Roman, the nice structure you've post here can be replaced with
4 pin low-drop ( typical 0.25, max 0.5V) liner regulator KA78R12 from
Motorola, in TO220 package, at current lower than 1A.
Jeff, I'm realy interested about your experiment results at 8A with this
schematic/structure.

regards, Vasile


On Fri, 11 Jan 2002, Roman Black wrote:

> 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.
>
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