A Variable Low Drop out low noise Linear post Regulator

[ali_asadzadeh]

Hi,
I have designed this Circuit, I wanted to be able to adjust the Voltage and Current in the output of my DIY power supply. I wanted it to be Low noise, (under 500uA and 500uV) and be able to adjust the Output voltage and current, But It seems that my circuit would dissipate about 3V across it and it's a ton , in what ways Can I improve it's performance? any idea's and hints are welcomed

Also I want to figure out a way to adjust the output with a DAC from my MCU

[Zero999]

The LT3081 will do this and can be connected in parallel, if more current than 1.5A is required.
http://cds.linear.com/docs/en/datasheet/3081fc.pdf

To get down to 0V a minimum load current is required, which can be achieved by adding a current mirror to the output. R1 needs to pass more than 4mA. Tr2 will also pass approximately 4mA down to fairly close to 0V.

[dmills]

IIRC Linear published a complete bench supply around that basic idea ("Linear applications journal"?).

Regards, Dan.

[iainwhite]

LT journal power supply article:

http://www.linear.com/docs/45095

[macboy]

That 10k resistor feeding current into the BD681... if you can connect it to a voltage higher than Vin, then you can reduce the dropout voltage significantly.
The current limit is very basic, and it necessarily results in a minimum of 0.6 V of dropout. You will need an amplifier to do better. 
I'd move the current shunt and limiter to the input side of the pass transistors.  Connect the 10k resistor mentioned earlier to Vin (not the output of the current shunt). Then you reduce the dropout a little bit. Refer to my circuit below that I designed a few years ago and is very similar to yours. In my circuit, the extra gain from using two transistors in the current limiter makes it much more accurate/stable than a 1-transistor version. (R3 and R6 are a 1k pot)

[ali_asadzadeh]

Thanks all for your hints, specially macboy

[BU508A]

LT journal power supply article:

http://www.linear.com/docs/45095

I think, the bridge rectifier in Fig. 11 is a bit wrong.