When is a reverse current diode protection needed for linear regulators?
Under normal operation, the input voltage of an LT3015 should always be more negative than the output voltage, e.g. -12V in, -5V out.  What the datasheet is saying is that if the output is somehow driven 0.3V lower than the input, e.g. -6V at the output, -5.7V at the input, then a parasitic diode that's part of chip substrate may/will start conducting.  You would then have current flowing in an unexpected way, and this may damage your regulator or other connected circuitry.  For example, if you hooked the regulator up like you would a positive voltage regulator, then this:







simulate this circuit &ndash; Schematic created using CircuitLab

effectively becomes this:







simulate this circuit

Under these conditions, your regulator has a good chance of functioning as an short-lived electric heater, releasing smoke or otherwise failing.  That said, if you keep your polarities correct, this is unlikely to happen otherwise.

When the datasheet is talking about reverse input/output voltage protection, it means that the output can be ABOVE ground without damaging the regulator, i.e. "If IN is left open circuit or grounded, OUT can be pulled above GND by 30V";  "In dual supply applications where the regulator’s load is returned to a positive supply, OUT can be pulled above GND by 30V and still allow the LT3015 to start up and operate."  That sort of configuration would look like this.







simulate this circuit

While both supplies are connected, the regulator provides regulation, current protection and thermal protection.  If V2 is disconnected, the regulator will safely stop conducting, and if the regulator is shut down, the OUT pin can rise above ground without damage, and the regulator can later be turned on (without disconnecting power supplies).  The important thing is that the IN pin should always be more negative than the OUT, ADJ, and SHDN pins to keep current from flowing through the parasitic diode.  If you're concerned about protecting the regulator from reverse polarity, a diode in series with your power supply should suffice, though remember that the diode will increase the dropout voltage by ~1V.