Output Short Circuit Current Limit in a Super Regulator
A voltage regulator is designed to provide a fixed voltage to any load. Inherently this means it must provide any current through that load to satisfy Ohm's Law of I = V/R. As R approaches zero, for example when the output is shorted to ground, for a fixed V then I approaches infinity. Thus the regulator output will, without some imposed current limit, attempt to provide infinite current. Physics takes over and typically the output device vaporizes its internal wiring or fuses the device into a short circuit and some other component burns.
The Standard Solution
Monolithic voltage regulators typically use a current limiting technique as seen in the schematic here:
Note: this circuit was previously called a foldback limit but in fact is not one. A foldback circuit not only limits current in a short circuit but also reduces its value. Sorry for any confusion!
The small resistor in the emitter circuit of the output device is used to sense the current flowing to the load and, when current is high enough the voltage across it turns on the limit transistor. This limit transistor then steals drive current from the output device, which limits output current.
You can see a fundamental limitation to minimum output impedance is the
sense resistor used in the current limiter. The impedance can be no lower
than the sense resistor, which is typically between 0.1 and 1 Ohm
The Innovative Solution
Belleson has developed an ingenious solution to this problem which does not require a sense resistor in the emitter circuit. It instead senses a voltage difference internal to the regulator, thus allowing the emitter to connect directly to the load. It would be great if we could illustrate with a schematic this clever innovation but it is, unfortunately, proprietary information. Yet we can definitely say the speed, dynamics, stability, low noise and low output impedance performance of Superpower are not compromised at all until the protection circuit begins to limit the current.
There's always a compromise, what is it?
OK, you're right. There is a compromise, yet it's a small one and it's not at the output. The SPL drop-out voltage increases to about 4V at 2A vs. 1V at 2A for the unprotected SPJ.
How Does Protection Compare To Monolithics?
Monolithic regulators have, in addition to a short circuit current limiter, a thermal limiter. Superpower does not have a thermal limiter. This means under short circuit conditions the output device must be heat sinked adequately to dissipate power of Vin * Isc without overheating the output device.
Also, Superpower maximum short circuit current varies due to parameter variations in the component used to limit the current.