LTC-2728JD 0.28-inch Quadruple Digit Seven-Segment LED Display Datasheet - Digit Height 7.0mm - Forward Voltage 2.6V - Power Dissipation 70mW - Red Color - English Technical Documentation

1. Product Overview

The LTC-2728JD is a quadruple-digit, seven-segment alphanumeric display module designed for applications requiring clear, low-power numeric readouts. Its primary function is to visually represent numbers and some limited characters through the selective illumination of its LED segments. The core technology utilizes AlInGaP (Aluminum Indium Gallium Phosphide) high-efficiency red LED chips, which are fabricated on a non-transparent GaAs substrate. This construction contributes to the device's characteristic high brightness and contrast. The display features a gray faceplate with white segment markings, enhancing readability when the segments are off and improving contrast when illuminated.

The device is categorized as a common cathode, multiplexed display. This means all the cathodes (negative terminals) for the LEDs in a single digit are connected together internally, forming a common node for that digit. To display a number across four digits, an external controller rapidly cycles power (multiplexes) to each digit's common cathode in sequence, while simultaneously driving the appropriate segment anodes for the desired character on that specific digit. This multiplexing approach significantly reduces the number of required driver pins compared to a static drive method.

A key design objective for this component is low power consumption. The segments are specifically tested and matched for excellent performance at low drive currents, with operation possible at currents as low as 1mA per segment. This makes it suitable for battery-powered or energy-conscious devices.

2. Technical Specifications Deep Dive

This section provides a detailed, objective analysis of the device's key electrical and optical parameters as defined in the datasheet.

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 allowable power that can be dissipated as heat by a single LED segment under continuous operation.
• Peak Forward Current per Segment: 100 mA. This current is permissible only under pulsed conditions with a 1/10 duty cycle and a 0.1ms pulse width. It is significantly higher than the continuous current rating to allow for brief, high-intensity pulses in multiplexed applications.
• Continuous Forward Current per Segment: 25 mA at 25°C. This rating derates linearly at 0.33 mA/°C as the 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.
• Reverse Voltage per Segment: 5 V. Applying a reverse bias voltage greater than this value can damage the LED junction.
• Operating & Storage Temperature Range: -35°C to +85°C.
• Solder Temperature: Maximum 260°C for a maximum of 3 seconds, measured 1.6mm (1/16 inch) below the seating plane of the component.

2.2 Electrical & Optical Characteristics

These are the typical and maximum/minimum guaranteed performance parameters under specified test conditions (Ta=25°C unless noted).

• Average Luminous Intensity (I_V): 200 μcd (Min), 600 μcd (Typ) at I_F = 1mA. This quantifies the perceived brightness of a segment. The wide range indicates a binning process, where devices are sorted based on measured output.
• Peak Emission Wavelength (λ_p): 656 nm (Typ) at I_F = 20mA. This is the wavelength at which the optical output power is greatest.
• Spectral Line Half-Width (Δλ): 22 nm (Typ) at I_F = 20mA. This measures the spread of the emitted light's wavelengths; a smaller value indicates a more monochromatic (pure color) light.
• Dominant Wavelength (λ_d): 640 nm (Typ) at I_F = 20mA. This is the single wavelength that best represents the perceived color of the light to the human eye.
• Forward Voltage per Segment (V_F): 2.1 V (Min), 2.6 V (Typ) at I_F = 20mA. This is the voltage drop across an LED segment when conducting the specified current. It is crucial for designing the current-limiting circuitry.
• Reverse Current per Segment (I_R): 10 μA (Max) at V_R = 5V. This is the small leakage current that flows when the LED is reverse-biased within its maximum rating.
• Luminous Intensity Matching Ratio (I_V-m): 2:1 (Max) at I_F = 10mA. This parameter ensures uniformity; the brightness of the dimmest segment compared to the brightest segment within a single device will not exceed a 2:1 ratio.

Note on Luminous Intensity Measurement: The datasheet specifies that intensity is measured using a sensor and filter combination that approximates the CIE photopic luminosity function, which models the spectral sensitivity of the standard human eye under normal lighting conditions.

