This is an article published around 2005. I can’t remember where I found it. Thanks to the author.

Introduction
LEDs are current driven devices.It is relatively simple to drive several LEDs individually. However,as the number of LEDs increases,the amount of resources needed to operate these LEDs will grow to an unmanageable level. As such, LEDs are often arranged in matrices in order to make efficient use of resources.
In a matrix format, LEDs are arranged in rows and columns. This arrangement will be discussed in more detail later. What should be noted here is that the matrix arrangement demands that LEDs bedriven in multiplex. The multiplex sequence inevitably requires more complex processing but is more efficient compared to individually driving each LED.
This application note will also describe how the brightness of each individual LED can be controlled in multiplex mode. It involves dividing the LED driving sequence into three levels in the time domain. The last section will introduce several ICs that are widely used in driving LEDs.
This note is intended to support the design of messaging and video systems using LED tiles. However,the concepts and techniques introduced here apply to any LED matrix including arrays formed using discrete LEDs.This application note is especially relevant to these products and applications: read more