(sorry re-posted to fix the ascii circuit)
Byron A Jeff wrote:

> Now we're getting somewhere. This is the issue I want to address. Please
> take a stab at explaining the controller using the 7805. I know that it's
> possible to use the GND pin as an adjust pin. However I just don't see how
> to get over the dropout voltage issue. I just took a look at the National
> Semi LM340 datasheet, and even with an output current of 0A it starts to
> drop out about 1.5V above the regulation voltage. So how can it be used as a
> control element when Vin is in the dropout region?

Wow! That was a long post. :o)
Here's my views on building a low dropout series
regulator as simple as possible.

First thing is separate the thing into two parts:
* power (obviously a low sat power device)
* control

ONLY the power device needs to be low dropout. The
control circuit can be anywhere, and you put it in
the best place, referenced to the zero rail.


                      Low sat PNP
Vin -------------------------E C--------- Vout
(13.8v)     |           |     B     |     (Reg 12v)
            |           R     |     |
            |           |     |     |
            |           |-----|     R1
            |                 |     |
            |  7805           C     |
            |--I  O---------B       |
                G             E-----|
                |        NPN        |
                |                   R2
                |                   |
                |                   |
Gnd -------------------------------------


Our power stage is just a low sat PNP like a BD204
or better part. The entire control system is referenced
to Gnd, and a 7805 is used as a cheap easy precision
zener, giving exactly 5v no matter what the input
voltage does.

R1 and R2 are a voltage divider tied to Vout. They
set the NPN emitter at 5v-0.6 =4.4v whenever the
output is at the correct voltage. Any less than the
correct Vout, and the transistors will turn on as hard
as needed to cause regulation or saturation, whatever
comes first.

Make sure the R1/R2 divider passes more than 10x
the max expected PNP base current to keep regulation
fairly accurate. If you keep this current substantially
higher than the PNP base current the regulation will
be close to perfect, at all voltages right down to
saturation.
-Roman

PS. It will work just as good with a P type FET
as the power part, if Vin is 12v or higher.

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body