Still trying (and NO LONGER failing) with the LM2588... 5V -> +/- 12V

[Skimask]

Trust the datasheet on this one. It's seems like it's the classic transformer-instead-of-coupled-inductor case...
[/quote]

Not 100% sure what you mean by that...

I think you mean transformer as in exactly that...a step up transformer....vs. two inductors sitting effectively side-by-side that just happen to be doing things together...which from my reading in TAOE, is almost the same thing, but not quite.

[Skimask]

No, a feed-froward cap is quite common to reduce overshoot and ringing. The RC is not related to the switching frequency. Sometimes C_ff is mandatory for stability, but if the datasheet doesn't say so, you may check the ringing frequency and try out the cap value. If you still got that low impedance feedback voltage divider as before I don't think 10 pF will do any difference. For me, last time I think I tried with an RC =1/(2*pi*f), where f is the ringing frequency, and it worked out quite well. If C_ff gets too big, the step-response will be degraded.

The only other cap's the datasheet talks about is the snubber RC across the positive rail output diode, before where the feedback gets picked off.
Doesn't sound like the same thing.  Maybe the LM2588 has a feed-forward cap internally???

[Skimask]

Pictures of construction will help a lot. Switching regulators are heavily dependent on layout for success.

Gonna finish building it up tonight and take some pics.
I'm even going to try and make it...pretty...rather than just slapping it together...for a change

[CarlG]

author=Skimask link=topic=13333.msg180523#msg180523 date=1357943958]

Trust the datasheet on this one. It's seems like it's the classic transformer-instead-of-coupled-inductor case...

Not 100% sure what you mean by that...

I think you mean transformer as in exactly that...a step up transformer....vs. two inductors sitting effectively side-by-side that just happen to be doing things together...which from my reading in TAOE, is almost the same thing, but not quite.

I mean that if pins 1 and 2 are swapped, the transformer/inductor will act as a transformer when the regulator switch is "on", instead of storing energy in the primary winding. For the +12V side, this means that you'll get 2.5*V_in on the secondary output, and the diode will conduct, instead of having a negative, -2.5*V_in, output voltage and the diode reverse biased.

When correctly wired, working as a coupled inductor, energy is "stored" during the switch "on" phase, which is transferred to the secondary windings when the switch is turned off.

As you may know, the transformer/coupled inductor always have some leakage inductance, meaning that the coupling between the windings is not 100%, so there will be some energy left in the primary winding that has to be dissapated somewhere, and that turns out as an overshoot at the switch node. This may kill the regulator if it's not capable of handling the voltage peak. That's why you may need a snubber on the primary side. This will also reduce ringing on the secondary side, but it will probably also be some ringing when the diodes turns of (snap).

[CarlG]

No, a feed-froward cap is quite common to reduce overshoot and ringing. The RC is not related to the switching frequency. Sometimes C_ff is mandatory for stability, but if the datasheet doesn't say so, you may check the ringing frequency and try out the cap value. If you still got that low impedance feedback voltage divider as before I don't think 10 pF will do any difference. For me, last time I think I tried with an RC =1/(2*pi*f), where f is the ringing frequency, and it worked out quite well. If C_ff gets too big, the step-response will be degraded.

The only other cap's the datasheet talks about is the snubber RC across the positive rail output diode, before where the feedback gets picked off.
Doesn't sound like the same thing.  Maybe the LM2588 has a feed-forward cap internally???

Sorry, I misinterpreted your post. I see whay you mean (I think): you want to put another cap in parallell with the electrolytic one? No problem with that either.

[Skimask]

Sorry, I misinterpreted your post. I see what you mean (I think): you want to put another cap in parallel with the electrolytic one? No problem with that either.

Well, yes and no.
No, in that it's purpose wouldn't be primarily to help filter the higher frequency noise on the output.
Yes, in that it's purpose would be to help smooth out the noise on the feedback loop and hence the caps would be placed right next to the feedback resistor network vs. near the output cap's.

[Skimask]

Ok, done with the rebuild, #3, and it works much better...but not perfect yet.  (pictures to follow, might not get there until tomorrow though)

I'm pretty sure one of the problems was the mislabeled primary side polarity marking for the coil in the LM2588 datasheet.  The primary side is reversed from the Coilcraft Q4337-BL datasheet, and as far as I can tell, the secondary sides both match up.

With no external load, I can adjust the output voltage as required.  The positive side has a small bit of load from the feedback network and it's steady enough.  The negative side doesn't have any load on it when wired up according to the figure, so I added a 10K to ground, and that took care of that.

