LTD-4708JG LED Display Datasheet - 0.4-inch Digit Height - AlInGaP Green - 2.6V Forward Voltage - English Technical Document

1. Product Overview

The LTD-4708JG is a dual-digit, seven-segment alphanumeric display module designed for applications requiring clear, bright numeric readouts. Its primary function is to visually represent two digits (0-9) using individually addressable LED segments. The core technology utilizes AlInGaP (Aluminum Indium Gallium Phosphide) semiconductor material grown on a non-transparent GaAs substrate, which is known for producing high-efficiency green light emission. The device features a gray faceplate with white segment markings, enhancing contrast and readability under various lighting conditions.

The display is categorized as a common cathode type, meaning the cathodes of the LEDs for each digit are connected together internally. This configuration simplifies multiplexing in driving circuits, allowing control of multiple digits with a reduced number of microcontroller I/O pins. Its key advantages include excellent character appearance due to continuous uniform segments, high brightness and contrast, a wide viewing angle for visibility from different positions, and solid-state reliability inherent to LED technology. The package is compliant with RoHS directives, being lead-free.

2. Technical Parameter Deep-Dive

2.1 Photometric & Optical Characteristics

The optical performance is central to the display's functionality. At a standard test current of 1mA per segment, the average luminous intensity ranges from a minimum of 320 µcd to a typical value of 850 µcd. This parameter defines the perceived brightness. The dominant wavelength (λd) is specified at 572 nm, placing the emission firmly in the green region of the visible spectrum. The peak emission wavelength (λp) is 571 nm, with a spectral line half-width (Δλ) of 15 nm, indicating a relatively pure and saturated green color. Luminous intensity matching between segments within a similar light area is guaranteed to be within a 2:1 ratio, ensuring uniform brightness across the displayed character. Cross-talk, the unwanted illumination of non-selected segments, is specified to be ≤ 2.5%.

2.2 Electrical Characteristics

The electrical parameters define the operating boundaries and conditions for the device. The forward voltage (VF) per segment is typically 2.6V with a maximum of 2.6V at a forward current (IF) of 1mA. This value is crucial for designing the current-limiting circuitry. The absolute maximum ratings set hard limits: the continuous forward current per segment is 25 mA, derating linearly by 0.28 mA/°C above 25°C ambient temperature. A peak forward current of 60 mA is allowed under pulsed conditions (1/10 duty cycle, 0.1ms pulse width). The maximum reverse voltage per segment is 5V, intended only for testing reverse current (IR, max 100 µA at VR=5V) and not for continuous operation. The maximum power dissipation per segment is 70 mW.

3. Mechanical & Package Information

The display has a digit height of 0.4 inches (10.0 mm). The package dimensions are provided in a detailed drawing. Key mechanical notes include: all dimensions are in millimeters with a general tolerance of ±0.25 mm; the pin tip shift tolerance is ±0.4 mm; the recommended PCB hole diameter is 1.0 mm. Cosmetic specifications are also defined, limiting foreign material on segments to ≤10 mils, surface ink contamination to ≤20 mils, bending to ≤1/100, and bubbles within segments to ≤10 mils.

4. Pin Configuration & Internal Circuit

The device has a 10-pin configuration. The internal circuit diagram shows two common cathode nodes, one for each digit (Digit 1 and Digit 2). The anodes for segments A through G and the decimal point (D.P.) are brought out to individual pins. The specific pin assignment is: 1 (Anode C), 2 (Anode D.P.), 3 (Anode E), 4 (Common Cathode Digit 2), 5 (Anode D), 6 (Anode F), 7 (Anode G), 8 (Anode B), 9 (Common Cathode Digit 1), 10 (Anode A). This arrangement is essential for designing the external driver circuit.

5. Absolute Maximum Ratings & Operating Conditions

Strict adherence to these ratings is necessary to prevent permanent damage. The device can operate within an ambient temperature range of -35°C to +105°C and can be stored within the same range. For soldering during assembly, a condition of 260°C for 3 seconds at 1/16 inch (approximately 1.6 mm) below the seating plane is specified. Exceeding the maximum temperature rating during assembly must be avoided.

6. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves. While the specific graphs are not detailed in the provided text, such curves typically illustrate the relationship between forward current (IF) and luminous intensity (IV), showing how brightness increases with current up to the maximum rating. They may also show the forward voltage (VF) versus current and the variation of luminous intensity with ambient temperature. These curves are vital for designers to optimize the drive current for desired brightness while maintaining efficiency and longevity, and to understand performance derating at elevated temperatures.

7. Application Suggestions & Design Considerations

7.1 Typical Application Scenarios

This display is suited for applications requiring compact, bright, and reliable numeric indicators. Common uses include test and measurement equipment (multimeters, frequency counters), industrial control panels, consumer appliances (microwaves, ovens, washing machines), automotive dashboard readouts (for aftermarket accessories), and point-of-sale terminals. Its high brightness and wide viewing angle make it suitable for environments with high ambient light.

