Introduction to IC7447

The IC7447 is a BCD to 7-segment decoder/driver integrated circuit widely used in electronic displays. It converts binary-coded decimal (BCD) input into a format suitable for driving 7-segment LED or LCD displays. This guide will provide a comprehensive overview of the IC7447, its features, pinout, functionality, and practical applications.

What is a BCD to 7-segment decoder/driver?

A BCD to 7-segment decoder/driver is an electronic component that converts a 4-bit binary-coded decimal (BCD) input into a 7-bit output, which can directly drive a 7-Segment Display. The IC7447 is one such decoder/driver that simplifies the process of interfacing BCD inputs with 7-segment displays.

Why use the IC7447?

The IC7447 offers several advantages:

1. Simplifies the design process by integrating the decoder and driver functions into a single chip.
2. Provides a direct interface between BCD inputs and 7-segment displays.
3. Supports a wide range of supply voltages (4.75V to 5.25V).
4. Offers high output current capability (up to 40mA per segment).
5. Includes built-in input and output protection circuitry.

IC7447 Pinout and Pin Description

The IC7447 comes in a 16-pin DIP (Dual Inline Package) format. The following table describes the function of each pin:

Pin Number	Pin Name	Description
1	B	BCD input B
2	C	BCD input C
3	LT	Lamp test input (active low)
4	BI/RBO	Blanking input/Ripple blanking output
5	RBI	Ripple blanking input
6	D	BCD input D
7	A	BCD input A
8	GND	Ground
9	e	7-segment output e
10	d	7-segment output d
11	c	7-segment output c
12	b	7-segment output b
13	a	7-segment output a
14	g	7-segment output g
15	f	7-segment output f
16	VCC	Supply voltage (4.75V to 5.25V)

BCD Inputs (A, B, C, D)

The IC7447 accepts a 4-bit BCD input through pins A, B, C, and D. The BCD input represents a decimal digit from 0 to 9. The following table shows the BCD input combinations and their corresponding decimal values:

BCD Input (DCBA)	Decimal Value
0000	0
0001	1
0010	2
0011	3
0100	4
0101	5
0110	6
0111	7
1000	8
1001	9

7-Segment Outputs (a, b, c, d, e, f, g)

The IC7447 provides 7 output pins (a, b, c, d, e, f, g) that directly drive the segments of a 7-segment display. Each output pin corresponds to a specific segment of the display, as shown in the following diagram:

   a
f     b
   g
e     c
   d

The IC7447 outputs are active-low, meaning that a logic ‘0’ on an output pin will turn on the corresponding segment, while a logic ‘1’ will turn it off.

Control Inputs (LT, BI/RBO, RBI)

The IC7447 features three control inputs that enable additional functionality:

1. Lamp Test (LT): When the LT pin is pulled low, all 7-segment outputs are forced to logic ‘0’, turning on all segments of the connected display. This feature is useful for testing the display and ensuring all segments are functional.

2. Blanking Input/Ripple Blanking Output (BI/RBO): The BI/RBO pin serves a dual purpose. When used as an input (BI), pulling this pin low will force all 7-segment outputs to logic ‘1’, effectively blanking the display. When used as an output (RBO), this pin provides a ripple blanking signal to cascade multiple IC7447s.

3. Ripple Blanking Input (RBI): The RBI pin is used when cascading multiple IC7447s. It accepts the ripple blanking signal from the previous stage and propagates it to the next stage through the BI/RBO pin.

How the IC7447 Works

The IC7447 internally consists of a BCD to 7-segment decoder and a set of output drivers. The BCD to 7-segment decoder converts the 4-bit BCD input into the corresponding 7-bit output pattern required to display the decimal digit on a 7-segment display.

The output drivers provide the necessary current and voltage levels to drive the 7-segment display directly. The IC7447 can sink up to 40mA per segment, making it suitable for driving displays with a wide range of sizes and brightnesses.

Truth Table

The following truth table summarizes the input-output relationship of the IC7447:

BCD Input (DCBA)	7-Segment Output (abcdefg)
0000	1111110
0001	0110000
0010	1101101
0011	1111001
0100	0110011
0101	1011011
0110	1011111
0111	1110000
1000	1111111
1001	1111011

Note that the 7-segment output is active-low, so a ‘0’ in the truth table represents an illuminated segment, while a ‘1’ represents an off segment.

