LTP-2157AKD LED Dot Matrix Display Datasheet - 2.0 Inch Height - 5x7 Array - Hyper Red (650nm) - 40mW/Dot - English Technical Document

1. Product Overview

The LTP-2157AKD is a solid-state, alphanumeric character display module designed for applications requiring clear, bright, and reliable visual output. Its core function is to display characters and symbols using a grid of individually addressable light-emitting diodes (LEDs). The primary application domains include industrial control panels, instrumentation, point-of-sale terminals, medical equipment displays, and various consumer electronics where a simple, monochrome character readout is sufficient.

The fundamental operating principle is based on a 5x7 dot matrix configuration. This means each character is formed by illuminating a specific pattern within a grid of 5 columns and 7 rows of LED pixels. By selectively applying forward voltage to the anode (row) and cathode (column) lines corresponding to each desired pixel, specific dots are turned on to create recognizable shapes like letters and numbers. The device utilizes a multiplexed driving scheme, where rows are activated sequentially at a high frequency, creating the perception of a stable, fully lit character while minimizing the number of required driver pins and power consumption.

2. Technical Parameters: In-Depth Objective Interpretation

2.1 Photometric and Optical Characteristics

The optical performance is defined under standard test conditions at an ambient temperature (Ta) of 25°C. The key parameter, Average Luminous Intensity (Iv), has a typical value of 3500 microcandelas (µcd) when driven with a peak current (Ip) of 32mA at a 1/16 duty cycle. This indicates a high-brightness output suitable for well-lit environments. The minimum specified value is 1650 µcd, which defines the lower performance boundary for the product.

The color characteristics are determined by the AlInGaP (Aluminum Indium Gallium Phosphide) semiconductor material. The Peak Emission Wavelength (λp) is typically 650 nanometers (nm), placing it in the hyper-red region of the visible spectrum. The Dominant Wavelength (λd) is specified at 639 nm. The difference between peak and dominant wavelength, along with the Spectral Line Half-Width (Δλ) of 20 nm, describes the spectral purity and the specific shade of red emitted. A Luminous Intensity Matching Ratio of 2:1 (max) is specified, meaning the brightness of the dimmest segment in a character should be no less than half the brightness of the brightest segment under identical drive conditions, ensuring uniform appearance.

2.2 Electrical and Thermal Characteristics

The Absolute Maximum Ratings define the operational limits beyond which permanent damage may occur. The Average Power Dissipation per Dot is rated at 40 milliwatts (mW). The Peak Forward Current per Dot can reach 90mA for pulsed operation, while the Average Forward Current per Dot has a base rating of 15mA at 25°C, derating linearly by 0.2 mA/°C as temperature increases. This derating is critical for thermal management and long-term reliability. The maximum Reverse Voltage per Dot is 5V.

Under typical operating conditions (Forward Current IF=20mA), the Forward Voltage per Segment (VF) ranges from 2.1V (min) to 2.6V (max). This parameter is essential for designing the current-limiting circuitry in the driver. The Reverse Current (IR) is very low, typically 2.3 to 2.8 microamperes (µA) at the maximum reverse voltage of 5V, indicating good diode characteristics.

The device is rated for an Operating Temperature Range of -35°C to +85°C, with an identical Storage Temperature Range. This wide range makes it suitable for harsh environments. The maximum soldering temperature is specified as 260°C for a maximum duration of 3 seconds, measured 1.6mm (1/16 inch) below the seating plane, which provides clear guidelines for PCB assembly processes.

3. Mechanical and Packaging Information

The display has a stated matrix height of 2.0 inches (50.8 mm). The package dimensions are provided in a detailed drawing with all measurements in millimeters. Manufacturing tolerances are generally ±0.25 mm (0.01 inches) unless otherwise noted on the drawing. The physical package features a gray face with white dot color, which enhances contrast when the LEDs are off and diffuses light when they are on.

