LTC-2721JD LED Display Datasheet - 0.28-inch Digit Height - AlInGaP Red - 2.6V Forward Voltage - 70mW Power - English Technical Documentation

1. Product Overview

The LTC-2721JD is a compact, high-performance triple-digit seven-segment display designed for clear numeric readouts in electronic equipment. It features a 0.28-inch (7.0 mm) digit height, providing an excellent balance between size and readability. The device utilizes advanced AlInGaP (Aluminum Indium Gallium Phosphide) LED chip technology, specifically a High-Efficiency Red variant fabricated on a non-transparent GaAs substrate. This technology choice is key to its performance, offering superior brightness and efficiency compared to older LED materials. The display has a distinctive gray face with white segments, which enhances contrast and character appearance, making the numerals easy to read under various lighting conditions. Its primary target markets include consumer electronics, industrial control panels, instrumentation, test equipment, and office appliances where reliable, low-power numeric indication is required.

1.1 Key Features and Advantages

• Optimal Size: 0.28-inch digit height offers a clear display without occupying excessive panel space.
• Superior Optical Performance: Continuous uniform segments ensure consistent illumination. The combination of high brightness, high contrast, and a wide viewing angle guarantees legibility from multiple perspectives.
• Energy Efficiency: Low power requirement, driven by the efficient AlInGaP technology.
• Enhanced Reliability: Solid-state construction provides long operational life and resistance to shock and vibration.
• Quality Assurance: Devices are categorized for luminous intensity, ensuring consistent brightness levels across production batches.
• Environmental Compliance: The product is offered in a lead-free package compliant with RoHS (Restriction of Hazardous Substances) directives.

1.2 Device Identification

The part number LTC-2721JD specifically denotes a multiplexed common cathode display using AlInGaP High-Efficiency Red LEDs, featuring a right-hand decimal point. This configuration is standard for driving multiple digits with a reduced number of microcontroller I/O pins.

2. Technical Parameters: In-Depth Objective Interpretation

This section provides a detailed, objective analysis of the critical parameters defining the display's performance and operational limits.

2.1 Absolute Maximum Ratings

These are stress limits that must not be exceeded under any conditions, even momentarily. Operating at or beyond these limits may cause permanent damage.

• Power Dissipation per Segment: 70 mW. This is the maximum power a single segment can safely dissipate as heat.
• Peak Forward Current per Segment: 90 mA. This is allowed only under pulsed conditions (1/10 duty cycle, 0.1ms pulse width) for multiplexing.
• Continuous Forward Current per Segment: 25 mA at 25°C. This current derates linearly at 0.33 mA/°C as ambient temperature (Ta) increases above 25°C. For example, at 85°C, the maximum continuous current would be approximately: 25 mA - ((85°C - 25°C) * 0.33 mA/°C) ≈ 5.2 mA.
• Temperature Range: Operating and storage temperature range is -35°C to +85°C.
• Solder Conditions: Wave or hand soldering must be performed 1/16 inch (≈1.59 mm) below the seating plane. The maximum recommended soldering temperature is 260°C for 5 seconds or 350°C ±30°C for manual soldering within 5 seconds.

2.2 Electrical & Optical Characteristics

These are typical performance parameters measured at Ta=25°C and specified forward current (IF).

• Average Luminous Intensity (I_V): Ranges from 200 to 600 μcd (microcandelas) at IF=1mA. The display is binned for intensity, meaning parts are sorted into groups based on measured output to ensure consistency.
• Forward Voltage per Segment (V_F): Typically 2.6V, with a maximum of 2.6V at IF=20mA. Designers must ensure the driving circuit can provide sufficient voltage.
• Peak Emission Wavelength (λ_p): 656 nm. This is the wavelength at which the optical output power is greatest.
• Dominant Wavelength (λ_d): 640 nm. This is the single wavelength perceived by the human eye, defining the color (red).
• Spectral Line Half-Width (Δλ): 22 nm. This indicates the spectral purity of the emitted red light.
• Reverse Current per Segment (I_R): Maximum 100 μA at V_R=5V. Critical Note: This parameter is for test purposes only. The device is not designed for continuous reverse bias operation, and such a condition must be prevented by the driving circuit.
• Luminous Intensity Matching Ratio: Maximum 2:1 for segments within a similar light area. This ensures uniform brightness across all segments of a digit.
• Cross Talk: Specified as ≤2.5%. This refers to the unintended illumination of a segment when an adjacent segment is driven, which should be minimal.

