Hi,
What does your task need to do?
If you need to regulate bright of LED then why not use duty cycle? PWM is common used way for such purpose. And if you know that subject well enough then why do you not use this simplest way?
If you need to regulate bright of LED using another technique then I may suggest to use something like DAC:
- an ULN2803, connected to one of MCU`s ports;
- R/2R resisters from ULN output to the cathode of LED;
- another LED pin is tied to +5V.

By this way you will be able to regulate up to 256 steps of LED`s bright.

Regards,
Oleg