Overview of the 7447 IC
The 7447 integrated circuit is a popular decimal decoder driver that converts binary-coded decimal (BCD) inputs into 7-segment outputs suitable for driving common anode or common cathode LED or LCD displays. Some key features of the 7447 IC include:
- Decodes 4-bit BCD input into 7-segment outputs
- Has active LOW outputs suitable for driving common anode displays
- Features on-chip pull-up resistors for simplified segment driving
- Operates across a wide 4.5V to 5.5V supply voltage range
- High noise immunity with hysteresis on inputs
- Low power consumption (15mW typical @5V)
With its built-in decoding logic and drive capabilities, the 7447 IC greatly simplifies interfacing microprocessors, counters, or other digital logic to numeric 7-segment displays. This makes it useful in applications like digital clocks, electronic meters, counting/timing circuits, scoring displays, and other embedded systems using numeric readouts.
Pinout Diagram
The diagram below shows the standard 16-pin DIP pinout and signals for the 7447 BCD to 7-segment decoder/driver IC.
| Pin | Symbol | Function |
|---|---|---|
| 1 | BLANK | Blank display input (active LOW) |
| 2 | LT | Lamp test input (active LOW) |
| 3 | RBI | Ripple blanking input (active LOW) |
| 4 | RBO | Ripple blanking output (active LOW) |
| 5 | GND | Ground |
| 6 | C | Carry out |
| 7-10 | A-D | BCD digit inputs |
| 11-14 | a-g | Cathode segment outputs |
| 15 | VCC | Positive supply voltage |
| 16 | OE | Output enable (active LOW) |
Functional Block Diagram
The internal block diagram of the 7447 IC consists of BCD input latching and decoding logic driving the 7-segment output buffers. Active LOW blanking and output enable controls are also provided.
7447 Internal Block Diagram (Image Source: electronictutorials.ws)
Logic Table
The logic table below shows the 7447’s BCD digit input to 7-segment output decoding scheme. A ‘0’ activates that segment output when enabled.
| BCD Digit Input | Segments Activated |
|---|---|
| 0000 | 6 |
| 0001 | 2,5 |
| 0010 | 5 |
| 0011 | 2,4,5,6 |
| 0100 | 1,2,4 |
| 0101 | 2,4 |
| 0110 | 2,3,4,6 |
| 0111 | 2,3,4 |
| 1000 | 1,2,3,5 |
| 1001 | 2,3,5,6 |
| 1010 | 1,3,5,6 |
| 1011 | 1,3,5,6 |
| 1100 | 1,2,6 |
| 1101 | 3,6 |
| 1110 | 1,6 |
| 1111 | None (blank) |
Electrical Characteristics
The DC electrical characteristics for the 7447 IC operating at 4.5V to 5.5V supply voltage include:
| Symbol | Parameter | Min | Typ | Max | Unit |
|---|---|---|---|---|---|
| VCC | Supply voltage | 4.5 | – | 5.5 | V |
| ICC | Supply current | – | 15 | 30 | mA |
| IOL | LOW level output current | – | – | 25 | mA |
| VIH | HIGH level input voltage | 2.0 | – | VCC+0.5 | V |
| VIL | LOW level input voltage | -0.5 | – | 0.8 | V |
| VOL | LOW level output voltage | – | – | 0.4 | V |
| IOH | HIGH level output current | -4 | – | – | mA |
Typical Applications
The 7447 IC is commonly used to interface microcontrollers to 7-segment LED or LCD displays in embedded systems. Some typical applications include:
- Digital clocks
- Electronic counters/timers
- Frequency meters
- Multimeters and other measurement equipment
- Numeric displays in appliances, instruments, etc.
A basic application circuit driving a common anode 7-segment LED display is shown below. The microcontroller provides digitized BCD data to the 7447, which activates the appropriate segments. Current limiting resistors are used for each LED segment.
7447 controlling common anode 7-segment LED display (Image Source: electronictutorials.ws)
Multiple 7447 ICs can also be cascaded to drive multi-digit displays as needed for counters, clocks, etc. Additional logic is required to multiplex the digit enables when scanning through multi-digit displays.
Comparison with Other ICs
Related display driver ICs comparable to the 7447 include:
- 7446 – Also a BCD to 7-segment decoder, but has common cathode outputs
- 7448 – BCD to 7-segment decoder with active HIGH outputs
- 74249 – BCD to 7-segment decoder with buffered outputs
- MAX7219 – LED display driver with serial interface
The 7447 provides a simple, low-cost solution suitable for most small LED or LCD numeric display applications. Where higher drive currents are needed, the 74249 or MAX7219 offer buffered outputs that can source more current per segment.
Buying the 7447 IC
The 7447 can be purchased from major electronic component distributors including Digi-Key, Mouser, Future Electronics, and Arrow. It is available in through-hole DIP and surface mount SOIC packages from multiple manufacturers. Some part number examples:
- NXP 7447N – DIP-16 package
- Texas Instruments SN74LS47N – DIP-16 package
- ON Semiconductor MC74VHC4477FN – SOIC-16 package
Pricing for the 7447 in single quantities is around $0.60 to $1.20 depending on manufacturer and package type.
Frequently Asked Questions
What is the difference between the 7447 and 7448?
The main difference is the 7447 has active LOW outputs suitable for common anode displays, while the 7448 has active HIGH outputs for driving common cathode displays.
How do I drive a multi-digit display with the 7447?
Use multiple 7447 ICs, one per digit, and digit enable logic/transistors to selectively activate one 7447’s outputs at a time. Shift register ICs like the 74LS164 can be used to control the digit enables.
What precautions should be taken when using the 7447?
- Add current limiting resistors for each LED segment
- Limit output current to 25mA per segment
- Decouple the power supply pins with a capacitor
- Ensure input voltages remain within specified limits
Can the 7447 directly drive LCD displays?
No, the 7447 cannot provide enough drive current for most LCDs. Use transistor buffers or a specialized driver IC like the HD44780 LCD controller.
What is the RBI and RBO pins used for?
RBI and RBO allow cascading multiple 7447 to create longer rippling blanking effects when desired. Leave unconnected if this feature is not needed.
