boundaries and conditions for the display. The forward voltage (Vf) for any single LED dot typically falls between 2.1 and 2.6 volts when a forward current (If) of 20mA is applied. The reverse current (Ir) is specified at a maximum of 100 microamperes (µA) when a reverse voltage (Vr) of 5V is applied, indicating the leakage in the off-state.

Current handling is critical. The absolute maximum ratings specify an average power dissipation per dot of 33 milliwatts (mW). The peak forward current per dot must not exceed 90mA. The average forward current per dot is rated at 13mA at 25°C, with a derating factor of 0.17 mA/°C, meaning the permissible continuous current decreases as ambient temperature rises above 25°C to prevent overheating and ensure longevity.

2.3 Thermal and Environmental Ratings

The device is designed for robust operation across a range of conditions. The operating temperature range is from -35°C to +85°C, allowing deployment in both cold and moderately hot environments. The storage temperature range is identical. For assembly, the solder temperature must not exceed 260°C for a maximum duration of 3 seconds, measured at a point 1.6mm (1/16 inch) below the seating plane of the component, which is a standard guideline for wave or reflow soldering processes to prevent damage to the LED chips or package.

3. Binning System Explanation

The datasheet indicates that devices are categorized for luminous intensity. This implies a binning process where units are sorted based on measured light output (e.g., the 1300-3000 µcd range). Binning ensures consistency within a batch, so designers can expect predictable brightness levels when using multiple displays in an array. While not explicitly detailed for wavelength or voltage in this document, such categorization is common in LED manufacturing to group parts with closely matched optical and electrical properties.

4. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves, which are essential for detailed design. Although the specific graphs are not provided in the text, such curves typically include:

• Forward Current vs. Forward Voltage (I-V Curve): Shows the nonlinear relationship between current and voltage, crucial for designing the current-limiting driver circuit.
• Luminous Intensity vs. Forward Current (L-I Curve): Illustrates how light output increases with current, helping to optimize drive current for desired brightness and efficiency.
• Luminous Intensity vs. Ambient Temperature: Demonstrates how light output decreases as the junction temperature of the LED increases, which is vital for thermal management in the application.
• Spectral Distribution: A graph showing the relative intensity of light emitted across different wavelengths, confirming the dominant and peak wavelengths.

These curves allow engineers to predict performance under non-standard conditions and design robust systems.

5. Mechanical and Package Information

The display has a matrix height of 2.0 inches (50.80 mm). The package dimensions drawing (referenced but not detailed in the text) would show the exact length, width, thickness, and lead spacing. All dimensional tolerances are ±0.25 mm (0.01 inches) unless otherwise specified. The pin connection details are provided in a table, mapping 14 pins to specific anode columns and cathode rows of the 5x7 matrix. This pinout is essential for designing the PCB footprint and the multiplexing driver circuit.

6. Internal Circuit Diagram and Driving Method

The internal circuit diagram shows the arrangement of the 35 individual LEDs (5 columns x 7 rows). Each LED's anode is connected to a column line, and its cathode is connected to a row line. This common matrix architecture requires multiplexed driving. The display is not constantly illuminated; instead, the controller rapidly cycles through the rows (or columns), energizing the appropriate column anodes for each active row cathode. The 1/16 duty cycle mentioned in the test condition is a typical multiplexing ratio. Proper design of the scan rate is necessary to avoid visible flicker and ensure uniform brightness.

7. Soldering and Assembly Guidelines

As per the absolute maximum ratings, the soldering process must be carefully controlled. The maximum allowable solder temperature is 260°C, and the exposure time at the lead should not surpass 3 seconds. This is to prevent thermal shock to the LED chips, which can cause cracks in the semiconductor material or degrade the wire bonds, leading to premature failure. Using a pre-heat stage during reflow soldering is recommended to minimize thermal stress. Proper ESD (Electrostatic Discharge) handling procedures should always be followed during assembly, as LEDs are sensitive to static electricity.

8. Application Suggestions

8.1 Typical Application Scenarios

This display is ideal for applications requiring a single, highly visible character or symbol. Common uses include:

• Industrial control panels for status indicators (e.g., showing a process step letter).
• Test and measurement equipment for displaying units or channel identifiers.
• Consumer appliances where a simple status code or identifier is needed.
• As a building block for multi-character displays by horizontally stacking multiple units.

8.2 Design Considerations

Designing with this display requires attention to several factors:

• Driver Circuit: A microcontroller or dedicated LED driver IC capable of multiplexing is required. The circuit must provide sufficient current (up to the peak rating) during the active scan time and include current-limiting resistors or a constant-current source to protect the LEDs.
• Power Supply: The supply voltage must be high enough to overcome the forward voltage of the LEDs plus any drops in the driver circuitry. A voltage of 5V is commonly used with appropriate current-limiting.
• Thermal Management: While the display itself may not generate excessive heat, the derating curve must be respected. In high ambient temperatures, the average current must be reduced. Ensuring good airflow around the display in enclosed spaces is advisable.
• Software/Firmware: The controller needs to include a character font map (compatible with ASCII or EBCDIC as noted) and the multiplexing scan routine. The refresh rate should be high enough (typically >60 Hz) to prevent perceptible flicker.