According to the figure in the datasheet, it's rated for 300mA on each rail.  I'm not getting that without a significant voltage drop.
With 82ohms on each rail (because that's what I've got on hand), I get 8.6v on each side, equating to about 104mA per rail, 208mA total.  And a quick current measurement off the battery pack shows about 570mA average draw, which works out close enough for me when I do the math.

I'm feeding the circuit with a 4 AA cell NiMH battery pack.  I've got a 4 D cell NiMH battery pack on charge and will test that in about an hour.
The meter is showing a constant 4.5v under load from the pack.  On the 'scope, I get about 2v of noise on the V_in line.  I'm pretty sure one big issue here is the really tiny gauge wires coming from that 4 cell AA holder.

So...

Input = 4.5v @ 570mA = ~2.57 watts

Output + and - = 8.6v @ 82 ohms = 104mA = .89 watts x 2 channels = 1.78 watts

1.78 watts out / 2.57 watts in = ~70% efficient.

[Skimask]

Pics of the "completed" board...(this camera phone is worse than I thought when doing close up stuff...)

[SeanB]

A magnifying glass held in front helps with that problem.

[CarlG]

Good to hear you're making progress:) From what I can see there's no obvious component choice that explains why the circuit doesn't work properly. But you'll never get good performance using that kind of bread board. Try to get something like this bare Cu board instead.

Meanwhile, you can use copper tape to improve the ground plane.

It might be noise injection that disturbs the regulator, at the FB or COMP nodes. The current limit may set in due to improper ground/decoupling. What does the outputs look like on the scope? And winding/diode nodes? It'll be interesting to see scope pictures!

The component placement doesn't seem optimal.
- connect Cin (1uF) directly between Vin and GND pins (is Cin the one to the right of the electrolytic in front? Doesn't work placing it there...)
- connect the bottom feedback R directly between 2588 FB and GND pins
- connect the top feedback R directly to the feedback pin to minimise the feedback node, route the wire from +12V away from high current paths, tie it to output cap or just by it on the way to the load)
- get rid of the pots to reduce problem sources, and route +12V from the output cap to the top feedback R (you can add the pots later when you get the feeling of the basic circuit)
- tie Cc directly to the 2588 GND pin
- a small R (1-10 ohm) between main Vin and 2588 Vin pin/Cin may help isolate the regulator internal circuitry from the noise on the 100 uF

Use the backside of the board for above.

The output routing seems reasonable local (although I think you can improve it), but the input seems to make up a large current loop from 2588 GND to the input cap (100u). Always minimize current loops!

Good luck!
//C

[Strada916]

Firstly get rid of those plugs. Solder the wires directly.

A point to remember,

What goes in, must come out. Kirchhoff's second law,

So +12V@0.3A and -12V@0.3A = 7.2Watts, lets assume 1 or 2 watts is dissapated in the circuit, transformer loses, heat etc. therefore we have about 10 or so watts of power used by your circuit.

Now ohms laws says that 10w @ 5v is about 2amps.

Can those batteries supply 5v @ 2amps?? And for how long can they do it?

[Skimask]

But you'll never get good performance using that kind of bread board. Try to get something like this bare Cu board instead.

I've never had a problem with these boards (www.protostack.com), even when using them with PICs @ 64Mhz, then again, haven't tried using them with anything that's analog and so 'touchy' as this.
I've got a decent shop to work with out here, and even have a CNC machine to route out raw copper PCBs.  I've tried the toner-transfer method as well in the past.  Just haven't found a good method for making PCBs that works reliably and is repeatable.

Meanwhile, you can use copper tape to improve the ground plane.

I've got multiple extra wires placed here and there to help out the grounds, same thing with the higher current Vin rails.  It's not a ground plane, but it's a start.

It'll be interesting to see scope pictures!

I'll work on it.  I've got a Tek2245A, but the Holdoff knob doesn't seem to work well these days.  More troubleshooting for another time...

The component placement doesn't seem optimal.

Ya, no kidding!  I was shooting more for 'tight' than 'optimal' since I'm going to be adding a few MCUs and OpAmps to the circuit.

