So, keeping things up to date...I hope nobody thought I'd given up.  I've made a few changes following input from you guys, particularly Jon.  I now have a supply which will go to the full 15V and 3A!

Note the iMon current and U3/U4 temperatures: I did indeed blow those analog pins when I was probing but as far as I can tell the Linear Regulators are fine.  Fortunately, the zener diodes (I think) protection on the 4Duino saved the chip.

The issue that I was having was caused by an LT and data sheet application note error in the schematic.  They have tied the iLim pins of the linear regulators together with a 3k92 Ohm in parallel with a 5K pot, both then in series with a 100Ohm.  The calculation they conveniently provide buried in the data sheet is 360mA / 1kOhm + 450Ohms = output current and even that isn't clear!  That gives current = (2197 + 100 + 450) / 1000 * 0.36 = 0.989A per linear regulator.  I'd used a 3K3 (1%) and a 5K pot (actually 4973Ohms) so I could achieve: (1984 + 100 + 450) / 1000 *.36 = 0.912A per Linear Regulator = 1.82A which, not coincidentally, was what I was achieving with the 5Ohm load.  Clearly that's enough for 10Ohms which only needs 1.5A.

The annoying thing is that using LT Spice with LT components and an LT schematic (and my updated schematic) shows it working perfectly fine with the parallelised resistors!  The lesson here is that you should still do the calculations if they are given, you can find them and understand them! 

Doing the calculation with just the 5K pot would give a total of 3.996A.  I desoldered the 3K3 and bingo.  Sort of....rise time to nearly 3A (actually 2.9 eventually) was around a minute.  In fact it never got there and was stuck at <15V and <3A (actual values following ohms law based on 5Ohm load)  Not good - I have access to a very simple dummy load that was able to draw 3A out of it fairly easily - it wasn't stuck with 5Ohm resistance - so I knew it could do it.

I'd also looked at the feedback circuit with some more simulation - I mention this in a comment above in response to Jon.  I can't yet change the Inductor or the Output cap but I have bodged in a 22nF cap across one of the 100K feedback resistors and re-routed the 1nF capacitor with a bit of wire (and cut the trace to its original destination).  Firing it up and it shoots immediately to 15V, 3A!  Yay.  It still drops to 0V and 0A as well through the voltage and current controls.

Ok, so I think I've resolved the output functionality, leaving the noise.  Here's some piccies.  Firstly, ripple at the output terminal with the 5Ohm load.  Pretty poor BUT I don't see the switching spikes.

Next, ripple at the output terminal with the 10Ohm load:

If you look carefully it is possible to see a bit of the switching noise just before the rise to a peak - I'd assume it is there on the 5Ohm waveform as well but the timebase is too large to notice it.  Still poor ripple but the switching noise looks a lot more under control in my opinion (I'd be interested in other's opinion though.)

I also checked the voltage on the feedback pin of the Switching Regulator, Vfb.  The whole point is to try and bring that under a bit more control.  Prior to the bodges:

This is ranging from 460mV to 2.16V (10Ohm load).  After the bodges with the 10Ohm load:

It's now ranging between 0.97V and 1.37V - firmly in the range for a 200kHz operation - I should say, I think it is still switching at 100kHz, I need to do more measurement.  LTSpice (which I'm not sure whether I trust or not anymore) has this at 1.15 to 1.22V but I haven't modelled the output cap ESR or parasitics as per the actual build.  Waveform is similar though.  It gives me a bit more confidence that changing the Inductor and output cap for low ESR along with, say, a 20nF on the output, the Vfb would improve.

There's still the output ripple though.  I need to find a way to squash that so this is what I intend to do:

• Redesign the PCB layout, particularly parts position and the grounding. 
• Add in low ESR Input and Output caps for the Switching Regulator - I've found some that are 28mOhm.
• Add in a 20nF cap to sit alongside the Output Cap
• Provision optional pads for 2 more Output caps of 100uF (but could be other values)
• Change the inductor to 50uH to help reduce output ripple
• Test points for various pins so I don't blow it up anymore.
• Use small pots for the feedback circuit so I can make adjustments.  I think I'd ultimately want to swap these for resistors at some point so I shall design around that.

I'm hoping to make use of LT's samples provision to get replacement parts to save cost!  I want to keep the existing board as-is for comparison purposes and I can't be sure the parts I have are not damaged in some way.  Remember, the first thing I did was solder the Schottky in the wrong way and blew up the Mosfet and sense resistor - it could just as easily have damaged the switching regulator. 

