LTF-2502KR LED Display Datasheet - 0.26-inch Digit Height - AlInGaP Super Red - 2.6V Forward Voltage - English Technical Document

1. Product Overview

The LTF-2502KR is a five-digit, seven-segment alphanumeric display module. Its primary function is to provide a clear, bright numerical readout for electronic equipment. The core technology utilizes AlInGaP (Aluminum Indium Gallium Phosphide) LED chips grown on a GaAs substrate, which is known for producing high-efficiency red light. The device features a black face with white segment markings, creating a high-contrast appearance suitable for various lighting conditions. It is designed as a multiplex common anode display, meaning the anodes of each digit are connected together internally, requiring a time-division multiplexing drive scheme to illuminate each digit sequentially.

1.1 Key Features and Advantages

• Compact Digit Size: Features a 0.26-inch (6.8 mm) digit height, offering a balance between readability and space efficiency.
• Optical Quality: Provides continuous uniform segments, excellent character appearance, high brightness, high contrast, and a wide viewing angle.
• Energy Efficiency: Designed with a low power requirement, contributing to overall system energy savings.
• Reliability: Benefits from solid-state reliability inherent to LED technology.
• Consistency: Devices are categorized (binned) for luminous intensity, allowing for matched brightness in multi-display applications.
• Environmental Compliance: The package is lead-free and complies with RoHS directives.

1.2 Device Identification

The part number LTF-2502KR specifically denotes a multiplex common anode display using AlInGaP Super Red LED chips, configured with a right-hand decimal point.

2. Technical Specifications Deep Dive

This section provides a detailed, objective analysis of the device's operational limits and performance characteristics under standard test conditions (Ta=25°C).

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.

• Power Dissipation per Segment: 70 mW maximum.
• Peak Forward Current per Segment: 90 mA maximum, permissible only under pulsed conditions (1/10 duty cycle, 0.1ms pulse width).
• Continuous Forward Current per Segment: 25 mA maximum. This rating derates linearly above 25°C at a rate of 0.33 mA/°C.
• Operating Temperature Range: -35°C to +85°C.
• Storage Temperature Range: -35°C to +105°C.
• Solder Reflow Condition: The device can withstand a soldering temperature of 260°C for 3 seconds at a point 1/16 inch (approximately 1.6 mm) below the seating plane.

2.2 Electrical & Optical Characteristics

These are the typical performance parameters under normal operating conditions.

• Average Luminous Intensity (I_V): Ranges from 320 µcd (min) to 900 µcd (typ) at I_F=1mA. At I_F=10mA, the typical intensity is 11700 µcd. Measurement follows the CIE eye-response curve with a 15% tolerance.
• Peak Emission Wavelength (λ_p): 639 nm (typical) at I_F=20mA.
• Spectral Line Half-Width (Δλ): 20 nm (typical) at I_F=20mA, indicating the spectral purity of the red light.
• Dominant Wavelength (λ_d): 631 nm (typical) at I_F=20mA, with a tolerance of ±1 nm.
• Forward Voltage per Chip (V_F): 2.0V (min) to 2.6V (max) at I_F=20mA, with a tolerance of ±0.1V.
• Reverse Current per Segment (I_R): 100 µA maximum at V_R=5V. Note: This is a test condition; continuous reverse bias operation is prohibited.
• Luminous Intensity Matching Ratio: 2:1 maximum for segments within a similar light area at I_F=1mA.
• Cross-talk Specification: ≤ 2.5%, indicating the level of unwanted illumination in non-selected segments.

3. Binning System Explanation

The LTF-2502KR employs a luminous intensity binning system to ensure consistency. Devices are sorted into bins (F, G, H, J, K) based on their measured light output at a specific test current. This allows designers to select displays from the same bin to achieve uniform brightness across multiple units in an assembly, preventing noticeable hue or brightness variations. The bin ranges are defined by minimum and maximum luminous intensity values in microcandelas (µcd).

4. Performance Curve Analysis

The datasheet includes typical characteristic curves (graphical data) which are essential for detailed design analysis. These curves visually represent the relationship between key parameters, helping engineers optimize performance.