The pin connection diagram is crucial for correct interfacing. The device has a 14-pin configuration. The internal circuit is a standard common-cathode arrangement for the columns, with the anodes for the 7 rows brought out to individual pins. It is important to note the internal connections: Pin 4 and Pin 11 are internally connected (both are Cathode for Column 3), and Pin 5 and Pin 12 are internally connected (both are Anode for Row 4). These connections are made to simplify the internal bonding and do not affect the external driving logic.

4. Performance Curve Analysis

The datasheet references typical electrical and optical characteristic curves. While the specific graphs are not detailed in the provided text, standard curves for such devices would typically include:

• Forward Current vs. Forward Voltage (I-V Curve): This graph shows the exponential relationship, crucial for determining the necessary supply voltage for a given drive current.
• Luminous Intensity vs. Forward Current: This curve shows how light output increases with current, typically in a near-linear relationship within the operating range before efficiency drops at very high currents.
• Luminous Intensity vs. Ambient Temperature: This demonstrates the thermal quenching effect, where LED output decreases as the junction temperature rises. Understanding this is key for applications with high ambient temperatures or poor heat sinking.
• Spectral Distribution: A plot of relative intensity versus wavelength, showing the concentration of light output around the 650nm peak with the defined 20nm half-width.

These curves allow designers to optimize performance and understand trade-offs between brightness, efficiency, drive current, and thermal management.

5. Soldering and Assembly Guidelines

Assembly must adhere to the specified soldering profile to prevent damage. The maximum allowable soldering temperature is 260°C, and the component should be exposed to this temperature for no more than 3 seconds. This is typically achieved using a controlled reflow soldering process with a profile that ramps up temperature, holds at peak, and cools down within specified limits. Hand soldering with an iron requires extreme care to localize heat and avoid exceeding these parameters.

For storage, the device should be kept within the specified temperature range of -35°C to +85°C in a low-humidity environment. It is recommended to use the components within their shelf life and to follow standard ESD (Electrostatic Discharge) precautions during handling to protect the sensitive semiconductor junctions.

6. Application Suggestions and Design Considerations

6.1 Typical Application Circuits

The LTP-2157AKD requires an external driver circuit. A common design uses a microcontroller with sufficient I/O pins or a dedicated LED display driver IC (like the MAX7219 or similar). The driver must implement multiplexing: it sequentially activates each of the 7 row anode lines while providing the column cathode data for that specific row. The refresh rate must be high enough (typically >100Hz) to avoid visible flicker. Current-limiting resistors are mandatory for each column line (or integrated into the driver) to set the forward current to the desired value (e.g., 20mA for typical brightness). The calculation for the resistor value is R = (Vcc - Vf - Vdriver_sat) / If, where Vcc is the supply voltage, Vf is the LED forward voltage (~2.6V max), Vdriver_sat is the driver's saturation voltage, and If is the desired forward current.

6.2 Design Considerations

• Power Supply: Ensure the power supply can deliver the peak current required. With 5 columns potentially lit per row and a peak current of up to 20mA per dot, a single row could draw 100mA. The supply must handle this pulsed load.
• Heat Dissipation: While individual dots dissipate only 40mW max, continuous operation of many dots can generate significant heat. Ensure adequate ventilation or heatsinking if operating at high ambient temperatures or maximum ratings.
• Viewing Angle and Contrast: The gray face/white dot design offers good off-state contrast. Consider the intended viewing angle when mounting the display; LED brightness is often highest on-axis.
• Software: The microcontroller firmware needs to include a font map (a lookup table defining the 5x7 pattern for each character) and the multiplexing routine.

7. Technical Comparison and Differentiation

The primary differentiator of this display is its use of AlInGaP Hyper Red LED technology. Compared to older GaAsP (Gallium Arsenide Phosphide) red LEDs, AlInGaP offers significantly higher luminous efficiency, resulting in greater brightness for the same drive current, or equivalent brightness at lower power. It also provides better color purity and stability over temperature and time.

