LTP-1557AKD LED Dot Matrix Display Datasheet - 1.2 inch (30.42mm) Height - Hyper Red (650nm) - 40mW per Dot - English Technical Document

1. Product Overview

The LTP-1557AKD is a single-digit, alphanumeric display module built using a 5x7 dot matrix of AlInGaP (Aluminum Indium Gallium Phosphide) Hyper Red light-emitting diodes (LEDs). This configuration is standard for displaying ASCII and EBCDIC character sets, making it suitable for applications requiring clear, single-character readouts. The device features a gray face with white dots, enhancing contrast for improved readability. Its core design principle is based on a common-cathode column and common-anode row matrix architecture, allowing efficient multiplexing to control individual LEDs with a reduced number of I/O pins.

1.1 Core Advantages and Target Market

The primary advantages of this display include its solid-state reliability, wide viewing angle due to the single-plane design, and low power requirement. The 1.2-inch (30.42 mm) character height provides good visibility. It is categorized for luminous intensity, allowing for brightness binning. The device is stackable horizontally, enabling the creation of multi-character displays. Its primary target markets include industrial control panels, instrumentation, test equipment, point-of-sale terminals, and other embedded systems where a simple, reliable, and low-power character display is required.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

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

• Average Power Dissipation per Dot: 40 mW. This limits the continuous thermal load on each individual LED chip.
• Peak Forward Current per Dot: 90 mA. This is permissible only under pulsed conditions with a 1/10 duty cycle and a 0.1 ms pulse width, used for achieving higher instantaneous brightness in multiplexed schemes.
• Average Forward Current per Dot: The base rating is 15 mA at 25°C. This rating derates linearly at 0.2 mA/°C as ambient temperature (Ta) increases above 25°C, a critical consideration for thermal management.
• Reverse Voltage per Dot: 5 V. Exceeding this can break down the LED's PN 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 package.

2.2 Electrical & Optical Characteristics

These parameters are measured at an ambient temperature (Ta) of 25°C and define the device's typical performance.

• Average Luminous Intensity (I_V): Ranges from 800 µcd (minimum) to 2600 µcd (typical). Measured under a pulsed condition of I_p=32mA and a 1/16 duty cycle. The wide range indicates the effect of the luminous intensity categorization (binning).
• Peak Emission Wavelength (λ_p): 650 nm (typical). This defines the primary color of the emitted light as Hyper Red.
• Spectral Line Half-Width (Δλ): 20 nm (typical). This indicates the spectral purity or bandwidth of the emitted red light.
• Dominant Wavelength (λ_d): 639 nm (typical). This is the wavelength perceived by the human eye, slightly different from the peak wavelength.
• Forward Voltage (V_F) any Dot:
  • 2.1V to 2.6V (typical range) at I_F=20mA.
  • 2.3V to 2.8V (typical range) at I_F=80mA (pulsed). Shows the positive temperature coefficient and dynamic resistance of the LED.
• Reverse Current (I_R) any Dot: Maximum 100 µA at V_R=5V.
• Luminous Intensity Matching Ratio (I_V-m): Maximum 2:1. This specifies the maximum allowable ratio between the brightest and dimmest LED in the array under the same drive conditions, ensuring uniform appearance.

Measurement Note: Luminous intensity is measured with a sensor and filter combination approximating the CIE photopic eye-response curve, ensuring the values correlate with human visual perception.

3. Binning System Explanation

The datasheet explicitly states the device is "Categorized for Luminous Intensity." This is a critical binning parameter.

• Luminous Intensity Binning: The typical I_V range of 800-2600 µcd suggests multiple intensity bins. Designers must select the appropriate bin based on application brightness requirements and ensure consistency when using multiple displays.
• Wavelength Consistency: While not explicitly binned for wavelength, the tight typical specifications for λ_p (650nm) and λ_d (639nm) indicate good manufacturing control, resulting in consistent red color across devices.
• Forward Voltage: The specified V_F range (e.g., 2.1-2.6V) implies a spread. For designs with many displays or stringent power requirements, consulting the manufacturer for voltage binning options may be necessary.

