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these LED drivers are very common and allow you to drive multiple (8 in this case) LEDs or groups of LEDs with a constant current.
this 'constant' means you can specify it (usually set by resistor value or in this case by combination of resistor value and configuration).
voltage is not generated by the IC, voltage is something that you provide from suitable power source (power supply or battery for example).
but IC can control current flowing from power source, through LED(s) into IC outputs (here outputs are sink type).
clarification about voltage:
if you want to drive several LEDs at once, you can connect them in a string and use single resistor (or constant current source).
however adding more LEDs means that you need to increase supply voltage accordingly. for example if you have LEDs with forward voltage drop of Vf=2.5V and your voltage source is 19.2V, then you connect 7 LEDs in a string (resistor or constant current source will drop remaining voltage, in this case 7*2.5=17.5V then 19.2V-17.5V = 1.7V is the difference taken by resistor or IC).
many ICs used in such applications can drive single LED per output because the outputs only tolerate voltage up to 5V.
this IC can tolerate up to 20V so you can connect multiple LEDs into a string. all LEDs in a string work together and are connected to one output. since this IC has 8 outputs, you can connect 8 strings of LEDs. 20V/2.5V = 8 but because output has some voltage drop too, you can not have 8 LEDs per string, but 7 will be fine.
however, you are not driving strings of LEDs, you are driving Background Light of a LCD display. you can connect multiple outputs together, or all eight if you like. suppose you set current to 5mA then just by enabling outputs you can vary current through LED (background light) from 0-40mA in steps of 5mA (each output you turn on, adds 5mA in this case). of course you could also change CM configuration too but you get most out of it by changing Configuration Code.