LTF-2502KG Seven Segment LED Display Datasheet - 0.26-inch Digit Height - AlInGaP Green - 2.6V Forward Voltage - 70mW Power - English Technical Documentation

1. Product Overview

The LTF-2502KG is a five-digit, seven-segment LED display module designed for numeric readout applications. It features a digit height of 0.26 inches (6.8 mm), providing clear and legible characters. The device utilizes AlInGaP (Aluminum Indium Gallium Phosphide) LED chips grown on a GaAs substrate, which are known for their high efficiency and brightness in the green spectrum. The display presents a high-contrast appearance with white luminous segments against a black face, enhancing readability under various lighting conditions. Its primary target markets include consumer electronics, industrial control panels, instrumentation, and any application requiring a compact, reliable numeric display with excellent visual performance.

1.1 Key Features

• Compact 0.26-inch (6.8 mm) digit height.
• Continuous and uniform segment illumination for consistent character appearance.
• Low power consumption, suitable for battery-powered devices.
• Excellent character appearance with high brightness and contrast.
• Wide viewing angle for visibility from different positions.
• High reliability due to solid-state construction.
• Luminous intensity is categorized (binned) for consistent performance.
• Lead-free package compliant with RoHS (Restriction of Hazardous Substances) directives.

1.2 Device Identification

The part number LTF-2502KG specifically denotes a multiplex common anode display with AlInGaP green LEDs and a right-hand decimal point configuration. This configuration is optimized for multiplexed drive circuits, which reduce the number of required microcontroller I/O pins.

2. Technical Parameters: In-Depth Objective Interpretation

This section provides a detailed analysis of the electrical and optical characteristics that define the display's performance envelope and guide proper circuit design.

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. This is the maximum power that can be safely dissipated as heat by a single LED segment.
• Peak Forward Current per Segment: 90 mA. This is allowed only under pulsed conditions (1/10 duty cycle, 0.1ms pulse width) to achieve very high instantaneous brightness without overheating.
• Continuous Forward Current per Segment: 25 mA at 25°C. This current derates linearly at a rate of 0.33 mA/°C as ambient temperature (Ta) increases above 25°C. For example, at 50°C, the maximum continuous current would be approximately 25 mA - (0.33 mA/°C * 25°C) = 16.75 mA.
• Operating Temperature Range: -35°C to +85°C. The device is guaranteed to function within this ambient temperature range.
• Storage Temperature Range: -35°C to +105°C.
• Solder Condition: The device can withstand wave soldering with the solder point 1/16 inch (≈1.6mm) below the seating plane for 3 seconds at 260°C.

2.2 Electrical & Optical Characteristics

These are the typical performance parameters measured at Ta=25°C under specified test conditions.

• Average Luminous Intensity (I_V): This is the primary measure of brightness.
  • MIN: 200 µcd, TYP: 540 µcd at I_F = 1 mA.
  • TYP: 5940 µcd at I_F = 10 mA. This shows the highly non-linear relationship between current and light output.
• Peak Emission Wavelength (λ_p): 571 nm (Typical). This is the wavelength at which the spectral power output is maximum, placing it in the green region of the visible spectrum.
• Spectral Line Half-Width (Δλ): 15 nm (Typical). This indicates the spectral purity; a smaller value means a more monochromatic light.
• Dominant Wavelength (λ_d): 572 nm (Typical). This is the wavelength perceived by the human eye, closely matching the peak wavelength.
• Forward Voltage per Chip (V_F): 2.6V (Typical), with a tolerance of ±0.1V, at I_F = 20 mA. This is a critical parameter for designing the current-limiting circuitry.
• Reverse Current per Segment (I_R): 100 µA (Maximum) at V_R = 5V. This parameter is for test purposes only; continuous reverse bias operation is prohibited.
• Luminous Intensity Matching Ratio: 2:1 (Maximum). This ensures uniformity across segments, meaning the brightest segment will be no more than twice as bright as the dimmest segment under the same drive conditions.
• Cross Talk: ≤2.5%. This specifies the maximum amount of unintended light leakage from an unpowered segment when an adjacent segment is lit.

3. Binning System Explanation

The display employs a binning system for luminous intensity to guarantee a consistent brightness level within a single unit and across multiple units in an assembly. The bin codes (F, G, H, J, K) represent specific ranges of minimum luminous intensity in microcandelas (µcd) measured at I_F = 1 mA.

• Bin F: 200 - 320 µcd
• Bin G: 321 - 500 µcd
• Bin H: 501 - 800 µcd
• Bin J: 801 - 1300 µcd
• Bin K: 1301 - 2100 µcd

Design Implication: For applications using two or more displays in one assembly, it is strongly recommended to use displays from the same bin code to avoid noticeable differences in brightness (hue unevenness) between them.

