>> You can probably get a very acceptable result with a bipolar PNP and
>> two resistors.

> Isn't that more complex and with greater losses than two Schottky diodes?

Complexity did get a tip of the hat above.

With a BC337-40 (I grabbed the TH datasheet) you get typically about
0.25V Vsat at 10 mA for 0.1 mA base current.

Vsat under 0.1V (graph shows ~-0.08V) at Ibase =3D 0.2 mA
0.05V at 0.3 mA
~~~=3D 0.02V at 1 mA drive (getting desperate)

At 100 mA load you get about Vsat =3D 0.15V at 1 mA drive - quite acceptabl=
e.

So [tm] it very much depends on how much load current you want, what %
you are prepared to "waste" in base drive etc.

_________

A properly specified MOSFET can do the battery switch with NO added
parts and you use a diode on the power supply.
Rdson then becomes the arbiter of performance.
Rgs_th needs careful selection.

To get 0.1V at 10 mA you need Rdson of under R =3D V/I =3D 10 Ohms so a
MOSFET made from wet spaghetti would almost be good enough.
P Channel.
***DRAIN*** to backup battery (reversed to normal)
Source to load.
Gate to main +5V supply
Reverse fet:  FET gets forward biased but is off when supply drives
load at above Vbackup and in this mode the body diode blocks. Used
with usual polarity you get backfeed from supply. .

5V supply feeds via a Schottky to load.
Vgs_th  >=3D 2V but as low as possible above that.
Cap on load.
As 5V falls it turns on FET as =3D%V supply falls through ABOUT 3V.
Vth too low and bsupply is above battery and you get a brief backfeed.
Too large and you lose 5V before backup is enabled (briefly).
The load cap handles that (done properly)
With a FET with Rdson of  <=3D say 0.1 Ohm you have <=3D 1 mV supply drop a=
t 10 mA.
If you MUST have 100 uV drop then 10 milliOhm Rdson FETS are available enou=
gh.

A small MOSFET and a small Schottky cost about the same in
manufacturing volume if all costs are considered.



            Russell
--=20
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