Delete comment from: Ken Shirriff's blog
Really great review.
The "voltage sag" in the Apple adapters is a common technique. In the literature you'll see it mentioned as adaptive voltage positioning, line-load regulation or sometimes as non-zero load line control.
It's been around for a very long time. Here is one paper that describes it. The example is specific to CPUs but the idea holds for any power source and load. You didn't measure transient response, which is admittedly tough to do right, so you wouldn't see the advantage it provides.
http://web.cecs.pdx.edu/~tymerski/ece446/paperDroopControl3.pdf
Apr 29, 2014, 4:18:28 AM
Posted to A dozen USB chargers in the lab: Apple is very good, but not quite the best