Voltage regulator with alternate voltage source...

[Eric Smith]

On Sun, Apr 10, 2016 at 8:54 PM, drlegendre . <drlegendre at gmail.com> wrote:
> I've often wondered why the back-current diode isn't incorporated into the
> die of all common linear regs. Is it that costly, or simply impractical due
> to die space or other considerations?

Because it would introduce additional voltage drop, and thus
additional power dissipation.

> In fact, I believe that some of the more modern regs do employ it.. just
> not the old-standby like LM317, 7805/7905, and so on.

Some of the more modern regulators may be protected to some degree
against votlage supplied to their output terminal, but it's almost
certainly not being done by a diode in series internally.