Voltage regulator with alternate voltage source...

[drlegendre .]

Eric,

No, not a series diode (in series with the In or Out), but a diode
connected from Out to In, with the band on the In terminal. It provides a
moderately high current path in the event that the voltage on the Out
exceeds that on the In.

This might occur if the input voltage suddenly drops (shorted input?),
while there is a charged large-ish cap on the (lightly-loaded) output side.
The diode provides a safe path for the reverse current flow, which would
otherwise possibly damage low-current pathways within the regulator device.

On Mon, Apr 11, 2016 at 12:02 AM, Eric Smith <spacewar at gmail.com> wrote:

> 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.
>