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Old 19th August 2006, 02:37 PM   #1
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Default Two regulators in a linear design?

I was wondering if there would be any real advantage to a linear PS design in using two regulators in series, say two LT108x types. The output of the first would be the input to the second. It seems like there might be some potential positive side effects (noise?), but apart from additional cost I suspect there are probably downsides as well. Any guru enlightenment appreciated.
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Old 19th August 2006, 05:51 PM   #2
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I'm no guru, but:

Yes, it's a good idea, and is common enough that you can even find it in the LM317 datasheet on page 19.
http://cache.national.com/ds/LM/LM117.pdf

This technique is described as a "tracking preregulator" and you can search on this if you like. Here's a really good thread:
Improving the LM3x7 regulator circuit
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Old 20th August 2006, 09:20 PM   #3
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Quote:
Originally posted by leadbelly
[B]I'm no guru, but:

Yes, it's a good idea, and is common enough that you can even find it in the LM317 datasheet on page 19.
Thanks leadbelly. I couldn't find anything like this on the Linear website.
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Old 20th August 2006, 10:20 PM   #4
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There is no value in doing this. All it will do is cost more money, and dissipate more power, because the second regulator needs a few volt across it in order to operate correctly.
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Old 20th August 2006, 10:49 PM   #5
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sawreyrw,

Take a second look. There are some advantages... real ones. With two regs, one can handle line variations, the other load variations. If done right, with sufficient capacitance between the regs, significant gains can be made in line rejection.

Look at the graphs for line rejection versus frequency for typical 3 term regs. They get pretty lame around at 20 kHz; 40 dB rejection being common. You can effectively double this by using regs in series.

Now whether or not a given application requires this is another story altogether. An amp with 80dB PSRR across the band probably woulkd benefit little.

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Old 20th August 2006, 11:10 PM   #6
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Quote:
Originally posted by poobah
With two regs, one can handle line variations, the other load variations.
Now that's a good idea I hadn't thought of. The requirements for line and load regulation are rather different, so it ought to be possible to better the doubling of rejection you've suggested, even if you design the first regulator to be a low drop-out voltage regulator (to minimise the inefficiency alluded to by sawreyrw).
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Old 20th August 2006, 11:20 PM   #7
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Yep,

Along slighlty different lines... one of my favorite tricks for distributing power and signals from board to board in bundled wiring is to send out a constant current from the PSU. Then, placed a shunt reg. at the point of use. This keeps the power current in the bundled group stable and balanced.

It does present some problems with open sockets and hot-swap boards etc... you need to clamp your upstream current source.

I think JC uses the same concept in the Blowtorch, but for other reasons. The concept of splitting the line/load regulation in two remains constant though.

Do Anode and Cathode like the new venue?


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Old 20th August 2006, 11:36 PM   #8
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Together, my female cats (Anode and Cathode) weigh as much as my partner's cat, Oscar. Unfortunately, he's a bit of a wusscat and backs away when threatened by a cat half his size. Interesting times lie ahead...
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Old 20th August 2006, 11:51 PM   #9
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Suppose you might rename Oscar? Something like "Grid" maybe?

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Old 21st August 2006, 12:30 AM   #10
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No, terrible. There's no way the Wusscat is a control grid. And '"She who must be obeyed" wouldn't allow it.

But to get back on topic... Cascading regulators is a good thing except for the efficiency.
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