Compared to a simple 7-segment display, a 5x7 dot matrix offers far greater flexibility, capable of displaying the full alphanumeric character set (A-Z, 0-9, symbols) and even simple graphics, whereas a 7-segment display is limited primarily to numbers and a few letters. The trade-off is increased complexity in both hardware (more driver pins) and software (character generation).

8. Common Questions Based on Technical Parameters

Q: Can I drive this display with a constant DC current without multiplexing?
A: Technically, yes, but it is highly inefficient and not recommended. Driving all 35 dots simultaneously at 20mA would require a 700mA current draw and significant heat generation. Multiplexing is the standard and intended method, reducing power and pin count.

Q: What is the difference between Peak Emission Wavelength (650nm) and Dominant Wavelength (639nm)?
A: The peak wavelength is the point of maximum intensity in the spectral output. The dominant wavelength is the single wavelength of monochromatic light that would appear to have the same color to the human eye. The difference arises because the LED emits a spectrum of light (20nm wide), not a single pure wavelength.

Q: The pinout shows two pins for Column 3 and two pins for Row 4. Do I need to connect both?
A: No, these pins are internally connected. Connecting to either one is sufficient. This internal connection is a manufacturing artifact to simplify the die bonding. Connecting both does not cause harm but is unnecessary.

Q: How do I calculate the average power consumption?
A: For a multiplexed display, the average current is roughly (Number of lit dots in a fully lit character / Total number of dots) * Current per dot * Duty Cycle. For example, for a character using 24 dots lit at 20mA with a 1/7 duty cycle (7 rows): Avg Current ≈ (24/35) * 20mA * (1/7) ≈ 2mA per character. Multiply by the number of characters for total load.

9. Practical Application Case Study

Scenario: Designing a simple single-character temperature readout for an industrial oven.
The requirement is to display a temperature from 0 to 199°C. A microcontroller reads a temperature sensor. One LTP-2157AKD display is used. The microcontroller has limited pins, so it uses a serial-in/parallel-out shift register (like 74HC595) to drive the 5 column cathodes and uses 7 of its own I/O pins to drive the row anodes via transistor switches (to handle the higher row current).

The firmware includes a 5x7 font for digits 0-9 and the degree symbol. The multiplexing routine runs on a timer interrupt. The display shows stable, bright red numbers readable from several feet away in a factory setting. The wide operating temperature range of the display (-35°C to +85°C) ensures reliability even if the external enclosure of the oven controller becomes warm. The high contrast of the gray face prevents washout under bright industrial lighting.

10. Technology Principle and Trends

10.1 Underlying Technology Principle

The light generation is based on electroluminescence in a semiconductor p-n junction. When a forward voltage is applied, electrons from the n-type AlInGaP layer recombine with holes from the p-type layer. This recombination event releases energy in the form of photons (light). The specific energy bandgap of the AlInGaP material determines the wavelength (color) of the emitted photons, in this case, red light around 650nm. The non-transparent GaAs substrate absorbs any downward-emitted light, improving overall efficiency by reflecting more light out of the top of the device. The 5x7 array is created by depositing and patterning multiple tiny LED chips or a single patterned die on a common substrate, with interconnected anode and cathode lines.

10.2 Industry Trends

While discrete 5x7 dot matrix displays like the LTP-2157AKD remain relevant for specific, cost-sensitive applications, broader industry trends are moving towards integrated solutions. These include:
- Surface-Mount Device (SMD) Arrays: Smaller footprint, easier automated assembly.
- Integrated Controller/Driver Displays: Modules with built-in controllers that communicate via SPI or I2C, drastically reducing the microcontroller resource burden.
- Higher Resolution and Color: Movement towards finer pitch matrices and full-color RGB displays for more detailed graphics.
- Alternative Technologies: In some applications, OLED (Organic LED) displays offer superior contrast and viewing angles, though often at higher cost and with different lifetime characteristics.
The enduring advantages of discrete LED matrices like this one are their extreme robustness, long lifetime, high brightness, simplicity, and low cost for monochrome character display tasks, ensuring their continued use in industrial and embedded systems.