3. Binning System Explanation

The datasheet indicates the device is "Categorized for Luminous Intensity." This refers to a binning or sorting process post-manufacturing. Due to inherent variations in semiconductor fabrication, individual LEDs will have slightly different forward voltages and, more noticeably for the user, different luminous intensities at the same drive current.

To ensure consistency for the end-user, manufacturers test each unit (or segments within a unit) and sort them into different "bins" based on their measured output. The specified range of 200-600 μcd at 1mA suggests that devices are grouped according to their actual measured brightness into specific intensity bins. When designing a product, engineers can specify a particular bin code to guarantee a minimum brightness level or a tighter brightness range across all displays used, which is critical for achieving a uniform appearance in multi-display products.

4. Performance Curve Analysis

The datasheet references "Typical Electrical / Optical Characteristic Curves." While the specific graphs are not detailed in the provided text, standard curves for such a device would typically include:

• Relative Luminous Intensity vs. Forward Current (I_V vs. I_F): This curve shows how brightness increases with drive current. It is generally linear at lower currents but may saturate at higher currents due to thermal effects.
• Forward Voltage vs. Forward Current (V_F vs. I_F): This exponential curve is fundamental for driver design, showing the voltage required to achieve a desired current.
• Relative Luminous Intensity vs. Ambient Temperature (I_V vs. T_a): LED output typically decreases as junction temperature rises. This curve helps designers account for brightness loss in high-temperature environments.
• Spectral Distribution: A graph showing the relative power emitted across the wavelength spectrum, centered around the peak wavelength of 656 nm with a typical half-width of 22 nm.

5. Mechanical & Package Information

5.1 Package Dimensions

The device is presented with a detailed dimensional drawing. Key notes from the drawing include that all dimensions are in millimeters (mm) and standard tolerances are ±0.25 mm (0.01 inches) unless a specific feature calls for a different tolerance. The drawing would define the overall length, width, and height of the display module, the spacing between digits, the size and position of the mounting pins, and the segment window cutouts.

5.2 Pin Connection & Internal Circuit

The device has a 16-pin configuration. The pinout is as follows: Pin 1 (Common Cathode Digit 1), Pin 2 (Anode C), Pin 3 (Anode DP), Pin 4 (No Pin), Pin 5 (Anode E), Pin 6 (Anode D), Pin 7 (Anode G), Pin 8 (Common Cathode Digit 4), Pins 9,10,12 (No Pin), Pin 11 (Common Cathode Digit 3), Pin 13 (Cathode A), Pin 14 (Common Cathode Digit 2), Pin 15 (Anode B), Pin 16 (Anode F).

The "Internal Circuit Diagram" shows the multiplexed common cathode architecture. It depicts four common cathode nodes (one for each digit), each connected to the cathodes of all seven segments (A-G) plus the decimal point (DP) for that specific digit. The anode for each segment type (e.g., all 'A' segments from digits 1-4) is connected together internally and brought out to a single anode pin. This structure enables the multiplexing drive scheme.

6. Soldering & Assembly Guidelines

The primary guidance provided is the absolute maximum rating for solder temperature: 260°C for a maximum of 3 seconds, measured at a point 1.6mm below the seating plane of the component. This is a standard rating for wave or reflow soldering processes using lead-free (SnAgCu) solder. Exceeding this time or temperature can damage the internal wire bonds, the LED chips, or the plastic package. It is recommended to follow standard JEDEC/IPC guidelines for the reflow profile, ensuring a gradual preheat, a controlled time above liquidus, and a controlled cooling rate to minimize thermal shock.

For storage, the specified temperature range of -35°C to +85°C should be adhered to, and components should be kept in moisture-barrier bags with desiccant if they are moisture-sensitive (the datasheet does not specify an MSL rating).

7. Application Suggestions

7.1 Typical Application Scenarios

This display is ideal for applications requiring a clear, multi-digit numeric readout with low power consumption. Common uses include:

• Test and measurement equipment (multimeters, power supplies).
• Industrial control panels and counters.
• Consumer appliances (microwaves, ovens, scales).
• Automotive aftermarket displays (voltmeters, timers).
• Battery-powered portable instruments.

