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Thread: Power supply project
www.piclist.com/techref/power/actodc.htm?key=power
face picon face BY : email (remove spam text)(Olin Lathrop)



Apptech wrote:
> I used an arrangement a few years ago which I saw also used
> recently in a circuit from AFAIR Olin. I had a very nice LDO
> linear regulator  with a dropout of a few tenths of a volt.
> I placed a transistor across the regulator (say NPN for the
> following example) with emitter to ouput and base via  a
> resistor to input. Collector via a resistor to a higher
> voltage. When across-regulator voltage exceeds Vbe the
> transistor turns on and pulls the collector down to Vout. Or
> use a PNP and reverese base+R and emitter connections and
> collector will pull high from ground when regulator drop is
>  > 1 x Vbe. Use this to control smps pre-regulator. Worked
> very well..

Yep, I've done this a bunch of times and it's always worked well.  One
example I can show publicly is in the new ReadyBoard-02
(http://www.embedinc.com/products/ready02/index.htm) schematic.  You can see
the PNP transistor around the 5V LDO on page 1 and the same topology around
the 3.3V LDO on page 3.  The feedback signal from page 1 is also used to
control the buck switcher on page 2 as either of these could be producing
the 6.3V main power rail.

One additional wrinkle in this case is that you have to consider the
behavior of the linear regulator with partial input power.  This is
generally not specified.  It is possible that the output stays off until the
input gets to some threshold.  In that case the PNP would come on because
the input is more than the E-B drop, which would prevent the output from
ever coming up.

There are several strategies to get around this.  One obvious one from
looking at the schematic are the dividers on the PNP collectors.  The output
of the dividers go into a the comparators in the 10F, with the other side of
the comparators tied to the internal 600mV fixed reference.  So for the
switcher output to be considered high, not only must the PNP transistor
conduct, but the switcher output must be high enough to be 600mV after the
divider.  This guarantees that the linear regulator input is reasonably
close to its valid range where it should be producing some output.  In
practise I know these MCP1700 regulators have FET pass elements, and
essentially pass input to output even if the input is a few 100mV less than
the regulation voltage.


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