I agree this is a quick intro to digital power management. There is indeed a cost addition to implementing ADC and DAC in the power management IC.
@t.alex: the software version of this in a PLD would not necessarily give you the cost benefits and ease of implementation.
I would recommend looking up Power One's Z-One digital power management app notes, it has much better description of digital power management using I2C.
Dr. MP Divakar
This is a good quick review or introduction to PID-based feedback. Only takes a few minutes too! However, many other implementations are possible (for example the Lattice Power Manager devices have a closed loop trim function that uses a different algorithm), but it's good to keep in mind what PID does and how it does it.
Digital feedback and PID filtering is great, but not without cost. That little block called "ADC" might be a modest little thing or it might be large, complex beast, depending on the application requirements and parametric tolerances that need to be met.
Just because something analog can be replaced with something digital does not always mean that it should be.