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Old 18th October 2011, 06:12 AM   #1
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Default Need a little help with a regulator circuit if you are willing.

I bought a little low voltage regulator kit from Glassware Audio design for a project a while back and ended up not using it. I had specified a 5v output. I would like to use it to power a DAC but I need 15v out. I looked up the LD1085P datasheet to figure out how to bias this for a different voltage but I'm having a little trouble figuring out what Vref represents. I want to use this reg kit with a 16 volt transformer. The rectifier is a full wave bridge so I will have approximately (16x1.41) 22.56 volts going into the regulator and I want 15v out. The math is: Vo = Vref (1+R2/R1). The equation is not a problem I just don't get what Vref represents. Here is a link to the datasheet:

Download Datasheet | Datasheet Archive

Any help is appreciated.

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Old 18th October 2011, 06:36 AM   #2
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O, that is a bad datasheet! They forgot to add the line specifying the Vref value.

Still, the LD1085 is obviously a copy of the LT LT1085 (3A, 3-terminal regulator). This, along with many others of its kind, use a Vref of 1.25V. you should find it works well if you plug 1.25 into your formula.

http://www.linear.com/docs/Datasheet/108345fg.pdf
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Old 18th October 2011, 06:49 AM   #3
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ST seem to be too inclined to bad datasheets lately. They produce some pretty good transistors, which are spoiled by lack of proper specification limits.

There is another big concern about the LD1085 - the "stability" plots on pages 12 and 13.

The meaning of these is not well defined, but suggest that you have to choose an output capacitor with a controlled ESR and value (within fairly narrow limits) - or the chip is unstable (or has bad transient response). This "spec" pretty much narrows down to using a 1 to 68uF tant, or a 10uF ceramic with a 1-ohm resistor in series.

This kind of output cap will be the last thing you need in a DAC, (which draws plenty of sudden current spikes).

These kind of shortcomings in 3-terminal regulators are not unusual, and help to explain why many DIYers find the trouble of building a discrete shunt regulator brings big sonic rewards!
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Old 18th October 2011, 07:01 AM   #4
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Thanks for the replies guys.1.25 it is. The schematic of the power supply shows a 680uF cap on the output side of this regulator Is that going to be a problem for a DAC with its own onboard regulators? It says in the description that I just need to supply a clean 15Vs.
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Old 18th October 2011, 07:31 AM   #5
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It's worth having a look with a scope, to see that the regulator reacts OK to load steps (start/stop of loud music).

It will be fun to experiment with the caps. Try the 680uFs, and then the 10uF tantalum recommended by the chip vendor. The one that sounds best is the one to keep!
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