9. Technical Comparison and Differentiation

The key differentiators of this specific display, based on the datasheet, are its use of AlInGaP technology and its 2.0-inch height. Compared to older GaAsP or GaP LEDs, AlInGaP offers significantly higher luminous efficiency, resulting in brighter output for the same input current. The 2.0-inch character height makes it suitable for applications where viewing distance is several meters, offering better long-range readability than smaller 0.5-inch or 1-inch displays. The gray face/white dot design enhances contrast compared to all-black or all-green packages. Its stackability is a practical mechanical feature for multi-digit designs.

10. Frequently Asked Questions (Based on Technical Parameters)

Q: What does "1/16 Duty" in the luminous intensity test condition mean?
A: It means each individual LED dot is only powered on for 1/16th of the total scan cycle time during measurement. The specified intensity is the average value over the full cycle. In actual use, you must design your multiplexing driver to achieve a similar or higher effective duty cycle to reach the rated brightness.

Q: Can I drive this display with a constant DC current without multiplexing?
A: Technically, you could, by connecting each of the 35 LEDs with its own current-limiting resistor to a power supply. However, this would require 35 driver channels, which is highly inefficient in terms of component count and power. Multiplexing is the standard and intended method, drastically reducing the required number of control pins and simplifying the design.

Q: The pin connection table seems to have duplicates (e.g., Anode Column 3 on pins 4 and 11). Is this an error?
A> This is likely not an error but a feature of the internal matrix wiring. It may indicate that certain column or row lines are brought out to more than one pin on the package. This can provide layout flexibility on the PCB, allowing the designer to choose the most convenient pin for connection. Always refer to the internal circuit diagram to verify the connections.

Q: How do I calculate the appropriate current-limiting resistor for my driver?
A> You need to know your supply voltage (Vs), the LED forward voltage (Vf, use max of 2.6V for safety), and the desired forward current (If, not exceeding the average rating of 13mA at your operating temperature). The resistor value R = (Vs - Vf) / If. Remember, in a multiplexed setup, the peak current during the active scan time will be higher than the average current. Ensure the peak current does not exceed 90mA.

11. Design and Usage Case Example

Scenario: Building a 4-digit production counter for a factory workstation.
Four LTP-2857JD displays are stacked horizontally on a PCB. A low-cost 8-bit microcontroller is used as the controller. The microcontroller has enough I/O pins to directly drive the rows (7 pins) and columns (5 pins per digit, but since they are stacked, the column lines of all digits are connected together, requiring only 5 column pins total). The microcontroller runs a routine that:

1. Scans through the seven row lines, activating one at a time.
2. For the active row, it sets the state of the 5 column lines for each of the 4 digits based on the character to be displayed (e.g., a number).
3. It repeats this scan at a rate of 200 Hz, making flicker imperceptible.
4. The count value is incremented by an external sensor input.

Current-limiting resistors are placed in series with each column line. The power supply is 5V. The average current per LED dot is kept below 10mA to provide a safety margin below the 13mA rating and ensure long-term reliability.

12. Operating Principle Introduction

The fundamental principle is electroluminescence in a semiconductor p-n junction. When a forward voltage exceeding the diode's threshold is applied, electrons from the n-type region and holes from the p-type region recombine in the active region (the AlInGaP layer). This recombination releases energy in the form of photons (light particles). The specific composition of the AlInGaP alloy determines the bandgap energy, which in turn dictates the wavelength (color) of the emitted light—in this case, red. The 5x7 matrix is formed by placing 35 of these microscopic p-n junctions in a precise grid pattern. The gray faceplate acts as a diffuser and contrast enhancer, while the white dots define the segments that become visible when illuminated.

13. Technology Trends and Context

Displays like the LTP-2857JD represent a mature, reliable technology for character-based information display. While modern graphic OLEDs or TFT LCDs offer far greater flexibility for displaying arbitrary graphics, 5x7 and similar dot-matrix LED displays retain advantages in specific niches: extreme environmental robustness (wide temperature range), very high brightness for sunlight readability, simplicity of interface, and long operational life with no backlight to fail. The shift from older LED materials to AlInGaP, as seen in this device, was a major trend that improved efficiency and brightness. Current trends might involve integrating the driver electronics more closely with the display module or exploring even more efficient materials like InGaN for different colors, but the basic multiplexed matrix architecture remains a proven and effective solution for many industrial and instrumentation applications.