Professional IC Distribution & Technical Solutions

The SN74LS247N is a BCD-to-seven-segment decoder/driver manufactured by Motorola (MOTO).

Specifications:

• Logic Family: LS (Low-Power Schottky)
• Function: BCD-to-Seven Segment Decoder/Driver
• Output Type: Open-Collector
• Supply Voltage (VCC): 4.75V to 5.25V
• Operating Temperature Range: 0°C to +70°C
• Package Type: PDIP-16 (Plastic Dual In-Line Package)
• Output Current (High): -0.4mA
• Output Current (Low): 8mA
• Propagation Delay: Typically 25ns

Descriptions:

The SN74LS247N is designed to convert a 4-bit Binary Coded Decimal (BCD) input into a seven-segment display output. It features active-low outputs for driving common-anode LED displays. The device includes ripple-blanking input/output for display blanking and lamp test functionality.

Features:

• BCD Input to Seven-Segment Output Conversion
• Active-Low Outputs for Common-Anode Displays
• Lamp Test Input (LT) for segment testing
• Ripple Blanking Input (RBI) and Output (RBO) for leading/trailing zero suppression
• Open-Collector Outputs for direct LED drive capability
• High Noise Immunity typical of LS family

This information is based on Motorola's datasheet for the SN74LS247N.

# SN74LS247N: Practical Applications, Design Pitfalls, and Implementation Considerations

## Practical Application Scenarios

The SN74LS247N is a BCD-to-seven-segment decoder/driver IC designed to convert binary-coded decimal (BCD) inputs into signals suitable for driving common-cathode seven-segment displays. Its primary applications include:

1. Digital Display Systems

• Used in instrumentation panels, calculators, and digital clocks where numeric output is required. The IC drives seven-segment LEDs directly, eliminating the need for additional current-limiting resistors due to its built-in open-collector outputs.

2. Industrial Control Interfaces

• Integrates into control panels for machinery, where BCD data from microcontrollers or PLCs must be visually displayed. Its ability to sink up to 24 mA per segment makes it suitable for driving high-brightness displays.

3. Embedded Systems Prototyping

• Commonly employed in educational and prototyping environments to simplify display interfacing. The active-low outputs and lamp test functionality facilitate quick debugging.

4. Automotive Dashboards

• Supports legacy dashboard designs where BCD signals from sensors (e.g., odometers) are decoded for segmented displays. Robustness against electrical noise is critical in these applications.

## Common Design Pitfalls and Avoidance Strategies

1. Incorrect Display Type Compatibility

• The SN74LS247N is designed for common-cathode displays. Using a common-anode display without additional inversion circuitry will result in failure.
• Solution: Verify display specifications before integration.

2. Overloading Outputs

• Exceeding the maximum sink current (24 mA per output) can damage the IC or cause dim segments.
• Solution: Ensure the total current per output remains within limits, especially when driving multiple segments simultaneously.

3. Improper BCD Input Handling

• Inputs beyond 1001 (decimal 9) produce undefined output patterns, leading to display errors.
• Solution: Implement input validation logic (e.g., clamping circuits or software checks) to restrict inputs to valid BCD values.

4. Inadequate Power Supply Decoupling

• Transient current spikes during segment switching can introduce noise, causing erratic behavior.
• Solution: Place a 0.1 µF decoupling capacitor close to the VCC and GND pins.

## Key Technical Considerations for Implementation

1. Output Configuration

• Open-collector outputs require pull-up resistors if interfacing with logic circuits. However, direct LED driving typically omits these resistors due to current-limiting features.

2. Input Termination

• Unused BCD inputs (A0–A3) must be tied to GND or VCC to prevent floating-state noise.

3. Dynamic vs. Static Display Driving

• For multiplexed displays, ensure the IC’s propagation delay (typically 25 ns) aligns with the refresh rate to avoid flickering.

4. Thermal Management

• High segment currents can cause heat buildup. Ensure proper PCB layout for heat dissipation, especially in high-duty-cycle applications.

By addressing these considerations and pitfalls, designers can leverage the SN74LS247N effectively in both legacy and