7segment Display driver IC selection and connection

7 segment display are of type common:

1. Common Anode
   
2. Common Cathode
   

Display can be:

1. Single Digit
   
2. Multiple multiplexed digits
   

In these cases, the current to be driven through these segments and digits is more as compared to the current source/ sink capacity of micro-controller GPIO pins. Hence the drivers come in to picture.

Drivers are of types:

1. HIGH side driver IC (UDN2981)
2. LOW side driver IC (ULN2803)

Common anode configuration:

1. It is better to go with LOW side driver, as the cathode of each segment can be connected to separate darlington pair inside driver IC to sink the current.
2. At the anode side, single PNP(BC557) transistor can  be connected. In case of multiple digits, separate PNP transistors to be connected for each digit. 
3. Base of each transistor will be controlled by GPIO through a series base resistor
4. GPIOs will also be connected to driver input through a series current limiting resistor

Common cathode configuration:

1. It is better to go with HIGH side driver, as the anode of each segment cab be connected to separate darlington pair inside driver IC to source the current.
2. At the cathode side, single NPN (BC547) transistor can  be connected. In case of multiple digits, separate NPN transistors to be connected for each digit.
3. Base of each transistor will be controlled by GPIO through a series base resistor
4. GPIOs will also be connected to driver input through a series current limiting resistor

In both cases care to be taken:

1. Care should be taken that only 1 digit is ON at a time.
2. All n-digits can be displayed by maintaining high refresh rate of the display (eg. 180Hz)
3. Based on Vce voltage and Isource/sink, the Power(Pd) dispation through every pin of driver can be estimated. Pd(W) * (degC/W) rating of the driver should be less than the Maximum junction temperature rating of  the driver. Otherwise driver IC is on the "Stairway to heaven" :)

More better option,

1. There are also driver IC options like MAX7219, which have both the HIGH side and LOW side driver in the package. 
2. MCU communicate to driver over SPI, sending digit to be displayed.
3. Driver takes care of refresh rate (eg. 500Hz for 8 digits)
4. Driver handles the intensity of digiti display (Layman terms  'brightness')
5. MCU dont need to update all digits at all time, even if one digit in the number is to be modified.

Diagram where all options are merged into one: