It's not a separate NFB mechanism. The way the error is sensed is the same, only the internal pass transistor is not the only one under control. The external transistor is controlled by the chip's input current.ruffrecords said:So it seems to me there is a separate feedback mechanism operating where the the BJT senses the regulator output current independent of the feedback via the diff amp in the regulator.
Some regulator IC's provide connections for this configuration. Just read recently about it, haven't kept the link though...To me, the more obvious topology would have been to use and NPN BJT as an emitter follower so the regulator only has to provide bas current like this:
The regulator raises the NPN base to ensure it is turned on enough to maintain 1.25 across the resistor between the OUT and ADJ terminals and the NPN definitely IS in the feedback loop.
But of course this will not work because with a three terminal regulator we cannot connect the actual supply output to the diff amp input so you have to invent some other method (such as the one in the first pic). To do the emitter follower version you need a four terminal regulator??
EDIT: found the link
https://www.analog.com/en/technical-articles/increasing-output-current-of-the-ultralow-noise-ultrahigh-psrr-lt3042-200ma-linear-regulator.html#