7.2 Design Guidelines

When integrating this display, several factors must be considered. Current Limiting: External current-limiting resistors are mandatory for each anode or cathode line to set the forward current per segment, typically between 1-20 mA depending on the required brightness and power budget. The resistor value can be calculated using R = (Vcc - VF) / IF, where VF is the typical forward voltage. Multiplexing: For dual-digit common cathode displays, a multiplexing drive scheme is most efficient. This involves sequentially enabling one digit's common cathode at a time (via a transistor switch) while applying the correct anode patterns for that digit's desired segments. The refresh rate must be high enough (usually >60 Hz) to avoid visible flicker. PCB Layout: Follow the recommended hole size of 1.0 mm for reliable soldering. Ensure adequate trace width for the segment current. Viewing Angle: Position the display considering its specified viewing angle to ensure optimal visibility for the end-user.

8. Technical Comparison & Differentiation

Compared to older technologies like standard GaP (Gallium Phosphide) green LEDs, AlInGaP offers significantly higher luminous efficiency, resulting in greater brightness for the same drive current. The use of a non-transparent GaAs substrate improves contrast by reducing internal light scattering. The gray face with white segments is a design choice that enhances contrast compared to all-black or all-gray faces. Being a dedicated seven-segment package, it offers a more integrated and mechanically robust solution compared to using discrete LEDs to form digits, saving assembly time and ensuring consistent segment alignment.

9. Frequently Asked Questions (FAQs)

Q: What is the purpose of the common cathode configuration?
A: It simplifies circuit design for multiplexing multiple digits. Instead of needing a separate ground connection for each of the 14+ segments per digit, you only need one per digit, drastically reducing the required number of driver lines.

Q: How do I calculate the current-limiting resistor value?
A: Use Ohm's Law: R = (Supply Voltage - LED Forward Voltage) / Desired Forward Current. For a 5V supply, a VF of 2.6V, and a desired IF of 10mA: R = (5 - 2.6) / 0.01 = 240 Ohms. Always use the nearest standard value and verify power rating.

Q: Can I drive this display with a constant voltage source without a current limit?
A: No. LEDs are current-driven devices. Their forward voltage has a tolerance and decreases with temperature. Connecting directly to a voltage source exceeding VF will cause excessive current to flow, potentially destroying the segment. A series resistor or constant current driver is essential.

Q: What does \"categorized for luminous intensity\" mean?
A: It indicates that devices are binned or sorted based on their measured luminous output. This allows designers to select parts with consistent brightness levels for their application, which is critical for multi-digit displays where uniformity is important.

10. Practical Design Case Study

Consider designing a simple two-digit counter using a microcontroller. The microcontroller would have 8 I/O pins connected to the segment anodes (A-G and DP) through current-limiting resistors. Two additional I/O pins would control NPN transistors (or similar switches) connected to the two common cathode pins (Digit 1 and Digit 2). The firmware would implement a multiplexing routine: turn on the transistor for Digit 1, output the segment pattern for the first digit's value on the anode ports, wait a short interval (e.g., 5ms), then turn off Digit 1's transistor. Next, turn on the transistor for Digit 2, output the segment pattern for the second digit, wait, and turn it off. This cycle repeats continuously. The timing must ensure the peak current per segment is not exceeded and the average current meets the desired brightness.

11. Operating Principle

The device operates on the principle of electroluminescence in a semiconductor p-n junction. When a forward bias voltage exceeding the diode's turn-on voltage (approximately 2.05-2.6V for this AlInGaP material) is applied, electrons from the n-type region and holes from the p-type region are injected into the active region where they recombine. In AlInGaP LEDs, this recombination releases energy primarily in the form of photons with a wavelength corresponding to green light (around 572 nm). The specific alloy composition of Aluminum, Indium, Gallium, and Phosphide determines the bandgap energy and thus the color of the emitted light. The seven-segment structure is formed by patterning multiple such LED chips on a substrate and connecting them with bonding wires to the external pins.

12. Technology Trends

While seven-segment LED displays remain a robust and cost-effective solution for numeric readouts, the broader display technology landscape is evolving. There is a general trend towards higher integration, such as displays with built-in driver ICs (e.g., TM1637 compatible modules) that communicate via simple serial protocols (I2C, SPI), reducing the microcontroller resource burden. In terms of materials, while AlInGaP is highly efficient for red, orange, amber, and green, InGaN (Indium Gallium Nitride) technology dominates for high-brightness blue, green, and white LEDs. For applications requiring alphanumeric or graphical capability, dot-matrix LED displays or OLEDs are increasingly common. However, for simple, bright, low-power numeric indicators in harsh environments, discrete seven-segment LED displays like the LTD-4708JG continue to offer an unbeatable combination of reliability, simplicity, and performance.