Cascading Multiple IC7447s

For applications that require displaying multi-digit numbers, multiple IC7447s can be cascaded together. The ripple blanking feature of the IC7447 simplifies the cascading process.

To cascade IC7447s, connect the BI/RBO pin of the first IC7447 to the RBI pin of the second IC7447, and so on. The last IC7447 in the chain should have its BI/RBO pin connected to ground or left unconnected.

When any of the cascaded IC7447s receives a blanking signal on its BI pin, it will blank its own display and propagate the blanking signal to the next IC7447 through its RBO pin. This ensures that all the displays in the chain are blanked simultaneously.

Interfacing with Microcontrollers

The IC7447 can be easily interfaced with microcontrollers to create dynamic displays. To interface an IC7447 with a microcontroller, follow these steps:

1. Connect the VCC and GND pins of the IC7447 to the appropriate power supply rails.
2. Connect the BCD input pins (A, B, C, D) of the IC7447 to the microcontroller’s GPIO pins configured as outputs.
3. If required, connect the control input pins (LT, BI/RBO, RBI) to the microcontroller’s GPIO pins or to the appropriate logic levels.
4. Connect the 7-segment output pins (a, b, c, d, e, f, g) of the IC7447 to the corresponding segments of the 7-segment display.
5. In the microcontroller’s firmware, set the GPIO pins connected to the BCD inputs to the desired BCD value to display the corresponding digit on the 7-segment display.

By changing the BCD input dynamically, the microcontroller can control the displayed digit on the 7-segment display.

Practical Applications

The IC7447 finds applications in various electronic projects and devices that require displaying numeric information. Some common applications include:

1. Digital clocks and watches
2. Timers and counters
3. Scoreboards and displays in gaming systems
4. Instrumentation and measurement equipment
5. Elevator and floor indicators
6. Vending machines and price displays
7. Automobile dashboards and odometers
8. Industrial control panels and status indicators

In these applications, the IC7447 simplifies the process of converting binary or BCD data into a format suitable for driving 7-segment displays, making it an essential component in many electronic display systems.

Frequently Asked Questions (FAQ)

1. What is the difference between the IC7447 and other BCD to 7-segment decoders like the IC7446 or IC7448?

The main difference between the IC7447 and other BCD to 7-segment decoders lies in their output configuration. The IC7446 has open-collector outputs, while the IC7448 has active-high outputs. The IC7447, on the other hand, has active-low outputs, which are the most common type used with 7-segment displays.

2. Can the IC7447 drive common-anode 7-segment displays?

No, the IC7447 is designed to drive common-cathode 7-segment displays. For common-anode displays, you would need to use a different decoder/driver, such as the IC7446 with open-collector outputs, and add pull-up resistors to each segment.

3. What is the maximum supply voltage for the IC7447?

The IC7447 is designed to operate with a supply voltage range of 4.75V to 5.25V. Exceeding this voltage range may damage the device or cause unreliable operation.

4. How much current can the IC7447 sink per segment?

The IC7447 can sink up to 40mA per segment, which is sufficient for most common 7-segment displays. However, it is essential to ensure that the total current drawn by all the segments does not exceed the maximum rating of the IC7447 package.

5. Can I use the IC7447 with a 3.3V microcontroller?

Yes, you can use the IC7447 with a 3.3V microcontroller, provided that you use appropriate level shifters or voltage dividers to convert the 3.3V logic levels to the 5V levels required by the IC7447’s inputs. Alternatively, you can use a 3.3V compatible BCD to 7-segment decoder/driver, such as the CD4511 or 74HC4511.

Conclusion

The IC7447 is a versatile and essential component for driving 7-segment displays in various electronic applications. Its ability to convert BCD inputs into the appropriate 7-segment output format greatly simplifies the design process and reduces the number of components required.

By understanding the IC7447’s pinout, functionality, and interfacing requirements, beginners can easily incorporate this device into their projects and create informative and visually appealing displays. With its wide range of applications and ease of use, the IC7447 remains a popular choice among hobbyists and professionals alike.