LTP-2257KA 5x7 Dot Matrix LED Display Datasheet - 1.97 Inch Height - AlInGaP Red-Orange - English Technical Document

1. Product Overview

The LTP-2257KA is a single-digit, alphanumeric display module designed for applications requiring clear, reliable character output. Its core function is to visually represent data, typically ASCII or EBCDIC coded characters, through a grid of individually addressable light-emitting diodes (LEDs). The device is engineered for integration into systems where low power consumption, solid-state reliability, and wide viewing angles are critical performance factors.

The primary market for this component includes industrial control panels, instrumentation, point-of-sale terminals, basic information displays, and embedded systems where a simple, robust character readout is needed. Its stackable design allows for the creation of multi-character displays horizontally, providing flexibility for showing words or numbers.

The core technological advantage lies in its use of Aluminium Indium Gallium Phosphide (AlInGaP) semiconductor material for the LED chips. This material system is known for producing high-efficiency light emission in the red to amber-orange spectrum, offering good visibility. The display features a black face, which provides high contrast against the illuminated white dots, significantly enhancing readability in various ambient lighting conditions.

2. Technical Specifications Deep Dive

This section provides a detailed, objective analysis of the key electrical, optical, and physical parameters defined in the datasheet.

2.1 Photometric and Optical Characteristics

The optical performance is central to the display's function. The key parameters are measured under standardized test conditions (Ta=25\u00b0C) to ensure consistency.

• Average Luminous Intensity (I_V): Ranges from a minimum of 2100 \u00b5cd to a maximum of 5000 \u00b5cd, with a typical value implied. This intensity is measured per dot under a pulsed drive condition of I_p=32mA at a 1/16 duty cycle. The 1/16 duty cycle is typical for multiplexed matrix drives, where each row is active for only a fraction of the time. The sensor used approximates the CIE photopic luminosity function, ensuring the measurement correlates with human eye sensitivity.
• Peak Emission Wavelength (\u03bb_p): Typically 621 nanometers (nm). This indicates the wavelength at which the optical power output is greatest. It falls within the red-orange region of the visible spectrum.
• Dominant Wavelength (\u03bb_d): 615 nm. This is the single wavelength perceived by the human eye that matches the color of the LED's output. It is slightly lower than the peak wavelength, which is common due to the shape of the emission spectrum.
• Spectral Line Half-Width (\u0394\u03bb): Approximately 18 nm. This parameter defines the bandwidth of the emitted light, specifically the width of the spectral curve at half its maximum power. A value of 18 nm indicates a relatively narrowband, monochromatic source, which is characteristic of AlInGaP LEDs and results in a saturated color.
• Luminous Intensity Matching Ratio (I_V-m): 2:1 maximum. This is a critical parameter for display uniformity. It specifies that the luminous intensity of any individual dot will not be more than twice that of any other dot within the same display module. This ensures consistent brightness across all segments of a character.

2.2 Electrical Characteristics

The electrical parameters define the interface and power requirements for the device.

• Forward Voltage (V_F): Ranges from 2.05V (min) to 2.6V (max) per dot at a test current (I_F) of 20mA. This is the voltage drop across the LED when it is conducting. Designers must ensure the driving circuit can provide this voltage. The typical value is not stated but lies within this range.
• Reverse Current (I_R): Maximum of 100 \u00b5A at a reverse voltage (V_R) of 15V. This is the small leakage current that flows when the LED is reverse-biased. It is generally negligible in operation but must be considered in circuit protection design.
• Average Forward Current Per Dot: The rated average current is 13 mA. However, a derating factor of 0.17 mA/\u00b0C applies linearly above 25\u00b0C. This means the maximum allowable average current must be reduced as the ambient temperature increases to prevent overheating and premature failure. For example, at 85\u00b0C, the maximum average current would be: 13 mA - [0.17 mA/\u00b0C * (85-25)\u00b0C] = 13 - 10.2 = 2.8 mA.

