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Old 27th February 2004, 09:19 AM   #11
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Thumbs up And I was starting to think every one was asleep

"this example, which raises the question: how much does changing the value of the Vout-Vadj resistor value affect the output characteristics?"

You may have asked the magic question. The fairly large programing current makes the internal 50uA current that appears from the adjustment terminal look smaller in terms of an error current. For a adustment current of 1.25 V divided by 240 ohm we get 5.2mA. The current from the ADJ terminal (used to bias the regulator's1.25 V voltage reference) represents about 1% error in relation to the 5.2mA current. This current can vary in initial value up to 100uA. When picking a set value of resistors this 1% variation must be added to the resistor tolerance for the error budget of how much the output voltage value will vary in production. Using a larger resistor value means less current and that the 50uA variation is a larger error term. The use of a voltage reference from ADJ to ground shows much less variation since the reference has a much lower impedance. An LM 329 has under 1 ohm which gives a much smaller change than varying current through a resistor of several kilohms. Using a pot to set the desired voltage works around this issue but is not practicle in large scale production, except by other means than a standard trim pot which would have to be tweakied by hand for the desired voltage.

A smaller current gives greater PSRR at low freqencies for a given sized cap from ADJ to ground, since the cap impedance smaller compared to a larger resistor value. I have seen values of 1K used instead of 120 or 240 ohms for this purpose in some audio regulators. Another thing the relatively large program current does is provide the minimum load current that the regulator needs to operate correctly. This load can easily achieved by a resistor across the output of the regulator where it is not compromising the dynamic performance of the regulator if not derived form voltage adjust circuit . The ap note circuit is designed to deal with the issue of voltage tolerance and minimum load which are easy work arounds for small quantity production. The data sheet has a good description of this current error term on page 8 of the data sheet:

http://www.national.com/ds/LM/LM117.pdf

The 1.2 Volts is even enough current for certain (very specific devices) transistor current sources to be used between Vout and ADJ, an approach I am investigating, stay tuned.............


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Old 27th February 2004, 11:25 AM   #12
rbroer is offline rbroer  Netherlands
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Default Re: And I was starting to think every one was asleep

Quote:
Originally posted by Fred Dieckmann
...The 1.2 Volts is even enough current for certain (very specific devices) transistor current sources to be used between Vout and ADJ, an approach I am investigating, stay tuned...
I've used the LM334 for this in the past, because it can operate with just 1V or up.
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Old 27th February 2004, 04:31 PM   #13
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Default Current source

I have had limited success with the LM334 from stability and noise issues but would not rule it out. I am a little reluctant to introduce another device with feedback into the feedback loop of the regulator and even would be careful about adding capacitance to this part of the circuit. I had something else in mind. A clue below............. (the graph is 1mA and 1 volt per division)
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Old 27th February 2004, 06:09 PM   #14
rbroer is offline rbroer  Netherlands
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I guess you mean something like this Fred:
(caps and gate stoppers not shown)
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Old 27th February 2004, 07:02 PM   #15
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Unhappy Uh Oh.............

You are going in the right direction.........

More questions to think about:

1. What happens when the output is shorted, will the jfet handle the peak current of the ADJ capacitor discharging through the now forward biased diode structure of the reversed polarity jfet? It very likely won't.

2. What the effect of a few picofards capacitance from the output to the noninverting input of the error amps in the regulator? As I remember Ben Duncan thought this was a significant issue when adding protection diodes across a three terminal regulator.

3. How to achieve the goal in number 1 while minimizing the effect of number 2?

4. Is the jfet still a constant current source at 1.2 Volts and less or has output impedance started to become a fairly low resistance?

5. how much does the internal current source move with temperature? Since it is a much larger portion of the current and the resistor to ground is larger, how much does the regulator output voltage change with temperature?

Not as easy as everyone thought, I'll bet. Really tweaking the last bit of performance from the LM 317 with a fairly simple circuit and with capacitor values on the ADJ terminal that won't damage the regulator is more involved than anyone thought I'll bet. The use of cap values below 5 uF would make the use of film caps practical, to avoid leakage currents in electrolytics that cause DC drift and noise. You are going to have to go further than the things the simple Spice model tells you about this part.......... That's what I get for working on protection circuit for a few years.
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Old 28th February 2004, 10:31 PM   #16
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Ive used the pretracking thingy as illustrated on the national spec sheet for the lm317 and its complementary other rail. I dont have anthing to compare it too appart from unregulated, but it was definatly better then no regs at all.
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Old 28th February 2004, 10:58 PM   #17
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Sorry that this is just a loose conceptual idea with no simulations
or measurements backing it up, and that I am taking the risk of
being wrong by posting late at night.