3. Mechanical & Package Information

3.1 Package Dimensions and Tolerances

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

• All dimensions are in millimeters (mm).
• General tolerance is ±0.20 mm unless otherwise specified.
• Pin tip shift tolerance is ±0.4 mm.
• Quality control limits are defined for foreign material (≤10 mils), ink contamination (≤20 mils), bending (≤1% of reflector length), and bubbles in the segment (≤10 mils).
• The recommended PCB hole diameter for the pins is 1.30 mm.

3.2 Pinout and Internal Circuit

The LTC-2721JD is a multiplexed common cathode display. It has three common cathode pins (one for each digit: pins 2, 5, 8) and individual anode pins for each segment (A-G, DP) and colon segments (L1, L2, L3). Pin 13 is a common cathode for the three colon LEDs. This architecture allows a microcontroller to illuminate a specific digit by grounding its common cathode while applying a forward voltage to the desired segment anodes. By cycling through the digits rapidly (multiplexing), all three digits appear to be continuously lit. The pin connections are as follows: 1(D), 2(CC1), 3(DP), 4(E), 5(CC2), 6(C/L3), 7(G), 8(CC3), 9(NC), 10-11(NP), 12(B/L2), 13(CC L1/L2/L3), 14(NP), 15(A/L1), 16(F).

4. Performance Curves and Characteristics

The datasheet references typical performance curves (though not displayed in the provided text). Based on standard LED behavior and the given parameters, these curves would typically illustrate:

• Forward Current vs. Forward Voltage (I-V Curve): Shows the exponential relationship, with the typical V_F of 2.6V at 20mA.
• Luminous Intensity vs. Forward Current: Demonstrates how light output increases with current, up to the maximum ratings.
• Luminous Intensity vs. Ambient Temperature: Shows the derating of light output as temperature increases, a critical factor for design.
• Spectral Distribution: A graph plotting relative intensity against wavelength, centered around 656 nm (peak) and 640 nm (dominant).

5. Reliability Testing

The device undergoes a comprehensive suite of reliability tests based on military (MIL-STD), Japanese (JIS), and internal standards to ensure robustness and longevity.

• Operation Life (RTOL): 1000 hours at maximum rated current under room temperature.
• Environmental Stress: Includes High Temperature/Humidity Storage (500 hrs at 65°C/90-95% RH), High Temperature Storage (1000 hrs at 105°C), and Low Temperature Storage (1000 hrs at -35°C).
• Thermal Cycling & Shock: Temperature Cycling (30 cycles between -35°C and 105°C) and Thermal Shock (30 cycles between -35°C and 105°C) test resilience to rapid temperature changes.
• Solderability: Solder Resistance (10 sec at 260°C) and Solderability (5 sec at 245°C) tests validate the package's ability to withstand assembly processes.

6. Soldering and Assembly Guidelines

6.1 Automated Soldering

For wave soldering, the recommended condition is to immerse the leads to a depth of 1/16 inch (1.59 mm) below the seating plane for a maximum of 5 seconds at 260°C. The body temperature of the display must not exceed the maximum storage temperature during this process.

6.2 Manual Soldering

When using a soldering iron, the tip should contact the lead (again, 1/16 inch below the seating plane) for no more than 5 seconds at a temperature of 350°C ±30°C. Using a heatsink on the lead between the joint and the package body is good practice.

7. Critical Application Cautions and Design Considerations

Important: Adherence to these cautions is essential for reliable operation and to prevent premature failure.

• Intended Use: Designed for ordinary electronic equipment. Consultation is required for safety-critical applications (aviation, medical, etc.).
• Ratings Compliance: The driving circuit must ensure that absolute maximum ratings (current, voltage, power, temperature) are never exceeded. The manufacturer is not liable for damage resulting from non-compliance.
• Current and Thermal Management: Exceeding the recommended forward current or operating temperature will cause severe, irreversible light output degradation and can lead to catastrophic failure.
• Circuit Protection: The driving circuit must incorporate protection against reverse voltages and voltage transients that can occur during power-up or shutdown. A series resistor or constant current driver is mandatory to limit current.
• Driving Method: Constant current driving is strongly recommended over constant voltage driving. This ensures consistent luminous intensity regardless of minor variations in forward voltage (V_F) between segments or units and provides inherent protection against current spikes. For multiplexed operation, the peak current must be calculated based on duty cycle to ensure the average current per segment remains within limits.

8. Practical Application Scenarios and Design Notes

8.1 Typical Applications

• Digital Multimeters (DMMs) & Test Equipment: Providing clear numeric readouts for voltage, current, and resistance.
• Industrial Timers & Counters: Displaying elapsed time, production counts, or setpoints.
• Consumer Electronics: Clocks, audio equipment displays, kitchen appliance readouts.
• Instrumentation Panels: For displaying sensor data like temperature, pressure, or speed in a compact format.