7.2 Design Considerations

• Driver Circuit: A dedicated LED display driver IC or microcontroller with sufficient current sink/source capability is required. The driver must implement the multiplexing sequence, cycling through the four common cathode pins while outputting the correct 7-segment code for each digit.
• Current Limiting: External current-limiting resistors are mandatory for each segment anode (or use a constant-current driver). The resistor value is calculated using R = (V_supply - V_F - V_{driver_sat}) / I_F. Use the maximum V_F from the datasheet (2.6V) for a worst-case design to ensure current does not exceed limits.
• Refresh Rate: The multiplexing frequency must be high enough to avoid perceptible flicker (typically >60 Hz per digit, so overall cycle >240 Hz). However, it must also be low enough to allow each segment to reach full brightness during its ON time.
• Viewing Angle: The datasheet claims a wide viewing angle, which is typical for LED seven-segment displays. This should be verified for the specific mechanical placement in the end product.

8. Technical Comparison & Differentiation

The key differentiating advantages of this specific display, as highlighted in its features, include:

• Low Current Operation: Its characterization and matching for low current (down to 1mA/segment) is a significant advantage for power-sensitive designs over displays that require higher currents for adequate brightness.
• AlInGaP Technology: Compared to older GaAsP or GaP LED technologies, AlInGaP offers higher efficiency, resulting in higher brightness and better color purity (more saturated red) at the same drive current.
• High Contrast & Uniform Segments: The gray face with white segments and the "continuous uniform segments" feature contribute to excellent readability in various lighting conditions.

9. Frequently Asked Questions (Based on Technical Parameters)

Q: Can I drive this display with a 5V microcontroller directly?

A: No, not directly. The forward voltage of a segment is typically 2.6V. Connecting 5V directly to the anode without a current-limiting resistor would destroy the LED due to excessive current. You must use series resistors or a constant-current driver. Furthermore, the common cathode pins must be driven by transistors or a driver IC capable of sinking the combined current of up to 8 illuminated segments (if the digit '8' and DP are on).

Q: What does a "2:1 Luminous Intensity Matching Ratio" mean in practice?

A: It means that within a single display unit, the dimmest segment will be no less than half as bright as the brightest segment when driven under the same conditions (10mA). This ensures visual consistency across the segments of one character.

Q: How do I achieve the typical brightness of 600 μcd?

A: The typical value is given at I_F=1mA. To achieve higher brightness, you can increase the drive current, but you must stay within the Absolute Maximum Ratings (25mA continuous per segment). The brightness will increase approximately linearly with current up to a point. Refer to the characteristic curve of I_V vs. I_F for guidance.

10. Design-in Case Study

Scenario: Designing a low-power, 4-digit voltmeter.

The LTC-2728JD is an excellent choice. The microcontroller's ADC reads the voltage, converts it to a number, and generates the corresponding 7-segment codes. A simple driver circuit using a transistor array (e.g., ULN2003) sinks current for the four common cathode pins, controlled by four microcontroller I/O pins. The seven segment anode lines connect to the microcontroller via current-limiting resistors. To conserve power, the multiplexing is performed, and the segment current can be set to 2-5mA, well within the device's efficient operating range, providing ample brightness while minimizing overall system current draw. The high contrast ensures readability in both indoor and moderately bright environments.

11. Operational 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 2.1-2.6V) is applied across an LED segment, electrons and holes are injected into the active region (the AlInGaP layer) where they recombine. This recombination process 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 light centered around 656 nm. The non-transparent GaAs substrate absorbs any light emitted downwards, improving overall contrast by preventing internal reflections that could "wash out" the displayed character.

12. Technology Trends

Seven-segment LED displays based on AlInGaP technology represent a mature and reliable solution for numeric displays. Current trends in the broader display field include a shift towards dot-matrix OLED or TFT-LCD modules that offer full alphanumeric and graphic capabilities. However, for dedicated numeric applications where extreme readability, wide viewing angles, high brightness, simplicity, robustness, and low cost are paramount, LED seven-segment displays remain highly competitive. Ongoing developments in LED efficiency (allowing even lower drive currents) and packaging (thinner profiles) continue to evolve this classic technology. The principle of multiplexing common-cathode or common-anode arrays remains a fundamental and efficient method for driving multi-digit displays.