LTP-1557KF 5x7 Dot Matrix LED Display Datasheet - 1.2-inch Height - Yellow-Orange Color - 20mA Drive Current - English Technical Document

1. Product Overview

The LTP-1557KF is a single-digit, alphanumeric display module built using a 5x7 dot matrix configuration. Its primary function is to display characters, symbols, or simple graphics by selectively illuminating individual LED dots. The core technology utilizes AlInGaP (Aluminum Indium Gallium Phosphide) semiconductor material to produce a yellow-orange light emission. This device is characterized by a gray faceplate with white dot coloration, enhancing contrast for improved readability. It is designed for low-power operation and offers a wide viewing angle, making it suitable for various indication and information display applications where clear, monochrome character output is required.

1.1 Core Advantages and Target Market

The key advantages of this display include its solid-state reliability, low power requirement, and compatibility with standard character codes like USASCII and EBCDIC. The single-plane design and wide viewing angle ensure good visibility from different perspectives. It is also categorized for luminous intensity, allowing for brightness matching in multi-unit applications, and is offered in a lead-free package compliant with RoHS directives. The primary target markets include industrial control panels, instrumentation, point-of-sale terminals, basic information displays, and embedded systems where a simple, reliable, and low-cost character display is needed.

2. In-Depth Technical Parameter Analysis

This section provides a detailed, objective interpretation of the electrical and optical parameters specified in the datasheet, explaining their significance for design engineers.

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. They are not for normal operation.

• Power Dissipation per Segment: 70 mW maximum. This limits the combined effect of forward current (I_F) and forward voltage (V_F) across any single LED dot.
• Peak Forward Current per Segment: 60 mA maximum, but only under pulsed conditions (1 kHz, 10% duty cycle). This allows for brief higher-current pulses for multiplexing or achieving higher instantaneous brightness.
• Continuous Forward Current per Segment: 25 mA maximum at 25°C. This is the key parameter for steady-state, non-pulsed operation. The derating factor of 0.28 mA/°C indicates that the maximum allowable continuous current must be reduced as ambient temperature (Ta) increases above 25°C to prevent overheating.
• Reverse Voltage per Segment: 5 V maximum. Exceeding this can break down the LED's PN junction.
• Operating & Storage Temperature Range: -35°C to +105°C. This defines the environmental limits for reliable operation and non-operational storage.
• Soldering Condition: 260°C for 3 seconds at 1/16 inch (approx. 1.6mm) below the seating plane. This is critical for wave or reflow soldering processes to avoid thermal damage to the plastic package and internal bonds.

2.2 Electrical & Optical Characteristics (at Ta=25°C)

These are the typical and guaranteed performance parameters under specified test conditions.

• Average Luminous Intensity (I_V): 55 to 170 μcd (min), 99 to 200 μcd (typ) at I_F=20mA. This wide range indicates the device is binned or categorized. Designers must account for this variation in system brightness planning. The test condition was revised from 1mA to 20mA, aligning the specification with a more standard drive current.
• Peak Emission Wavelength (λ_p): 611 nm (typ). This is the wavelength at which the spectral output is strongest.
• Spectral Line Half-Width (Δλ): 17 nm (typ). This measures the spread of the emitted spectrum; a smaller value indicates a more monochromatic (pure color) light.
• Dominant Wavelength (λ_d): 605 nm (typ). This is the single wavelength perceived by the human eye, defining the color as yellow-orange.
• Forward Voltage per Dot (V_F): 2.05V (min), 2.6V (typ) at I_F=20mA. This is crucial for designing the current-limiting circuitry. The driver supply voltage must be higher than V_F to regulate current properly.
• Reverse Current per Dot (I_R): 100 μA maximum at V_R=5V. A low reverse current is desirable.
• Luminous Intensity Matching Ratio: 2:1 maximum for similar light area. This means the brightest dot in an array should not be more than twice as bright as the dimmest dot, ensuring uniform appearance.

3. Binning System Explanation

The datasheet indicates the device is \"Categorized for Luminous Intensity.\" This implies a binning system is applied, though specific bin codes are not listed here.

