The LT part is very clever. What makes it unique is a reference current
instead of a reference voltage. You then develop a reference voltage with
an external resistor, which allows you to go down to 0V. You can also
drive the reference input with an external voltage source (like a DAC) to
get a programmable regulated supply.

The LDO capabilities could use some improvement. They have separate
supplies for the load current input and the remainder of the circuit. The
output is a voltage follower, so the output is always Vbe below the base.
I don't know why they did not use a PNP and pull the base down to make it
conduct. It seems like they'd have a lower dropout voltage that way.

On current sharing, I think that works with these better than LM317 style
regulators because the outputs are guaranteed to be precisely the same
voltage since the reference inputs are tied together. With voltage
reference chips, you have voltage tolerances on the reference causing a
mismatch. Here current tolerances on the reference current do not matter
if the reference pins are all tied together and a single voltage set
resistor used.

Very clever design! To me, the big deal is use of a reference current
instead of a reference voltage.

Harold





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