4. Performance Curve Analysis

While the specific graphs are not detailed in the provided text, typical curves for such a device would include:

• I-V (Current-Voltage) Curve: Shows the exponential relationship between forward voltage and current. The curve will have a knee voltage around 2.0-2.2V, after which current increases rapidly with a small increase in voltage, highlighting the need for current regulation, not voltage regulation.
• Luminous Intensity vs. Forward Current: A curve showing that light output increases super-linearly with current at lower levels and may approach saturation at higher currents. This informs the trade-off between brightness and efficiency/power dissipation.
• Luminous Intensity vs. Ambient Temperature: Typically shows a negative temperature coefficient, where light output decreases as junction temperature increases. This is crucial for designing in high-temperature environments.
• Forward Voltage vs. Ambient Temperature: Usually shows a negative temperature coefficient, meaning V_F decreases slightly as temperature rises.
• Spectral Distribution: A bell-shaped curve centered around 571-572 nm, with a width defined by the 15 nm half-width.

5. Mechanical & Package Information

5.1 Package Dimensions

The display has a standard dual in-line package (DIP) footprint. Key dimensional notes include:

• All dimensions are in millimeters (mm).
• General tolerance is ±0.25 mm unless otherwise specified.
• Pin tip shift tolerance is ±0.4 mm.
• Defect limits on the display face: foreign material ≤10 mils, ink contamination ≤20 mils, bubbles in segment ≤10 mils.
• Bending of the reflector is limited to ≤1% of its length.
• The recommended PCB hole diameter for the pins is 1.0 mm.

5.2 Pin Connection and Polarity

The LTF-2502KG is a multiplex common anode device. This means the anodes of the LEDs for each digit are connected together internally, while the cathodes for each segment type (A-G, DP) are connected across digits.

Pinout (16-pin DIP):

• Pin 1: Cathode E
• Pin 2: Cathode D
• Pin 3: Cathode DP (Decimal Point)
• Pin 4: Common Anode for Digit 3
• Pin 6: Cathode G
• Pin 8: Cathode C
• Pin 10: Common Anode for Digit 5
• Pin 11: Common Anode for Digit 4
• Pin 12: Cathode B
• Pin 13: Cathode F
• Pin 14: Common Anode for Digit 2
• Pin 15: Cathode A
• Pin 16: Common Anode for Digit 1
• Pins 5, 7, 9: No Connection (N/C)

Internal Circuit: The internal diagram would show five common anode nodes (one per digit), each connected to the anodes of 7 segments (A-G) and the decimal point (DP) for that specific digit. The cathode of each segment type (e.g., all 'A' segments) is connected together across all five digits.

6. Soldering & Assembly Guidelines

6.1 Wave Soldering Profile

A recommended wave soldering temperature profile is provided. Key parameters include:

• A preheat stage to a temperature between 100-110°C for a minimum of 2 minutes to reduce thermal shock.
• A peak solder temperature in the range of 250-260°C.
• The time spent within 5°C of this peak temperature should be 3 to 5 seconds to ensure proper solder joint formation without damaging the component.

7. Application Suggestions & Design Considerations

7.1 Design and Usage Cautions

These points are critical for reliable long-term operation:

• Drive Circuit Design: Constant current driving is strongly recommended over constant voltage to ensure consistent brightness and protect the LEDs. The circuit must be designed to accommodate the full range of forward voltage (V_F = 2.5V to 2.7V).
• Protection: The driving circuit must incorporate protection against reverse voltages and transient voltage spikes during power-up/shutdown, as these can cause metal migration and failure.
• Thermal Management: The operating current must be derated according to the maximum ambient temperature. Exceeding current or temperature ratings leads to severe light output degradation and premature failure.
• Environmental: Avoid rapid temperature changes in humid environments to prevent condensation on the display.
• Mechanical: Do not apply abnormal force to the display body during assembly. If a decorative film is applied, avoid letting it press directly against a front panel, as it may shift.
• Binning for Multi-Display Use: As stated, use displays from the same luminous intensity bin for uniform appearance.

7.2 Storage Conditions

To prevent pin oxidation and maintain solderability:

• Recommended: Store in original moisture-barrier packaging.
• Temperature: 5°C to 30°C.
• Humidity: Below 60% RH.
• Inventory Management: Avoid long-term storage of large quantities. Use first-in, first-out (FIFO) principles. Products stored outside these conditions may require re-processing before use.

8. Typical Application Scenarios

The LTF-2502KG is suited for a wide range of applications requiring clear, reliable numeric indication:

• Test and Measurement Equipment: Digital multimeters, frequency counters, power supplies.
• Industrial Controls: Process timers, counter displays, temperature readouts on machinery.
• Consumer Electronics: Audio equipment (amplifier volume/displays), kitchen appliance timers.
• Automotive Aftermarket: Gauges and displays for performance monitoring (where environmental specs are met).
• Medical Devices: Simple parameter displays on non-critical equipment (consult manufacturer for safety-critical uses).