• Forward Current vs. Forward Voltage (I-V Curve): This curve shows the non-linear relationship between the current flowing through the LED and the voltage drop across it. It is crucial for designing the correct current-limiting circuitry.
• Luminous Intensity vs. Forward Current: This graph illustrates how the light output increases with drive current, typically showing a region of linear relationship before potential saturation or efficiency drop at very high currents.
• Luminous Intensity vs. Ambient Temperature: This curve demonstrates the thermal derating of light output. As ambient temperature rises, luminous intensity generally decreases, which must be accounted for in thermal management and drive current selection.
• Spectral Distribution: A plot showing the relative intensity of light emitted across different wavelengths, centered around the dominant and peak wavelengths.

5. Mechanical & Package Information

5.1 Package Dimensions

The display conforms to a specific mechanical outline. All primary dimensions are provided in millimeters with a standard tolerance of ±0.25 mm unless otherwise specified. Key dimensional notes include a pin tip shift tolerance of ±0.4 mm and limits on visual defects such as foreign material (≤10 mil), ink contamination (≤20 mil), bubbles in segments (≤10 mil), and reflector bending (≤1% of length).

5.2 Pin Connection and Circuit Diagram

The device has a 16-pin configuration, though not all pins are active. The internal circuit diagram reveals a multiplexed common anode structure. The pinout is as follows:

• Pins 1, 2, 3, 6, 8, 12, 13, 15: Connect to the cathodes of specific segments (A-G and DP).
• Pins 4, 10, 11, 14, 16: Are the common anode pins for digits 1 through 5, respectively.
• Pins 5, 7, 9: Are marked as "No Connection" (N/C).

This arrangement requires an external driver circuit to sequentially enable each common anode (digit) while driving the appropriate segment cathode lines to form the desired number.

6. Soldering, Assembly & Storage Guidelines

6.1 Soldering and Assembly

• Adhere strictly to the recommended solder reflow profile (260°C for 3 seconds).
• Avoid applying abnormal mechanical force to the display body during assembly.
• If using a pressure-sensitive pattern film on the display surface, avoid letting it make close contact with the front panel/cover, as external force may shift the film.

6.2 Storage Conditions

Proper storage is critical to prevent oxidation of the pins and maintain performance.

• Recommended Standard Conditions: Temperature between 5°C and 30°C with relative humidity below 60% RH, while the product remains in its original moisture-barrier packaging.
• Long-term Storage: Avoid large, long-term inventories. Consume stock promptly.
• Exposure Mitigation: If the moisture-barrier bag is opened or absent for more than 6 months, it is recommended to bake the devices at 60°C for 48 hours and complete assembly within one week to remove absorbed moisture and prevent "popcorning" during reflow.

7. Application Recommendations

7.1 Intended Use and Design Considerations

The display is designed for ordinary electronic equipment in office, communication, and household applications. For safety-critical applications (aviation, medical, etc.), consultation with the manufacturer is required prior to use. Key design considerations include:

• Drive Circuit Design: Constant current driving is recommended for consistent brightness. The circuit must be designed to accommodate the full V_F range (2.0V-2.6V). It must also incorporate protection against reverse voltages and transient spikes during power cycling.
• Current and Thermal Management: Do not exceed the absolute maximum ratings for current and power. The operating current should be chosen based on the maximum ambient temperature, considering the derating specified. Excess current or temperature leads to rapid light degradation or failure.
• Multi-Display Applications: When assembling two or more displays in one set, select units from the same luminous intensity bin (see Section 3) to avoid uneven brightness (hue unevenness).
• Environmental Considerations: Avoid rapid temperature changes in humid environments to prevent condensation on the display.

7.2 Typical Application Scenarios

Due to its multiplexed design, medium brightness, and clear red digits, the LTF-2502KR is well-suited for:

• Consumer appliance displays (e.g., microwave ovens, coffee makers).
• Test and measurement equipment readouts.
• Industrial control panel indicators.
• Point-of-sale terminal displays.
• Any application requiring a compact, reliable, multi-digit numeric display.

8. Technical Comparison & Differentiation

Compared to older technologies like standard GaAsP (Gallium Arsenide Phosphide) red LEDs, the AlInGaP technology used in the LTF-2502KR offers significant advantages:

• Higher Efficiency and Brightness: AlInGaP provides superior luminous efficacy, resulting in brighter output for the same drive current or lower power consumption for the same brightness.
• Better Color Purity: The spectral characteristics (dominant wavelength of ~631nm) produce a more saturated, "true" red compared to the often orange-tinted red of GaAsP.
• Improved Thermal Stability: AlInGaP LEDs generally exhibit less performance degradation with temperature increases compared to older technologies.