The idea of using a CCS from Vout to Adj and a voltage reference,
a zener a string o LEDs or whatever, from Adj to ground has been
suggested both here and in an earlier thread that I linked to in
the "parent thread" of this one. One potential problem here, that
Fred also brought up, although as a question to us rather than
a claim, is about CCSs for such small voltages as 1.2V. There might
perhaps be JFETs working in the saturation region already for such
small voltages, at least if used at a sufficiently small Id, I don't know
since I haven't checked it. An altenative would be a BJT-based CCS,
but we will probably be out of the linear region also here for most
or all BJTs. Maybe this is still sufficiently good. Howver, maybe there
is a different approach to this.

Since the regulator attempts to maintain a fixed voltage difference
between Vout and Adj, it seems to me that it is not really necessary
that the CCS is connected from Vout to Adj. Couldn't it just as
well be connected from Vin to Adj, which would give it a larger
voltage drop to work with? This may on the other hand introduce
noise since Vin is not regulated. However, if one anyway uses
a preregulator this might perhaps be an interesting approach.
Alternatively one could use an extra dedicated regulator
(an LM317L would suffice) just to feed the CCS.
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Old 29th February 2004, 04:57 AM   #18
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Wink question to us rather than a claim..... the data sheets claim.

"One potential problem here, that Fred also brought up, although as a question to us rather than a claim, is about CCSs for such small voltages as 1.2V. There might perhaps be JFETs working in the saturation region already for such small voltages, at least if used at a sufficiently small Id, I don't know since I haven't checked it."

There are jfets that are still acting in the current source mode at voltages of a volt and below. One could also use voltage reference like a LM329 biased from the regulators output, filtered, and multiplied by an op amp running from the regulated output. There are many op amp who's output will swing to less than 1.2 volts from the supply, like the AD820 or (AD823 dual) for example. I was thinking along the lines of some thing simple that used 2 or three extra transistors and a larger R (and smaller C) from ADJ to ground. I actually have a design but would like to play with the proto before posting.
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Old 29th February 2004, 06:37 AM   #19
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What are the requirements that we need? Some kind of spec to meet? I like 3 terminal regulators because they are simple and don't take up much board space. Sometimes simple means reliable. Nice benefits of the IC regulators are packaging, current limit and thermal shutdown. If circuit size, current limit, or thermal shutdown is not a requirement, use a precision reference, OPA134, and a series pass transistor. Add one more transistor and have a low voltage dropout regulator. You can have current limit/shutdown, and thermal shutdown with more added discretes.

However, the top of discussion is how to improve the 3 terminal regulator. Maybe by the time you get it improved, you won't need it.

If you want lower noise at low current levels, use an RC low pass filter near the sensitive circuit. I usually use a 10, 50, or 100 ohm resistor in series and a .1u and .01u (or some other combination)in parallel to ground. Using surface mount parts, you can really reduce the noise. I use power planes above ground planes on most of my PCBs as good quality capacitance.

Using 2 or 3 different valued capacitors in parallel at the output of a 3 terminal regulator and good PCB layout techniques is good for enough for some applications.
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Old 29th February 2004, 11:33 AM   #20
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Fred,

OK sounds good if there are JFETs that make it possible to build
low-voltage-drop CCSs. To bad JFETs are getting hard to buy
easily for many of us.

My main point however was to point to a possible alternative
route to go. All designs I can remember seeing are essentially
based on the standard connection by replacing one or both
resistors in the voltage divider with other types of components.
My point was: Do we really need any component at all between
Vout and Adj if we have some kind of voltage reference connected
to Adj. It seems we don't. Yes, this is just a loose conceptual idea
as I said and, no, it is not a full design. If that bothers some people,
sorry. I seem however to remember one of our famous talented
commecial designers asking for "ideas, simulations or
measurements" of things
recently, but that must have been in the thread this one originated
from.
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