74LS47: Understanding The 74LS47 Data In Details

74LS47 Datasheet Overview

The 74LS47 datasheet provides all the necessary technical information about the IC, including its pinout, functional description, electrical characteristics, and typical applications. Let’s begin by examining the key sections of the datasheet.

Pinout and Pin Description

The 74LS47 comes in a 16-pin DIP (Dual Inline Package) with the following pinout:

The BCD inputs A, B, C, and D accept a 4-bit binary-coded decimal value that determines which segments of the 7-segment display will be illuminated. The lamp test input (LT) can be used to test the display by turning on all segments when pulled low. The blanking input (BI) and ripple blanking input/output (RBI/RBO) are used for controlling the display in multi-digit applications.

Function Table

The function table in the datasheet shows the relationship between the BCD inputs and the corresponding 7-segment display outputs. Here is the function table for the 74LS47:

In the table, ‘L’ represents a low input (0), ‘H’ represents a high input (1), and ‘X’ represents a “don’t care” condition. The outputs a, b, c, d, e, f, and g correspond to the individual segments of the 7-segment display, with a ‘1’ indicating that the segment is on and a ‘0’ indicating that it is off.

Electrical Characteristics

The electrical characteristics section of the datasheet provides information about the operating conditions, input/output voltage levels, and current requirements of the 74LS47. Some key specifications include:

• Supply Voltage (VCC): 4.75V to 5.25V
• Input Voltage Range: -0.5V to 7V
• Output Voltage Range: 0V to VCC
• Input Current: ±20µA
• Output Current (Sink): 8mA per segment
• Power Dissipation: 500mW

It is essential to ensure that the 74LS47 is operated within these specified limits to guarantee proper functionality and prevent damage to the device.

Typical Applications

The 74LS47 is widely used in applications that require displaying decimal digits, such as:

1. Digital clocks and timers
2. Counters and scoreboards
3. Instrumentation and measurement devices
4. Calculators and cash registers
5. Elevator floor indicators

In these applications, the 74LS47 is often combined with other components like BCD counters, multiplexers, and microcontrollers to create multi-digit displays and more complex systems.

Interfacing with the 74LS47

To use the 74LS47 in a project, you need to connect the appropriate inputs and outputs according to your application’s requirements. Here’s a simple example of how to display a single decimal digit using the 74LS47 and a common anode 7-segment display:

1. Connect the BCD inputs (A, B, C, D) to the appropriate binary signals representing the decimal digit you want to display.
2. Connect the 7-segment outputs (a, b, c, d, e, f, g) to the corresponding segments of the common anode display through current-limiting resistors (typically 220Ω to 1kΩ).
3. Connect the common anode of the display to the positive supply voltage (VCC).
4. Connect the lamp test input (LT) to VCC if not used, or to a switch for testing the display.
5. Connect the blanking input (BI) and ripple blanking input (RBI) to VCC if not used in a multi-digit application.
6. Connect the ground (GND) pin to the system ground.

By following these steps and providing the appropriate BCD inputs, you can control the 7-segment display to show the desired decimal digit.

Multi-Digit Displays and Multiplexing

For applications that require displaying multiple digits, you can use multiple 74LS47 ICs along with a multiplexing technique. Multiplexing allows you to drive several displays using a single set of 74LS47 outputs by rapidly switching between the digits and relying on the persistence of vision to create the illusion of a steady, multi-digit display.

To create a multiplexed display, you’ll need additional components such as a BCD counter, a multiplexer (e.g., 74LS157), and transistors to control the common anodes of the displays. The BCD counter generates the sequential digit values, the multiplexer selects the appropriate digit for each display, and the transistors switch the common anodes on and off in sync with the multiplexer.

The 74LS47’s ripple blanking input/output (RBI/RBO) is particularly useful in multiplexed displays. By connecting the RBO of one 74LS47 to the RBI of the next, you can create a ripple blanking chain that automatically suppresses leading or trailing zeros in the displayed value.

Frequently Asked Questions (FAQ)

1. What is the difference between the 74LS47 and other BCD to 7-segment decoders like the 7447?

The main difference between the 74LS47 and the 7447 is the logic family they belong to. The 74LS47 is part of the Low-power Schottky (LS) family, which offers lower power consumption and faster switching speeds compared to the original 7447. However, they are functionally equivalent and can be used interchangeably in most applications.

2. Can the 74LS47 drive a common cathode 7-segment display?

No, the 74LS47 is designed to drive common anode displays, where the individual segments are connected to ground through the IC’s outputs. To drive a common cathode display, you would need to use a different decoder/driver IC like the 74LS48 or 7448, which has inverted outputs.

3. What is the maximum number of digits that can be multiplexed using the 74LS47?

Theoretically, there is no hard limit to the number of digits you can multiplex. However, as the number of digits increases, the multiplexing frequency must also increase to maintain a flicker-free display. The practical limit depends on factors such as the display’s persistence, the switching speed of the multiplexer, and the desired refresh rate. Typically, multiplexing 4 to 8 digits is common in most applications.

4. How do I limit the current through the 7-segment display when using the 74LS47?

To limit the current through the display segments and prevent damage, you should use current-limiting resistors in series with each segment. The value of the resistor depends on the display’s specifications and the desired brightness. A common range is 220Ω to 1kΩ for standard 7-segment displays driven by the 74LS47.

5. Can I use the 74LS47 with a microcontroller to create a dynamic display?

Yes, you can interface the 74LS47 with a microcontroller to create dynamic displays. The microcontroller can generate the appropriate BCD values and control the multiplexing of multiple digits. By updating the BCD inputs and multiplexer control signals in software, you can display changing values, create animations, or respond to user input.

Conclusion

The 74LS47 is a versatile and widely used BCD to 7-segment decoder/driver IC that simplifies the process of displaying decimal digits on LED displays. By understanding its pinout, function table, electrical characteristics, and interfacing techniques, you can effectively use the 74LS47 in your projects to create single-digit or multiplexed multi-digit displays.

Whether you are working on a digital clock, a measurement device, or any other application that requires displaying numerical information, the 74LS47 provides a reliable and efficient solution. By following the datasheet specifications and considering factors like current limiting and multiplexing, you can design robust and visually appealing displays using this essential component.