8.2 Design Implementation Case Study

Scenario: Designing a 3-digit voltmeter display using a microcontroller.

1. Multiplexing Driver: The microcontroller will use 7-8 I/O pins for segment anodes (A-G, DP) and 3 I/O pins (configured as open-drain/low-output) for the digit cathodes (CC1, CC2, CC3).
2. Current Limiting: Place a current-limiting resistor in series with each segment anode line. The resistor value (R) is calculated using: R = (V_supply - V_F) / I_F. For a 5V supply, V_F=2.6V, and a desired I_F of 10 mA: R = (5 - 2.6) / 0.01 = 240 Ω. Use the nearest standard value (e.g., 220 Ω or 270 Ω).
3. Multiplex Timing: Program the microcontroller to activate one digit cathode at a time, illuminate the required segments for that digit, wait a short period (e.g., 2-5 ms), then move to the next digit. A refresh rate of 50-200 Hz prevents visible flicker.
4. Peak Current Check: If using a 10% duty cycle (3 digits), the peak current during the active time can be higher. For an average I_F of 10 mA, the peak current during the 1/3 duty cycle would be 30 mA. This must be checked against the Absolute Maximum Rating for Peak Forward Current (90 mA) and the Continuous Current derating at the operating temperature.

9. Technical Comparison and Differentiation

The LTC-2721JD's primary advantages stem from its AlInGaP technology:

• vs. Traditional GaAsP/GaP Red LEDs: AlInGaP offers significantly higher luminous efficiency, resulting in greater brightness for the same drive current or lower power consumption for the same brightness. It also provides better temperature stability and color purity.
• vs. Larger Displays: The 0.28-inch size offers a sweet spot between the very small (0.2-inch) displays that can be hard to read and larger (0.5-inch or more) displays that consume more power and board area.
• Common Cathode vs. Common Anode: The common cathode configuration is often preferred in systems driven by microcontrollers, as they can typically sink current (drive pins low) more effectively than they can source it (drive pins high).

10. Frequently Asked Questions (Based on Technical Parameters)

10.1 Can I drive this display with a 3.3V microcontroller?

Answer: Possibly, but with caution. The typical forward voltage (V_F) is 2.6V. With a 3.3V supply, there is only 0.7V headroom for the current-limiting resistor. This small voltage drop makes the current very sensitive to variations in V_F and the supply voltage. A constant-current driver circuit is highly recommended for 3.3V operation to ensure stable brightness. Direct connection to 3.3V GPIO pins without a driver risks overcurrent if V_F is at the lower end of its range.

10.2 Why is the maximum continuous current derated with temperature?

Answer: This is due to the negative temperature coefficient of the LED's forward voltage and the physical limits of the package. As temperature rises, the internal efficiency drops, and more electrical power is converted to heat instead of light. If the current is not reduced, the junction temperature can rise uncontrollably (thermal runaway), leading to rapid degradation and failure. The derating curve (0.33 mA/°C) is provided to prevent this.

10.3 What does "categorized for luminous intensity" mean?

Answer: It means the displays are tested and sorted into different brightness bins after production. For example, one batch may have I_V from 200-300 μcd, another from 300-400 μcd, etc. This allows designers purchasing large quantities to ensure uniform brightness across all units in their product. The specific bin code is often marked on the package (referenced as "Z: BIN CODE" in the module marking).

11. Operating Principle and Technology Trends

11.1 Basic Operating Principle

A seven-segment LED display is an array of light-emitting diodes arranged in a figure-eight pattern. Each segment (A through G) is an individual LED. By applying a forward bias voltage (exceeding the diode's V_F) and limiting the current with a resistor or constant-current source, electrons and holes recombine within the AlInGaP semiconductor's active region, releasing energy in the form of photons (light) at a wavelength characteristic of the material—in this case, red (~640 nm). Multiplexing takes advantage of the human eye's persistence of vision by illuminating only one digit at a time but cycling through them so quickly that they appear to be all on simultaneously.

11.2 Objective Technology Context

AlInGaP represents a mature and highly optimized material system for red, orange, and yellow LEDs. It offers excellent efficiency and reliability. The trend in display technology is towards higher integration (e.g., dot matrix displays, OLEDs, micro-LEDs) and direct integration with driver ICs. However, discrete seven-segment displays like the LTC-2721JD remain highly relevant due to their simplicity, low cost, high brightness, robustness, and ease of use in applications where only numeric data needs to be shown. Their design is well-understood, and they interface easily with low-cost microcontrollers, ensuring their continued use in industrial, consumer, and instrumentation fields for the foreseeable future.