9. Technical Comparison & Differentiation

Compared to other seven-segment display technologies:

• vs. Red GaAsP/GaP LEDs: AlInGaP green LEDs generally offer higher luminous efficiency and brightness, resulting in better visibility and potentially lower power consumption for the same perceived brightness.
• vs. LCDs: LEDs are emissive, providing their own light, which makes them far superior in low-light conditions and offer wider viewing angles without backlight complexity. They are also generally more robust and have a faster response time.
• vs. Larger Digit Displays: The 0.26-inch size offers a balance between readability and board space savings, making it ideal for compact devices where a larger display would be impractical.
• Key Advantage of this Part: The combination of AlInGaP technology (for efficiency), multiplex common anode configuration (for driver simplicity), and categorized luminous intensity (for consistency) makes it a well-rounded choice for cost-sensitive, volume production designs.

10. Frequently Asked Questions (Based on Technical Parameters)

1. Q: Why is constant current drive recommended instead of using a simple resistor with a voltage source?
   A: While a series resistor is common, it provides imperfect regulation because the LED's forward voltage (V_F) varies with temperature and between individual units. A constant current source ensures the current (and thus brightness) remains stable regardless of these V_F variations, leading to more uniform and reliable performance.
2. Q: Can I drive this display with a microcontroller directly?
   A: For multiplexing, yes, but not directly for segment current. Microcontroller pins have limited current sourcing/sinking capability (typically 20-25mA). You must use external drivers (transistors or dedicated LED driver ICs) to handle the segment current (up to 25mA continuous per segment) and the higher cumulative digit anode current.
3. Q: What does the 2:1 luminous intensity matching ratio mean for my design?
   A: It means that in the worst case, one segment might be twice as bright as another segment on the same display when driven identically. Good circuit board layout (equal trace lengths/resistance) and proper current regulation help minimize visible differences. For critical applications, software brightness calibration per segment is an option.
4. Q: The storage humidity is below 60% RH. What happens if it's stored in a more humid environment?
   A: High humidity can lead to oxidation of the tin/lead-free plating on the pins, resulting in poor solderability when the part is eventually used. This may cause defective solder joints during assembly.

11. Practical Design and Usage Case

Scenario: Designing a simple 5-digit timer.

1. Microcontroller Selection: Choose an MCU with enough I/O pins. For a 5-digit, 7-segment + DP multiplexed display, you need 5 pins for the digit anodes and 8 pins for the segment cathodes, totaling 13 control lines.
2. Driver Circuit: Use a low-side driver array (e.g., a ULN2003A Darlington transistor array) to sink the current for the 8 cathode lines. Use individual NPN transistors or a high-side driver to source current for the 5 anode lines.
3. Current Setting: Determine the required brightness. For indoor use, 5-10mA per segment might be sufficient. Calculate the current-limiting resistors for the anode drivers or configure your constant-current driver IC accordingly, remembering to derate for maximum ambient temperature.
4. Multiplexing Software: Write firmware that cycles through each digit, turning on its anode and setting the appropriate cathode pattern for that digit's value. The refresh rate should be high enough (e.g., >100Hz) to avoid visible flicker.
5. PCB Layout: Ensure power traces to the anode and cathode drivers are sufficiently wide. Keep the display close to the drivers to minimize trace inductance.

12. Operating Principle Introduction

The LTF-2502KG is based on semiconductor electroluminescence. When a forward bias voltage exceeding the diode's junction potential is applied across the AlInGaP p-n junction, electrons and holes are injected into the active region. Their recombination releases energy in the form of photons (light). The specific composition of the Aluminum, Indium, Gallium, and Phosphide layers in the epitaxial structure determines the bandgap energy, which directly defines the wavelength (color) of the emitted light—in this case, green at ~572 nm. The seven-segment format is created by placing individual LED chips (or chip arrays) in the shape of standard numeric segments, which are then electrically interconnected in a common-anode, multiplexed matrix to minimize external connections.

13. Technology Trends

Trends in seven-segment LED display technology focus on several key areas:

• Increased Efficiency: Ongoing material science improvements in AlInGaP and the rise of InGaN (for blue/green/white) aim to achieve higher lumens per watt, enabling brighter displays or lower power consumption.
• Miniaturization: There is a constant drive for smaller pixel pitches and higher density, allowing for more digits or information in the same footprint, though the 0.2"-0.5" range remains popular for human-factor readability.
• Integration: More displays are incorporating the driver IC and sometimes even a simple controller (like for clock functions) into the module package, simplifying the end-user's circuit design.
• Enhanced Reliability & Robustness: Improvements in packaging materials and epoxy resins enhance resistance to moisture, thermal cycling, and mechanical stress, expanding the range of operating environments.
• Color Options & RGB: While single-color displays like this green one are prevalent, there is growing use of multi-color or full RGB displays in segments, allowing for status indication (e.g., green for normal, red for alarm) within the same digit.

Despite the proliferation of dot-matrix and OLED graphical displays, the seven-segment LED remains a highly cost-effective, reliable, and easily readable solution for dedicated numeric output, ensuring its continued relevance in electronic design.