2.3 Absolute Maximum Ratings

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

• Average Power Dissipation Per Dot: 36 mW maximum. This is the product of the average forward current and forward voltage.
• Peak Forward Current Per Dot: 100 mA maximum. This is the highest instantaneous current allowed, typically relevant during very short pulses in multiplexed schemes.
• Reverse Voltage Per Dot: 5 V maximum. Exceeding this can cause junction breakdown.
• Operating & Storage Temperature Range: -35\u00b0C to +85\u00b0C. The device is rated for industrial temperature ranges.
• Solder Temperature: Maximum of 260\u00b0C for a maximum of 3 seconds, measured 1.6mm (1/16 inch) below the seating plane. This is crucial for wave or reflow soldering processes.

3. Binning and Categorization System

The datasheet explicitly states that the device is "Categorized for Luminous Intensity." This indicates that units are sorted, or "binned," based on their measured light output. The luminous intensity range (2100-5000 \u00b5cd) likely represents the spread across multiple bins. Manufacturers typically group LEDs into tighter intensity ranges (e.g., 2100-3000 \u00b5cd, 3000-4000 \u00b5cd, 4000-5000 \u00b5cd). This allows customers to select a bin for their specific brightness uniformity requirements. For a multi-unit display, using LEDs from the same intensity bin is essential to achieve uniform appearance. The datasheet does not specify binning for forward voltage or wavelength, though the provided min/max ranges for V_F and \u03bb_p define the total spread.

4. Performance Curve Analysis

The datasheet references "Typical Electrical / Optical Characteristic Curves." While the specific graphs are not provided in the text, we can infer their standard content and significance.

• Relative Luminous Intensity vs. Forward Current (I-V Curve): This graph would show how light output increases with drive current. It is typically non-linear, with efficiency dropping at very high currents due to thermal effects. The 32mA pulse test point is likely on the efficient, linear portion of this curve.
• Forward Voltage vs. Forward Current: This curve shows the diode's I-V characteristic. The voltage increases logarithmically with current. The specified V_F at 20mA is a single point on this curve.
• Relative Luminous Intensity vs. Ambient Temperature: This is a critical curve for understanding thermal performance. The light output of LEDs generally decreases as junction temperature rises. The derating specified for forward current is directly related to managing this thermal effect to maintain performance and reliability.
• Spectral Distribution: A plot of relative intensity vs. wavelength, showing a peak around 621nm and a width of approximately 18nm at half the peak intensity (FWHM).

5. Mechanical and Package Information

The device is a through-hole component with a standard DIP (Dual In-line Package) style form factor suitable for PCB mounting.

• Matrix Height: The defining physical feature is a 1.97-inch (50.15 mm) character height. This is a large-format display designed for viewing at a distance.
• Package Dimensions: The datasheet includes a detailed dimensioned drawing. All dimensions are in millimeters with a standard tolerance of \u00b10.25 mm unless otherwise specified. This drawing is essential for PCB footprint design and ensuring proper fit within an enclosure.
• Pin Connection: The device has 12 pins in a single row.
  • Pins 1-7: Correspond to Cathode Rows 1 through 7. In a common matrix configuration, these would be the scan lines.
  • Pins 8-12: Correspond to Anode Columns 5 through 1 (note the reverse order: Pin 8 is Column 5, Pin 12 is Column 1). These would be the data lines.
• Internal Circuit Diagram: The provided diagram shows a standard 5x7 matrix configuration. Each LED (dot) is located at the intersection of an anode column and a cathode row. To illuminate a specific dot, its corresponding anode line must be driven high (positive voltage) while its cathode line is driven low (ground). This matrix arrangement minimizes the number of required driver pins (12 instead of 35 for individually addressed dots).
• Polarity Identification: The pinout table clearly identifies anode and cathode connections. The package likely has a notch or marking on one end to indicate pin 1 orientation.

6. Soldering and Assembly Guidelines

The key assembly specification provided is for the soldering process.

• Reflow/Wave Soldering Parameters: The absolute maximum rating specifies that the device can withstand a solder temperature of 260\u00b0C for a maximum of 3 seconds. This measurement is taken 1.6mm below the seating plane (i.e., at the PCB level), not at the component body. This is a standard rating for leaded components and is compatible with typical wave soldering profiles. For reflow soldering with lead-free solder (which has higher melting points), the profile must be carefully controlled to ensure the component body temperature does not exceed the maximum storage temperature of 85\u00b0C for an extended period, even if the leads briefly see 260\u00b0C.
• Hand Soldering: If hand soldering is necessary, a temperature-controlled iron should be used. Contact time per pin should be minimized, ideally under 3 seconds, to prevent heat from traveling up the leads and damaging the internal wire bonds or epoxy.
• Cleaning: No specific cleaning instructions are given. Standard isopropyl alcohol or approved flux removers can be used, but aggressive solvents should be avoided as they may damage the plastic face or markings.
• Storage Conditions: The device should be stored within its specified temperature range of -35\u00b0C to +85\u00b0C in a dry, non-condensing environment. It is advisable to keep components in their original moisture-barrier bags until use to prevent moisture absorption, which can cause "popcorning" during soldering.