- connect Cin (1uF) directly between Vin and GND pins (is Cin the one to the right of the electrolytic in front? Doesn't work placing it there...)
- connect the bottom feedback R directly between 2588 FB and GND pins
- connect the top feedback R directly to the feedback pin to minimise the feedback node, route the wire from +12V away from high current paths, tie it to output cap or just by it on the way to the load)
- get rid of the pots to reduce problem sources, and route +12V from the output cap to the top feedback R (you can add the pots later when you get the feeling of the basic circuit)
- tie Cc directly to the 2588 GND pin
- a small R (1-10 ohm) between main Vin and 2588 Vin pin/Cin may help isolate the regulator internal circuitry from the noise on the 100 uF

-Cin is to the right of the electrolytic near the bottom.  The positive and negative battery inputs are tied to 7 each rows of pins to give me extra holes to work with.  The datasheet said .1uf, but I'll add another one in there anyway.
- feedback, noted...
- Cc - that was the only .47uf I could find in the whole shop, so that's why it's connected like that.
- small R between Vin-2588/Vin - that's what the datasheet said too.  Will work it...

The output routing seems reasonable local (although I think you can improve it), but the input seems to make up a large current loop from 2588 GND to the input cap (100u). Always minimize current loops!

Oh ya...it's not optimal by any stretch of my, or anybody else's, imagination!  The current loop you refer too is 'shortened' a bit by the two extra wires on the left, but like you say, is not optimal at all.

Rework time!

[Skimask]

Firstly get rid of those plugs. Solder the wires directly.
A point to remember,
What goes in, must come out. Kirchhoff's second law,
So +12V@0.3A and -12V@0.3A = 7.2Watts, lets assume 1 or 2 watts is dissapated in the circuit, transformer loses, heat etc. therefore we have about 10 or so watts of power used by your circuit.
Now ohms laws says that 10w @ 5v is about 2amps.
Can those batteries supply 5v @ 2amps?? And for how long can they do it?

Plugs - not for awhile yet.  Those plugs give me a way to pull the battery pack out in a hurry in case I see smoke!  But I understand where you're coming from.

Can those batteries supply 5v @ 2 amps?  Well, probably not for long, which is why I've got a handful of 4 AA cell NiMH packs on charge waiting to go.
On the upside, the 4 D cell NiMH pack came off charge a little while ago, so that's my next thing to try now that I know I won't be letting the smoke out of various components...

[Skimask]

Can't find any low ohm resistors with decent wattage to put between Vin and the LM2588 Vin...left it out for now...
Got rid of the pots.  Using 2x 4.7K resistors for the 'top' of the feedback resistor chain, and a 1K resistor for the 'bottom'.  In a perfect world, that should come out to 12.792 volts at the output.

No load results (practically no load...feedback network on the positive rail provides about ~10.4K, added 10K to ground on negative rail, about ~2.5mA idling load)
Vin = 5.35v measured at the LM2588 and coil primary side
Vout+ = 12.62v
Vout-  = 12.62v
V @ feedback pin = 1.217v

1.5K load on each rail (3 470 ohm resistors in series, in this case ~19.2mA total load post-regulator)
Vin = 5.34v measured at the LM2588 and coil primary side
Vout+ = 12.51v
Vout-  = 12.52v
V @ feedback pin = 1.207v

82 ohm load on each rail (in this case, ~210mA total load post-regulator)
Vin = 5.17v
Vout+ = 8.48v
Vout-  = 8.48v
V @ feedback pin = .829v

  Put the meter on the ground rails, positive rails, looking for any obvious resistance...
Didn't find anything measurable either in resistance or in voltage differences.
'scope pictures up next if I can get the Holdoff knob to work right and clear up the picture.

(EDITs vs. new posts each time I make 'progress' should keep this a bit cleaner...)

EDIT:  I can't get the 'scope pictures to clear up at all.  Secondhand Tek 2246A I've had for about 10+ years.  Never had it "tuned up" or calibrated other than using the built-in menus.

EDIT#2:  Added the "snubber" R & C to the positive output diode.  Made it worse.  Output voltage dropped down to + and - 8.1v.  Noise appeared to stay the same.  Removed previously mentioned snubber since all it was snubbing was my power!

EDIT#3:  Added a 1uf across LM2588-Vin to ground.  Output voltage started at 8.4v, but rose to about 9.1v after the circuit had "warmed up" for about 2 minutes (power resistor load and LM2588 w/heatsinks rose to about 30F above ambient, 105F).

EDIT#4:  What if the polarity marking for the coil was actually on pin #1 according to the LM2588 datasheet  vs. on pin#2 according to the Coilcraft datasheet?   Hmmm....
Edit#4a:  Nope that's not it.