4. Performance Curve Analysis

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

• Forward Current vs. Forward Voltage (I-V Curve): Shows the exponential relationship, crucial for designing current-limiting circuits. The curve shifts with temperature.
• Relative Luminous Intensity vs. Forward Current: Demonstrates that light output is relatively linear with current in the normal operating range before efficiency droop occurs at very high currents.
• Relative Luminous Intensity vs. Ambient Temperature: Shows the decrease in light output as junction temperature rises, highlighting the importance of thermal management, especially when driven at higher average currents.
• Spectral Distribution: A plot of relative intensity vs. wavelength, centered around 650nm with a ~20nm half-width, confirming the Hyper Red color.

5. Mechanical & Package Information

5.1 Package Dimensions

The device has specific physical dimensions provided in a drawing (referenced but not detailed in text). All dimensions are in millimeters with a standard tolerance of ±0.25 mm unless otherwise noted. This includes the overall height, width, depth, lead spacing, and dot matrix positioning within the gray face.

5.2 Pin Connection and Internal Circuit

The device uses a 14-pin configuration. The internal circuit diagram shows a standard 5x7 matrix where:
- Columns (1-5) are common-cathode groups.
- Rows (1-7) are common-anode groups.
To illuminate a specific dot (e.g., Row 3, Column 2), the corresponding row anode must be driven high (with current limiting) while the corresponding column cathode is pulled low. The pinout table is essential for correct PCB layout and driver circuit design.

6. Soldering & Assembly Guidelines

The key assembly specification is the soldering profile.

• Reflow Soldering: The maximum allowable temperature at the package body (1.6mm below seating plane) is 260°C, and this peak temperature must not be sustained for more than 3 seconds. Standard lead-free (SnAgCu) reflow profiles must be carefully controlled to stay within this limit to prevent damage to the LED chips, internal wire bonds, or the plastic package.
• Hand Soldering: If necessary, it should be performed quickly with a temperature-controlled iron, applying heat to the PCB pad and not directly to the component lead for an extended period.
• Cleaning: Use only cleaning agents compatible with the LED package material.
• Storage Conditions: Store in a dry, anti-static environment within the specified -35°C to +85°C temperature range to prevent moisture absorption and electrostatic discharge damage.

7. Application Suggestions

7.1 Typical Application Scenarios

• Industrial Readouts: Status indicators, error codes, or single-value displays on machinery.
• Test & Measurement Equipment: Displaying units (V, A, Hz), channel numbers, or simple codes.
• Consumer Electronics: Clock displays, appliance status indicators (though less common today).
• Prototyping & Education: Excellent for learning microcontroller interfacing and multiplexing techniques.

7.2 Design Considerations

• Driver Circuit: Requires a microcontroller or dedicated driver IC capable of multiplexing 12 lines (5 columns + 7 rows). Use transistors or integrated sink/source drivers to handle the required current.
• Current Limiting: Essential for every row or column line. Calculate resistor values based on the desired average current (e.g., 10-15mA per dot), supply voltage, and LED forward voltage. Remember the current is shared across multiple LEDs in a multiplexed frame.
• Refresh Rate: The multiplexing scan rate must be high enough (typically >100Hz) to avoid visible flicker. The 1/16 duty cycle mentioned in the specs implies a 16-step multiplexing scheme is suitable.
• Thermal Management: When operating near the maximum average current or at high ambient temperatures, ensure adequate ventilation. The 0.2 mA/°C derating for I_F is crucial for reliability.
• Viewing Angle: The wide viewing angle is beneficial but consider the mounting orientation relative to the user.