7. Application Suggestions

7.1 Typical Application Scenarios

• Industrial Control Panels: Displaying setpoints, process values (temperature, pressure, speed), error codes, or machine status.
• Test and Measurement Equipment: Showing numerical readings from multimeters, power supplies, or signal generators.
• Consumer Electronics (Legacy): Clocks, timers, basic calculators, or appliance displays.
• Embedded System Prototyping: A simple, direct output for microcontrollers (e.g., Arduino, PIC) to display debug information or user prompts.
• Stacked Multi-Character Displays: By placing multiple LTP-2257KA modules side-by-side, words, numbers, or simple scrolling messages can be created for basic information boards or signage.

7.2 Design Considerations

• Drive Circuitry: A dedicated LED driver IC or microcontroller GPIO pins with current-limiting resistors are required. Due to the matrix configuration, a multiplexing (scanning) scheme is necessary. The driver must source current to the anode columns and sink current from the cathode rows. The peak current per dot (100mA) and average current derating must be respected in the multiplexing timing calculations.
• Current Limiting: External resistors are mandatory for each anode column or cathode row (depending on the drive topology) to set the operating current. The value is calculated based on the supply voltage (V_CC), the LED forward voltage (V_F), and the desired current (I_F). For example, with a 5V supply, a V_F of 2.3V, and a target I_F of 20mA: R = (5V - 2.3V) / 0.02A = 135 Ohms. A standard 150 Ohm resistor would be suitable.
• Thermal Management: While the device is low power, the derating curve for forward current must be followed in high ambient temperature environments. Ensure adequate airflow if the display is enclosed. The average power dissipation per dot (36mW max) translates to a total maximum dissipation for an entire lit character, which should be considered for PCB thermal design.
• Viewing Angle: The "wide viewing angle" feature is beneficial, but for optimal readability, the display should be mounted facing the primary viewer. The black face/white dot design offers good contrast from most angles.

8. Technical Comparison and Differentiation

Compared to other display technologies available at its time of release (2000), the LTP-2257KA offered specific advantages:

• vs. Incandescent or Vacuum Fluorescent Displays (VFDs): LEDs are solid-state, offering far greater reliability, shock/vibration resistance, longer lifetime (typically tens of thousands of hours), and lower operating voltage/power. They also do not require heated filaments or high voltages.
• vs. Early LCDs: LEDs are emissive, meaning they produce their own light, making them clearly visible in low-light or dark conditions without a backlight. They have a much wider operating temperature range and faster response time. However, they consume more power than reflective LCDs and are not suitable for complex graphics.
• vs. Other LED Technologies: The use of AlInGaP material, compared to the older GaAsP or GaP, provided higher efficiency and better color purity (more saturated red-orange) for a given drive current. The specific 5x7 format with a large 1.97-inch height targeted applications needing easily readable characters from a distance.

9. Frequently Asked Questions (Based on Technical Parameters)

Q: Can I drive this display with a constant DC current on all dots simultaneously?
A: Technically yes, but it is highly inefficient and would exceed the average power ratings if all 35 dots were on. The standard and intended method is multiplexing, where dots are illuminated one row (or column) at a time at a high frequency, creating the illusion of a steady display while drastically reducing average current.

Q: What is the difference between peak and dominant wavelength?
A: Peak wavelength is where the LED emits the most optical power. Dominant wavelength is the single wavelength the human eye perceives as matching the LED's color. They are often close but not identical due to the asymmetry of the LED's emission spectrum. Dominant wavelength is more relevant for color perception.