I have been reading about proper grounding, including a paper from Analog Devices (LT.)  It's difficult: there are definitive recommendations for the use of a ground plane but just as definitive recommendations for star grounding (Gene, for example, is a definite fan of star grounding as he's commented above.)  Analog Devices are very much in the ground plane camp.  One thing I'm fairly sure of, and open to correction, is that copper fills on the top layer that are via'd to the ground plane and used as a grounding plane by the top layer components is not a good thing - I think that's a capacitor.  So I intend to tie ground pins with a short track and a via.  Is it worth, then, 'filling the gaps' with copper on the top layer?

Finally, I will re-layout the PCB but I have previously gone with 2-layer to keep the cost down.  That inevitably means I have to place tracks on the bottom layer which, if I go with a ground plane, will cause breaks - you can see in the images earlier in the comments.  Should I bite the bullet and go for a 4-layer board - I could keep the current carrying tracks on one layer and signal tracks on a different layer?

Thanks Jon - I did think you meant the error amplifier but the way you drew it I just wanted to check!

As for the output caps, it was really based on the existing schematic and the simulation on LTSpice.  I neglected to choose low ESR ones for the input and output capacitors.  Calculating ESR from tan-theta for these at 100Hz (so meh!) gives ESR of around 1.5Ohms.

I had seen the noise graph - naively I thought I could do better given the LTSpice simulation.  Where I'm getting to know is not dissimilar: see my follow on comment at the end of this thread where I've made a bit of progress. 

I had intended the project to be a learning exercise and to end up with a useful bench power supply.  As I said at the beginning, I'm right at the edge of my comfort zone (actually, fallen off the cliff now but I haven't hit the bottom yet!) so I expected to learn a fair bit and it is absolutely achieving that goal.  If I end up with something restricted in use then I guess so be it, but I haven't finished yet.  There's optimism for you  

When you mention 1.2V as the reference do you mean 1.19V or 1.28V?

I meant 1.19V. [They were obviously trying for 1.2V, missed slightly, and I'm being polite and pretending they hit their target.] 1.2V is very common and you'll see it a lot - it must be something to do with what's possible in a chip with a bandgap reference. The partial diff amp I drew is the one marked EA on their functional drawing - it's what is generating the error term.

The [controller] compensation doesn't roll things off quite as fast as I would have expected (assuming I'm simulating it right - I'm a bit confused by the additional bits and bobs hanging on the output of the transconductance error amplifier) - it looks like it's about 40kHz or 50kHz before the gain is down to its minimum value. [The internal compensation comes from the Cc capacitor and Rc resistor.]

Be careful of how much capacitance you add in the feedback circuit - too much and the dominant pole of the controller compensation won't necessarily be dominant anymore.

The feedback circuit is an amplifier with a gain of 2.5 (the potential divider at the base is a gain of 0.5 and the ratio of collector resistor to emitter resistor is 5, giving 2.5 overall). You might experiment with lowering the gain a bit to give the curve a lower slope. That might help with not running into the overvoltage comparator or, the other way, having the clock slow down.

There's an offset because of the Vbe. That may, possibly, be beneficial - I haven't looked at it carefully. If you wanted to remove it partially, you could add a diode in series with R5 to counter it - only partial cancellation because the current is lower.

How did you arrive at a value of 100uF for the output capacitor? [If you've got some spare ones, it would be quite easy to add a couple more if you scraped away resist. That would divide the ESR in three and triple the capacitance (roughly). Might be an interesting experiment to try.]

Do you think perhaps I'm flogging a dead horse with this feedback circuit and this Switching Regulator?  If so, I can head back to the drawing board and try and work out a different approach.

I suspect you may end up needing to give it more headroom than the original, in order to get around the problems that this controller has introduced, so you might find the regulator dissipation becoming an issue. It also depends on what you want it to do - were you hoping to control it from the microcontroller so that it works as a fast (variable) voltage source? Or is the output essentially static and just needs to respond quickly to load changes?

I think the different behaviour when it comes up is possibly the regulators catching up with the controller more quickly in your case than the original. But the way it dips in both cases isn't very nice for a PSU.

What are you going to be using it for? The last page of their design article shows the noise. They say it's good "for a mixed-mode" device. Whether that's true, I've got no idea, but it's poor compared to even a cheap analogue supply. They haven't made much attempt to deal with the high-frequency spike noise other than what comes from doing a reasonable layout. So you've got work to do there if you want it for powering sensitive analogue stuff (it would be fine for anything you'd normally power from a switching supply, obviously).

Thanks Jon, that's very useful and I can follow that easy enough.  When you mention 1.2V as the reference do you mean 1.19V or 1.28V?  The former seems more tied in to the adjustment of output (through the Error Amplifier), but the latter is how the functional diagram is shown with the OV comparator as per your sketch - but seems to be used to limit transient overshoots (1.19V + 7% = 1.28V).  I'm assuming you are rounding 1.19V for ease. 