8. Technical Comparison & Differentiation

Compared to older GaAsP or GaP red LED matrices, the AlInGaP technology in the LTP-1557AKD offers significantly higher luminous efficiency, resulting in brighter displays at the same current or lower power consumption for the same brightness. The Hyper Red (650nm) wavelength is more vibrant and distinct than standard red. Compared to modern graphic OLEDs or LCDs, this device is much simpler, more robust, lower cost, and operates over a wider temperature range, but is limited to pre-defined 5x7 characters. Its niche is in applications demanding extreme reliability, simplicity, and low cost for character display.

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 drivers, which is inefficient. Multiplexing is the standard and intended method, using the X-Y select architecture.
• Q: Why is the peak current (90mA) so much higher than the average current (15mA)?
  A: In multiplexing, each LED is only powered for a fraction of the time (duty cycle). To achieve a perceived average brightness equivalent to 15mA DC, a higher pulsed current is used during its active time slot. The 90mA rating ensures the LED can handle these brief pulses.
• Q: What does a "Luminous Intensity Matching Ratio of 2:1" mean for my design?
  A: It means the dimmest dot in the array could be half as bright as the brightest dot under identical drive conditions. For uniform-looking characters, you may need to select devices from a tighter bin or implement software brightness compensation if your driver allows individual dot control.
• Q: How do I interface this 14-pin device to a microcontroller with fewer I/O pins?
  A> You must use external shift registers (like 74HC595), I/O expanders, or a dedicated LED driver IC with multiplexing support. You cannot reduce the number of required control lines below 12 for full control of the 5x7 matrix.

10. Practical Design Case Study

Scenario: Designing a single-digit temperature readout for an industrial oven controller operating at up to 70°C ambient.

• Brightness: Select a luminous intensity bin from the higher end (e.g., 2000+ µcd) to ensure visibility in a potentially bright environment.
• Drive Current: Determine the derated average current. At Ta=70°C, the derating is (70-25)°C * 0.2 mA/°C = 9 mA. Therefore, the maximum safe continuous average current per dot is 15 mA - 9 mA = 6 mA. The design must use a pulsed current within the 1/16 duty cycle to achieve the required brightness while keeping the average current at or below 6mA per dot.
• Circuit: Use a microcontroller to generate the multiplexing signals. Employ low-side N-channel MOSFETs to sink column currents and high-side P-channel MOSFETs or a driver IC to source row currents. Calculate current-limiting resistors based on the supply voltage (e.g., 5V), the LED V_F at the pulsed current, and the desired pulsed current value needed to yield an effective average brightness.
• Layout: Place the display away from other heat-generating components on the PCB. Ensure the solder reflow profile during assembly strictly adheres to the 260°C for 3s limit.

11. Operating Principle

The device operates on the principle of electroluminescence in a semiconductor PN junction. When a forward bias voltage exceeding the diode's turn-on voltage (~2.1V) is applied across an individual LED cell (anode row high, cathode column low), electrons and holes recombine in the active AlInGaP region, releasing energy in the form of photons with a wavelength centered at 650 nm (red light). The 5x7 matrix arrangement and common anode/cathode architecture allow any of the 35 dots to be individually addressed by selecting the appropriate row and column lines, enabling the formation of characters through multiplexing.

12. Technology Trends

While discrete LED dot matrix displays like the LTP-1557AKD remain relevant for specific rugged and cost-sensitive applications, the broader trend is towards integration and advanced technologies. Integrated character LCD (LCD) and OLED modules with built-in controllers have become standard for more complex displays. For applications still requiring LEDs, surface-mount device (SMD) LED arrays and high-density, multi-color, addressable RGB LED matrices (e.g., using WS2812B-type LEDs) are increasingly popular for their flexibility and ease of use. However, the simplicity, high reliability, wide temperature range, and distinct, bright single-color output of traditional through-hole dot matrix LEDs ensure their continued use in industrial, automotive, and harsh environment applications where newer technologies may not meet all requirements.