Q: The forward voltage is 2.05-2.6V. Can I run it from a 3.3V logic supply?
A: Yes, absolutely. A 3.3V supply is sufficient to forward-bias the LED. You will need to recalculate the current-limiting resistor value based on the lower supply voltage (e.g., R = (3.3V - 2.3V) / 0.02A = 50 Ohms).

Q: What does "1/16 Duty" mean in the luminous intensity test condition?
A> It means the LED was pulsed with a 32mA current, but the pulse was only active for 1/16th of the total time period. The measured intensity is the average over the full period. This simulates the conditions in a 1:16 multiplexed drive scheme (e.g., 7 rows + 9 blanks = 16 time slots).

10. Practical Design and Usage Case

Case: Building a Simple 4-Digit Voltmeter Display. An engineer needs to display a voltage from 0.000 to 9.999 volts on a panel. They decide to use four LTP-2257KA modules stacked horizontally.

1. Circuit Design: A microcontroller with an ADC reads the voltage. Firmware converts the reading to four decimal digits. The microcontroller's I/O ports, combined with discrete transistors or a dedicated multiplexing driver IC (like the MAX7219), are configured to scan the four displays. Each display's cathode rows are connected in parallel, while the anode columns of each digit are controlled separately. This creates a 4-digit by 7-row matrix.
2. Current Setting: Using a 5V supply and aiming for a bright display, they choose an average current of 15mA per dot. Accounting for the multiplexing across 4 digits and 7 rows (effectively a 1/28 duty cycle for each dot when all are on), the peak pulse current during its active time slot would be higher (e.g., 15mA * 28 = 420mA), but this must be checked against the 100mA peak current rating. Therefore, they would need to adjust the timing or use a lower average current to keep the peak within spec.
3. Thermal Consideration: The panel is intended for a lab environment (25\u00b0C). The average current derating is not a concern here. However, they ensure the PCB has a ground plane to help dissipate heat from the driver circuitry.
4. Result: The final product shows a clear, bright, 4-digit readout with good viewing angle, meeting the requirement for a bench-top instrument.

11. Operating Principle

The LTP-2257KA operates on the fundamental principle of a light-emitting diode (LED) arranged in a passive matrix. Each of the 35 dots that form the 5x7 grid is an individual AlInGaP LED chip. When a forward bias voltage exceeding the diode's junction potential (roughly 2V) is applied across a specific anode (column) and cathode (row) pair, current flows through the LED at that intersection. This current causes electrons and holes to recombine within the semiconductor's active region, releasing energy in the form of photons—light—with a wavelength characteristic of the AlInGaP material (red-orange).

The matrix organization is a clever interconnect method. Instead of having 35 separate wires, the anodes of all LEDs in a vertical column are connected together, and the cathodes of all LEDs in a horizontal row are connected together. To light a single dot, its specific column is driven positive and its specific row is driven to ground. To display a pattern (like a character), a scanning algorithm rapidly sequences through the rows (or columns), turning on the appropriate column drivers for each row in turn. At a high enough frequency (typically >100Hz), persistence of vision makes the entire character appear steadily illuminated.

12. Technology Trends and Context

The LTP-2257KA represents a mature, well-established display technology. At the time of its release, dot matrix LED displays were a mainstream solution for alphanumeric output. The shift towards AlInGaP from older materials like GaAsP was a significant trend, offering improved efficiency and color.

Subsequent trends have moved towards:
Surface-Mount Device (SMD) Packages: Modern equivalents are almost exclusively SMD types, allowing for smaller, automated assembly.
Higher Density and Full-Matrix Displays: The basic 5x7 format has been largely supplanted by larger dot matrix modules (e.g., 8x8, 16x16) and full graphic panels that can display arbitrary shapes and text in multiple fonts.
Integrated Controllers: Modern LED matrix modules often include the driver, memory, and communication interface (like I2C or SPI) on a single board, greatly simplifying the design-in process for engineers.
Alternative Technologies: For many applications requiring simple character output, low-power LCDs (with or without backlights) and OLED displays have become more common, especially where power consumption, thinness, or graphical capability are priorities.

Despite these trends, through-hole LED dot matrix displays like the LTP-2257KA remain relevant in educational settings, for hobbyist projects, in legacy equipment maintenance, and in specific industrial applications where their simplicity, robustness, high brightness, and wide temperature range are decisive advantages.