• Luminous Intensity Binning: The specified I_V range (55-170 μcd min, 99-200 μcd typ) suggests products are sorted into groups based on measured light output at 20mA. Designers sourcing multiple units should specify or be aware of the bin to ensure consistent brightness across a multi-digit display.
• Wavelength/Color Binning: While not explicitly stated, typical LED manufacturing includes binning for dominant wavelength (color) to ensure visual consistency. The tight specs on λ_d (605nm) and λ_p (611nm) indicate a controlled process.
• Forward Voltage Binning: Less commonly highlighted for displays, but the V_F range (2.05-2.6V) defines the electrical parameter spread.

4. Performance Curve Analysis

The datasheet references \"Typical Electrical/Optical Characteristic Curves\" on the final page. While the specific graphs are not provided in the text, standard curves for such devices would typically include:

• I-V (Current-Voltage) Curve: Shows the exponential relationship between forward voltage and current. The knee voltage is around 2V, consistent with AlInGaP technology.
• Luminous Intensity vs. Forward Current (I_V vs. I_F): Would show that light output increases approximately linearly with current up to a point, after which efficiency drops.
• Luminous Intensity vs. Ambient Temperature (I_V vs. T_a): Would demonstrate the decrease in light output as junction temperature rises, highlighting the importance of thermal management and current derating.
• Spectral Distribution: A plot of relative intensity vs. wavelength, showing a peak near 611nm and a width of about 17nm at half the peak intensity.

5. Mechanical and Package Information

5.1 Package Dimensions and Drawing

The device has a standard dual in-line package (DIP) footprint. Key dimensional notes from the datasheet: all dimensions are in millimeters with a general tolerance of ±0.25 mm unless specified otherwise. A specific note mentions a pin tip shift tolerance of ±0.4 mm, which is important for PCB hole placement and soldering yield.

5.2 Internal Circuit Diagram and Pin Connection

The internal circuit is a standard 5x7 matrix. Rows (anodes) and columns (cathodes) are multiplexed. The pinout table is essential for correct PCB layout and driver circuit design:

• Pins 1, 2, 5, 7, 8, 9, 12, 14 connect to Anode Rows (1, 2, 3, 4, 5, 6, 7).
• Pins 3, 4, 6, 10, 11, 13 connect to Cathode Columns (1, 2, 3, 4, 5).

Note that some functions are duplicated on different pins (e.g., Anode Row 4 on pins 5 & 12, Cathode Column 3 on pins 4 & 11), which may offer layout flexibility. The pin numbering likely follows a specific orientation relative to the dot matrix viewing side.

6. Soldering and Assembly Guidelines

The primary guideline provided is the absolute maximum rating for soldering: 260°C for 3 seconds, measured 1.6mm below the seating plane. This is a standard wave soldering profile. For reflow soldering, a standard lead-free profile with a peak temperature not exceeding 260°C should be used. It is critical to avoid excessive thermal stress to prevent package cracking or delamination. Devices should be stored in the original moisture-barrier bag until use, especially if they are not moisture-sensitive level (MSL) rated, though the datasheet does not specify an MSL.

7. Application Recommendations

7.1 Typical Application Scenarios

This display is ideal for applications requiring a single line of alphanumeric characters: industrial equipment status displays (e.g., error codes, setpoints), consumer appliances, basic handheld test equipment, legacy system upgrades, and educational electronics kits.

7.2 Design Considerations

• Drive Circuitry: Requires a multiplexing driver (e.g., a dedicated display IC or microcontroller with sufficient I/O). Each row anode is driven sequentially while data is applied to the column cathodes.
• Current Limiting: External current-limiting resistors are mandatory for each column (cathode) line to set the I_F to a safe value, typically 20mA or lower depending on brightness and power requirements. Resistor value R = (V_supply - V_F) / I_F.
• Power Supply: Must provide a voltage higher than the max V_F (2.6V) plus the dropout voltage of any driver transistors. A 5V supply is common.
• Viewing Angle: The wide viewing angle is beneficial but consider the mounting position relative to the user.
• Brightness Consistency: Specify intensity bin if uniformity across multiple units is critical.