I don't want to change my circuit physically just yet - a colleague is bringing a DC load on Monday to try out and I want to make sure its working (he's bringing it from Yorkshire!).  Having blown Arduino analog pins yesterday I'm now unsure of the state of either the LTC1624 (from the incorrectly soldered Schottky which blew, at least, the Mosfet and sense resistor) nor the LT3081s.

What I have been doing is playing around with the feedback circuit in LTSpice and I can stablise the Vfb significantly - keeping it at 1.19V:

Before (compare to measured actual in my comment below which changes from 696mV to 1.46V):

Whether or not it improves things in reality is anyone's guess until I try it: Vout is stable in both setups so I can't really trust the simulation but I have nothing else to go on.  However, my thinking is that if the feedback isn't stable around the reference voltage of 1.19V (1.2V) then it's going to struggle with ith-run and compensating the output.

The changes that make the most improvements are re-routing C6 and adding C4 (this is the LT Spice circuit NOT the schematic!).  I have also set the ESR value for C8 to 0.028 Ohms (I can get a replacement cap with that value at 100kHz which if I've understood correctly would be better at a higher frequency) - the data sheet for the existing one doesn't state the ESR but judging from its tan-theta at 100Hz, it's almost certainly too high.  Changing the Inductor to 50uH makes a much smaller improvement.  I've already physically tried a 0.1uF capacitor alongside C8 but it made no discernible difference.

Ultimately, I will need to re-layout the board as well, change grounding and so on.  But the above changes are readily implementable so I could see the impact of the changes first to make a judgement call.

Do you think perhaps I'm flogging a dead horse with this feedback circuit and this Switching Regulator?  If so, I can head back to the drawing board and try and work out a different approach.

I'd appreciate if you could clarify the feedback circuit in some more detail.

Here's how I would look at it. Depending on how much electronics you are familiar with, you might need to

concentrate a little to follow it, but none of it is difficult and we can do it all in our heads without

using a calculator. You do need to understand Ohm's Law and, ideally, have an idea of how a bipolar

junction transistor works.

Here's the feedback circuit reproduced from your schematic

The dc-dc controller has a voltage control loop that will try to adjust its output voltage so that the

difference between Vfb and the internal reference voltage (1.2V) is zero. If Vfb is higher than the

reference, the output will be reduced until they match. If Vfb is less than the reference, the output will

increase until they match.

If you were to connect the output directly to Vfb, then the output would end up the same as the reference

voltage. If you had a potential divider made up of two equal value resistors, so that half the output

voltage was fed back to Vfb, the output would be adjusted to twice the reference (2.4V). It's fairly easy

to see how you could set it up to give whatever voltage you want with just a pair of resistors.

That's how the controller would normally be used. In this case the feedback circuit works in a different

way.

To analyse the feedback circuit you've got, let's assume that the whole thing does actually function

properly and the loop has settled with Vfb equal to the reference (1.2V). Working backwards through the

feedback circuit, that means the voltage across the 5k resistor (R4) must be 1.2V. We could then use that

to work out the current in the resistor, but we don't really need to - we simply observe that the current

in the 5k collector resistor is going to be more or less the same as the current in the 1k emitter

resistor R3. It's about 99.5% for the transistor you've got there, but that's close enough to 100% for

this rough analysis. [That comes from the basics of how a BJT works.] The voltage dropped by the 1k will

be a fifth of that dropped by the 5k. A fifth of 1.2V is 240mV. The voltage across the top 100k resistor

R5 must then be the 240mV plus the Vbe drop of the transistor. That drop will vary with base current, but

let's go for 0.6V. That makes the voltage across the top 100k resistor 840mV. If there were no base

current, the voltage across the other 100k resistor R6 would also be 840mv [same current in both

resistors], giving 1.68V across the pair from the dc-dc converter output to the linear regulator output.

In practice the voltage across that bottom resistor will be higher than that because it carries not just

the same current as the top resistor but also the base current, so in the circuit I would imagine that

you're seeing more than the 1.7V my rough analysis suggests.

Having followed it through in reverse, it becomes quite easy to see what happens in the forward direction

as the voltages at the dc-dc converter output and the linear regulator outputs vary.

If the difference between the two increases then the voltage across R5 increases, that increases the

voltage across R3, the current through R3 goes up, the current through R4 goes up, and so the voltage

across R4 (which is also Vfb) increases above the reference voltage. The converter will then reduce its

output and that should result in the difference going down.

Conversely, if the difference goes down, Vfb will fall below the reference voltage and the converter

should increase its output to increase the difference again.

So the feedback acts to maintain the difference. It doesn't matter what the absolute value of the DC-DC

converter output is because of the way the transistor behaves - the transistor doesn't mind what its

collector voltage is, it will just keep the same current flowing in R4 as R3.