EDIT#5:  Added a smattering of .01uf caps, one each at the input and output of each positive and negative rail diode, one at the Vin/Pin7 to go along with the 100uf & 1uf, and another at 'top' of the feedback resistor chain.  Also put a fat 4700uf cap across the battery input wires on the PCB itself.
Now...I'm getting 11.6v at the output with the 82ohm load...I think.  It's seeming like the regulator wants to be "warmed up" before it'll start giving me a decent output.
Using 2 4.7K resistors for the 'top' of the feedback, 1K resistor for the bottom of the feedback.
Vout     = 1.23 * ( 1 +   R1  /  R2   )
12.792 = 1.23 * ( 1 + 9400/1000 )
At any rate, 11.6v is within the tolerances from the resistors in the feedback chain as well as the tolerance of Vref inside the LM2588.  The calculated worst case each way gives me a range of 11.453v - 14.294v (5% resistors in the feedback, along with 2% feedback voltage tolerance according to the datasheet).

EDIT#6:  No dice...
Loaded each rail down with 44ohms (82ohm + 2x47ohm in parallel).  Got 7.4v on each output.  That's 168mA @ 7.4v, where I should be getting ~12v @ ~270mA. 
But, after the regulator/heatsink warmed up a bit, 130F (~60F above ambient), I got 9.58v output, with 4.84v on Vin.
If the 70% efficiency I calculated earlier holds up, that's 438mA load at the outputs, 626mA draw from the battery pack.

Rechecked it after the regulator had cooled down to 60F (at the heatsink) and the output was only 7.4v.  (Vbatt = 4.82v)
Re-recheck it after the regulator had heated up to about 100F (again at the heatsink), output up to 8.42v.  (Vbatt = 4.81v)
Re-Re-rechecked it with the regulator heated up to about 130F, output up to 9.58v.  (Vbatt = 4.81v)

That a repeatable response.  Did it 3 times, same temps, same voltages, a few millivolts lower on the battery pack each time though.

What the heck is going on here?

[Skimask]

Zzzzz....pissed off am I....unable to make it work properly I am....backwards I will talk.

Added 4,700uf across Vin and ground right where the battery pack is connected to the PCB.
Added 2 extra 220uF across the plus and minus outputs right before the loading power resistors.
Added 2 extra 1uf and .01uf cap's across those same 220uF caps.
Added another .047uF across the feedback resistor network right at the LM2588.

My waveforms still looking nothing like those in the datasheet, not nearly as clean, and almost unrecognizable if I didn't know what I was looking for in the first place.
Still getting bunches of the same noise on Vin, Vout+, Vout-, Vfeedback.
The voltage on the switch spikes (very VERY narrow spike) up to about 50+ volts, but hangs out between ground and 2xVin.  What gives here???
On every pin i checked, the noise seems to have a decent frequency component of about 5Mhz, other than the main switching frequency of ~100Khz.  Mine seems to hover around 109Khz.

Charging the battery packs for more tries again later.

[jahonen]

Believe it or not, but lack of stiff ground (i.e. plane) is #1 reason for strange and difficult-to-solve problems with switch mode regulators. Having few wires here and there for ground simply doesn't cut it. Do it "Jim Williams" style.

Regards,
Janne

[Skimask]

I couldn't agree more...well I could, but I'd sound stupider than I already am 
I don't have the facilities to make a really good ground plane...so trying to make due.  Ain't workin' out so good is it?   I'll get 'er figured out sooner or later...

[jahonen]

But certainly you could get a copper clad board (unetched single/double sided PCB)? That makes a very good ground plane. Then solder all your components there using "3D construction".

What I (and others have meant by copper clad construction) mean, look at Linear Technology AN-47 http://cds.linear.com/docs/Application%20Note/an47fa.pdf, somewhere around page 102 or so. Of course, the result will not be pretty or even mechanically very robust but I suspect that will work fine electrically.

BTW, I just remembered that have tried to do something similar in the past with LM2585, and I had also horrible problems (the PCB is now in the box of "unsolved problems projects"), so I think we have done same mistakes, no need to repeat them

Regards,
Janne

[Skimask]

That AN-47 is quite the read, but it covers a lot of stuff not in "the good book" (aka The Art Of Electronics).

Nonetheless, I don't have the capability at the moment to do anything close to what that AN47 shows.
I think the best I'm going to be able to do in the near future with the "Protostack" boards is to tie all of the unused buses together in and around the entire circuit back to ground.
As well, if I move a few parts around, it should shorten up any ground loops that might exist.
I've got a handful of single sided copper clad board.  You think if I drilled a handful of holes in it and connected it up as a ground plane, I might make progress?

[CarlG]

That AN-47 is quite the read, but it covers a lot of stuff not in "the good book" (aka The Art Of Electronics).