8. Technical Comparison and Differentiation

Compared to older GaAsP or GaP LED matrices, the AlInGaP technology in the LTP-1557KF offers higher efficiency and better color purity (more saturated yellow-orange). Compared to contemporary side-glow or high-density SMD matrices, this is a traditional, through-hole DIP device offering ease of prototyping and repair. Its main differentiation is the specific 1.2-inch character height, 5x7 format, and yellow-orange color, which may be chosen for legacy compatibility, specific visibility requirements (yellow/orange can be distinctive), or cost-effectiveness for simple applications where full-color or graphical capability is unnecessary.

9. Frequently Asked Questions (Based on Technical Parameters)

Q: Can I drive this display with a constant DC current on each dot?
A: Technically yes, but it would require 35 independent current sources (5x7). This is highly inefficient. Multiplexing (scanning) is the standard and intended method, dramatically reducing the required driver pins and power dissipation in the driver IC.

Q: Why is the Peak Forward Current (60mA) much higher than the Continuous Current (25mA)?
A: This allows for time-division multiplexing. A dot is only on for a fraction of the scan cycle (e.g., 1/7th for a 7-row scan). You can pulse a higher current during its brief \"on\" time to achieve a higher perceived average brightness without exceeding the average power (thermal) limits of the LED chip.

Q: The luminous intensity has a very wide range (55-200 μcd). How do I ensure consistent brightness in my product?
A: You must either: 1) Purchase devices from a single production lot or specified intensity bin, 2) Implement software brightness calibration or adjustment in your driver, or 3) Use hardware current adjustment per unit (impractical for volume). Discuss bin code availability with the distributor or manufacturer.

Q: Is a heat sink required?
A: For normal operation at or below 20mA per dot and within the ambient temperature range, a heat sink is not typically required for the display itself. However, proper PCB layout for heat dissipation from the driver components is important. Adhere to the current derating curve if operating in high-temperature environments.

10. Practical Design and Usage Examples

Case Study 1: Simple Microcontroller Interface. A basic 8-bit microcontroller can drive this display directly if it has at least 12 I/O pins (7 for rows, 5 for columns). The rows are connected via current-limiting resistors to microcontroller pins configured as outputs sourcing current (anodes). The columns are connected to pins configured as open-drain or active-low outputs (cathodes). Firmware implements a timer interrupt to scan through the rows, pulling one row high at a time while setting the column patterns for that row from a font table stored in ROM.

Case Study 2: Using a Dedicated Display Driver IC. For systems with limited microcontroller pins or to offload processing, a driver IC like the MAX7219 or HT16K33 can be used. These ICs handle all multiplexing, decoding, and brightness control via a simple serial interface (SPI or I2C), requiring only 2-4 pins from the host controller. They also often include features like digit blinking and multi-digit cascading, which aligns with this display's \"stackable horizontally\" feature.

11. Operating Principle Introduction

The LTP-1557KF is an array of 35 independent AlInGaP LED chips arranged in a grid of 5 columns and 7 rows, mounted behind a gray mask with 35 apertures (dots). Each LED's anode is connected to a common row line, and its cathode is connected to a common column line. To illuminate a specific dot, its corresponding row line is driven to a positive voltage (through a current limit), and its column line is connected to a lower voltage (ground). This matrix arrangement reduces the required connection pins from 35 (one per dot) to 12 (7 rows + 5 columns). Displaying a character involves rapidly scanning through the rows (1-7) and, for each row, turning on the appropriate column LEDs (1-5) that form part of the desired character shape. This multiplexing happens faster than the human eye can perceive, creating a stable, full character image.

12. Technology Trends and Context

Displays like the LTP-1557KF represent a mature, established technology. Current trends in indicator and alphanumeric displays are moving towards surface-mount device (SMD) packages for automated assembly, higher-density multi-digit modules, and the integration of controllers directly onto the display PCB (\"intelligent\" displays). Furthermore, full-color RGB LED matrices and OLED displays are becoming more cost-competitive for applications requiring color or superior contrast. However, simple monochrome dot matrix LEDs like this one remain highly relevant due to their extreme reliability, simplicity, low cost, high brightness, wide operating temperature range, and longevity—attributes critical in industrial, automotive, and outdoor applications. The shift to AlInGaP from older materials, as seen in this device, was a key step in improving efficiency and color performance within this classic form factor.