9. Frequently Asked Questions (Based on Technical Parameters)

Q1: Why is a multiplexed drive scheme used?

A1: Multiplexing significantly reduces the number of required driver pins. A non-multiplexed 5-digit, 7-segment display would need 5x8=40 pins (including decimal). This multiplexed version requires only 5 (anodes) + 8 (cathodes) = 13 active pins, simplifying PCB design and reducing cost.

Q2: What does "common anode" mean for my driver circuit?

A2: In a common anode configuration, you supply a positive voltage (through a current-limiting element or switch) to the anode of the digit you wish to illuminate. You then sink current to ground by pulling the cathodes of the desired segments low. The driver IC must be configured to source current for the anodes.

Q3: How do I select the appropriate current-limiting resistor?

A3: Use the formula R = (V_supply - V_F) / I_F. Use the maximum V_F (2.6V) from the datasheet to ensure sufficient current at the lower end of the tolerance range. Choose I_F based on your desired brightness, ensuring it does not exceed the continuous current rating (25 mA, derated for temperature).

Q4: Why is binning important?

A4: Manufacturing variations cause slight differences in light output between individual LEDs. Binning sorts them into groups with similar performance. Using displays from the same bin guarantees visual consistency in your product, which is critical for user perception of quality.

10. Design and Usage Case Study

Scenario: Designing a digital timer for a kitchen appliance requiring a 5-digit display (MM:SS or HH:MM format).

Design Steps:

1. Component Selection: The LTF-2502KR is chosen for its appropriate digit size, red color for good visibility, and multiplexed interface to save microcontroller pins.
2. Driver Circuit: A dedicated LED driver IC with multiplexing support is selected. The design uses constant current drivers set to 10 mA per segment to achieve good brightness (typ. 11700 µcd) while staying well within the 25 mA limit.
3. Thermal Consideration: The appliance's internal ambient temperature is estimated to reach 50°C. Using the derating factor (0.33 mA/°C above 25°C), the maximum allowable continuous current per segment is calculated: 25 mA - [0.33 mA/°C * (50°C-25°C)] = 25 mA - 8.25 mA = 16.75 mA. The chosen 10 mA is safe.
4. PCB Layout: The display is placed on the PCB with careful attention to the pinout. Decoupling capacitors are placed near the driver IC. Traces for the common anode lines are sized to handle the peak current of all segments in one digit (up to 8 segments * 10 mA = 80 mA).
5. Software: The microcontroller firmware implements a timer interrupt routine to refresh the display. It cycles through each digit (common anode), turning on the corresponding segments for that digit's value with a duty cycle that prevents flicker.
6. Procurement Note: The Bill of Materials (BOM) specifies "LTF-2502KR, Bin H" to ensure all displays for production have matched brightness.

11. Operating Principle

The fundamental principle is based on electroluminescence in a semiconductor p-n junction. When a forward voltage exceeding the diode's turn-on threshold is applied, electrons from the n-type AlInGaP layer recombine with holes from the p-type layer. This recombination event releases energy in the form of photons (light). The specific composition of the AlInGaP alloy determines the bandgap energy, which directly defines the wavelength (color) of the emitted light—in this case, red at approximately 631 nm. The seven-segment structure is formed by arranging multiple individual LED chips (or chip segments) in the classic "8" pattern, with each segment electrically isolated and independently addressable.

12. Technology Trends

While discrete seven-segment displays like the LTF-2502KR remain vital for specific applications, broader display technology trends are relevant:

• Integration: There is a trend towards integrating the LED driver, microcontroller, and sometimes even the display into more compact modules or smart displays.
• Materials Evolution: While AlInGaP is efficient for red/orange/yellow, InGaN (Indium Gallium Nitride) technology dominates the blue/green/white spectrum and continues to improve in efficiency and cost.
• Alternative Technologies: For more complex graphics or alphanumerics, dot-matrix LED displays, OLEDs, or LCDs are often preferred. However, seven-segment LEDs retain advantages in sunlight readability, ruggedness, simplicity, and cost for pure numeric applications.
• Smart Control: Drive schemes are increasingly leveraging advanced microcontrollers with PWM capabilities for dimming and intensity control, enhancing functionality beyond simple on/off.