Nonetheless, I don't have the capability at the moment to do anything close to what that AN47 shows.
I think the best I'm going to be able to do in the near future with the "Protostack" boards is to tie all of the unused buses together in and around the entire circuit back to ground.
As well, if I move a few parts around, it should shorten up any ground loops that might exist.
I've got a handful of single sided copper clad board.  You think if I drilled a handful of holes in it and connected it up as a ground plane, I might make progress?

So you got it? Well, then there's no problem. You don't need to drill anything, solder ground pins to the copper and "hardwire" the rest in the air! It will look messy, but that's part of the beauty!

[jerry507]

Bad physical construction is the downfall of every switching regulator. No amount of added capacitors anywhere will fix the problem. Somewhere Jim Williams kept a tally of the causes of problems that he got calls about. Well over half of them were caused by layout problems, another big chunk by crappy parts.

Every time I see one of these posts and the person just wants to keep slapping parts of to fix the problem, I sigh. They always have some kind of non-ideal layout that causes a huge amount of inefficiency.

[Skimask]

Can't disagree with you there jerry507, especially when I pulled all those extra cap's out of there and found the overall noise went DOWN!, then again so did the 'regulate-ability' of the output voltage (is that even a word? )

I'll get it figured out eventually.
That is unless somebody else has a more easily workable idea for generating a split rail supply for opamps using (preferably) a 4 cell (or 8 cell, got both, either would be nice vs using a PC/AT PSU) battery pack.

[grumpydoc]

If you just want to split the supply then a "virtual ground" buffer should do the trick - in fact wasn't the circuit in the "perfboard prototyping" thread just such a beast?

[Skimask]

Ya, I saw that.  But I'm not sure if a "virtual ground" buffer is the right tool for me in this application, e.g. the advantages and disadvantages of using a virtual ground.  I'm going to do a bunch of reading tonight on the "virtual ground" subject, see what I can come up with...

My project is the battery powered curve tracer using a PIC to do the A/D work and an LCD for the display.  Been working on it for awhile now.  Guess a little bit of a circuit description is in order.
The basics are done up already on the PIC, LCD, A/D, D/A, etc. side of things.  That's all down to tightening up the programming, making it more user friendly, etc.

It's the power supply that's tripping me up.

I want +15v / -15v to drive the opamps (along with another 3.3v for the PICs/LCD/etc).  Could make due with +/-12v, heck even as low as +5/-5 volts would probably do for my purposes.  Using TL084 opamps for now since I happen to have a tube of 20 sitting in the parts bins.
After I get the PSU issues sorted, I'll switch over to something with more current source capability and can drive closer to the rails.

I want to run it off a 4 cell NiMH pack, whether it's a 4 cell pack of AA's or D's doesn't matter (AA's are preferable, but D's will do).
An 8 cell AA NiMH pack would be ok too but would increase the size/weight of the whole thing.
What I don't want is to have to run the thing off a PC/AT PSU just to get a decent +/-12v and have to plug it in everywhere I go.

Maybe, in my situation (with the lack of proper facilities, PCB building materials, etc.) the way to go would be to boost that input battery volts to ~30, split that with the virtual ground, then post regulate that back down with a couple of linear regulators leaving plenty of headroom while under load.  I have successfully boosted a 6 cell pack up to 12v using an LM2576 (?partnumber?) awhile back.  Didn't have any problems with that.  But all that was doing was pulsing a solenoid.

Amateur hour for me... 

EDIT:  It was the LM2577 that I used successfully for the 7.2v -> 12v step-up converter that I made back in the day.  Worked good enough to run a hard drive for the MP3 player I built and that was on a protoboard board very similar to what I'm using now.

[Skimask]

I give up on the LM2588 in the flyback configuration.  I don't have the proper PCB making equipment to make a 'clean' layout to make it work correctly and cleanly.

New plan...In flux...still working out the details...

Use the LM2588 in the boost configuration, not a flyback config.  5v in, 12v out, Page 22 of the LM2588 datasheet.
http://www.ti.com/lit/ds/symlink/lm2588.pdf

Use an LM675 and make a virtual ground with that.  Page 2 of the LM675 datasheet.
http://www.ti.com/lit/ds/symlink/lm675.pdf

The ground of the LM2588 section becomes V-, the LM675 provides a Vgnd, the LM2588 booster gives me V+.
Keep the grounds separated as required between the PSU and the actual logic doing the work.

Whether or not this will work in this configuration using the same layout methods, I don't know.  I do know, as I said in the last post, I've used the LM2577 as a booster practically identical to this and it worked, using the same layout methods I'm going to use now, but a fair bit cleaner (I didn't know what I doing back then anyways).

We'll see what happens.  I gotta order a few LM675's.  Closest thing I've got to that is an LM386 and that ain't gonna cut it.