some questions about switcher regulator (LM267x family) from National - diyAudio
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Old 11th August 2006, 01:56 PM   #1
jarthel is offline jarthel  Australia
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Default some questions about switcher regulator (LM267x family) from National

I have downloaded "Swtichers Made Simple" software from National.

Well I ran some simulations using the software

- 13V to 15V Vinput
- 1 output
- 8V Voutput
- 2.55A Ioutput

My questions relate to Cin, Cout and the choke.

1. The one of the suggested Cin is a Panasonic FC 1mF 50V. I was looking at the datasheet and the ripple current is around 2.55A. Should I make some allowance for this? Can I use a bigger Cin like perhaps a 15mF 16V?

2. same question as no.1 but for Cout

3. The suggested choke values is 33uH but the current rating is very close to 2.55A. Is it okay to select a bigger one like 3A to 3.5A rating. I assume I need to maintain the 33uH? what if I cannot find a 33uH with a higher current rating? perhaps 22uH can be a substitute?

4. Also the suggested chokes brand are impossible to find locally. how do I know if a choke can be use for this circuit?

Thank you very much.
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Old 11th August 2006, 02:36 PM   #2
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National has all the design equations on their website - the value of the inductor is inversely proportional to the current so specify the minimum current, run the sim, then specify the maximum current.

33uH shouldn't be difficult to obtain -- it's a standard value.

the capacitor specifications take into account their ESR -- start from their initial values -- or pick alternates which can handle the ripple current, etc.

the biggest difficulty you will encounter is circuit layout -- National has an application note which discusses this in general. but the specific part may have a demo board layout which you can use for your project. National is pretty good about this stuff.
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Old 11th August 2006, 03:12 PM   #3
jarthel is offline jarthel  Australia
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Quote:
Originally posted by jackinnj
National has all the design equations on their website - the value of the inductor is inversely proportional to the current so specify the minimum current, run the sim, then specify the maximum current.
I'll try this.

Quote:
Originally posted by jackinnj

33uH shouldn't be difficult to obtain -- it's a standard value.
it is difficult to find 33uH in our country. I have a look at the 2 biggest online electronics shop and it's hard to find them with the necessary current rating.

I found inductors made by Pulse which one of those specified by National but the values they stock does not include 33uH.

Quote:
Originally posted by jackinnj

the capacitor specifications take into account their ESR -- start from their initial values -- or pick alternates which can handle the ripple current, etc.
From what I have seen in various datasheets from dfferent manufacturers, ESR decreases and ripple current rating increases as capacitance goes up. using a 15mF (take note of "m") 16V, it'll definitely have better ESR and higher ripple current rating than a 1mF 50V that was suggested by the simulation.

Quote:
Originally posted by jackinnj

the biggest difficulty you will encounter is circuit layout -- National has an application note which discusses this in general. but the specific part may have a demo board layout which you can use for your project. National is pretty good about this stuff.
does that mean I need to make my own board? I was hoping I can use something like this: http://www.dse.com.au/cgi-bin/dse.st...uct/View/H5112

it is pre-drilled and the traces are running down (longest side of the pcb).


thanks for the help
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Old 12th August 2006, 04:41 PM   #4
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Default Nomographs

Jarthel-

1mF (1000mF) and 1.5mF (1500mF) are much more than sufficient for the LM267X chips. Note, they switch much faster (260kHz) than the LM2575 (1A) or the 2576 (3A) chips (52kHz). Hence the lower inductances for the energy-storing inductor. Ditto for the smaller Cin & Cout.

As for capacitor and inductor values, please refer to p.13 of the National Datasheet. There several nomographs for different operating voltages and output currents. And, the corresponding coils for them are on p.14. I know, I know, its kinda takes some of the fun out of designing it yourself because it removes some of the hardcore math involved, but it does speed up the design process.

I have worked with almost all of the SimpleSwitcher lines, and by far, the LM2677s are the easiest. I love the 2677 the best because of its 5A rating and the ability to synch it with other 2677s. My most ambitious was three LM2677s synched together, providing +3.3, +5.0, and +12V output. Here is the link to the chip's datasheet:

http://cache.national.com/ds/LM/LM2677.pdf

And here's the link on the datasheet for the demo boards.

http://www.national.com/an/AN/AN-1135.pdf

Let us know how this info works out.

Steve
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Old 12th August 2006, 05:03 PM   #5
jarthel is offline jarthel  Australia
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Default to n-channel

1. is it required that I must use design a PCB for this regulator?

I'm hoping I can use a "vero board" (pcb with pre-drilled holes).

2. In regards to the inductor chosen by SMS, the current rating is either below or equal to the current load. shouldn't there be any allowance?

3. why can't I use larger value caps for Cin? they usually have better ESR and larger ripple current rating.

4. how about the schottky? the suggested part was 1N5820. but it's not available. but I can easily get 1N5822.

ps. I'm beginning to think a switching psu is more complicated to work with (at least when it comes to getting the recommended parts). it's fine if you have a shop with extensive stock like mouser or digikey. but in australia, that is not possible
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Old 12th August 2006, 07:14 PM   #6
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Default Re: to n-channel

Quote:
Originally posted by jarthel
1. is it required that I must use design a PCB for this regulator?

I'm hoping I can use a "vero board" (pcb with pre-drilled holes).

Probably NOT a good idea with a SMPS chip operating at 267kHz. If you don't want to make a full blown PCB you can use a dremel tool, or perhaps a utility knife to cut the copper to approximate a PCB -- or you can use copper foil from 3M.

There are 10 nano-Henries per centimeter of copper trace.
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Old 17th August 2006, 02:08 PM   #7
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Jarthel-

Jack is right about the NO vero board thing. As these emit much RFI, any stray trace on the power path would act like an antenna, radiating its 267kHz signal out into free space. Not good.

I have used the SMT versions of the LM2677 & '2678 almost exclusively, beacuse making the pc board is soooooo much simpler. You only have to do one side of the board, and you don't have to "invert" the design because of working on the opposite side of the board. Also, you eliminate the need for drilling the board. I use the dry transfers from Radio Schmuck (Alas, they don't make 'em anymore ) to make the small, critical traces and ground plane borders, then use clear packing tape to mask off the rest of the board area. Two advantages here: 1) Since I'm etching less actual board, the process doesn't take as long; and 2) I can re-use my etchant (Fe-Cl3) many more times.

You could use larger values for C(in), but this would be unnecessary and a waste of board space. Believe me, at 267kHz, you can supply relatively alot of current with smaller inductors and capacitors. Splitting C(in) up into smaller caps is good to lower ESR, but I think the critical area to be concerned with is C(out), as it is sized to work with the built-in compensation network to nearly optimize system response to changing line and load. Too big a cap will slow down the regulator's ability to respond to suddenly changing loads, and too small a cap will result in unacceptable ripple on the output.

I would most definietly use a Schottky, because of its lower V(f). The 1N5822 would be a good choice for loads less than 3.0A, continuous. I use the MBR1645 (16A, 45PIV) just because I have an excess of them here. I realize that 16A may be a bit of overkill, but I have absolutely no doubt, then, of the diode's ability to do the job.

Actually, a switching supply is much easier than one might think, thanks to National's SimpleSwitcher series and the SMS software. I will agree not having parts readily available will complicate things a bit, but from a purely design and fabrication point, the SS's make life a bit easier.

I really can't believe that distrbutors are that rare in Australia, as you guys are a HUGE part of the DIY market, and many of the good parts houses that supply the world are from down unda'.
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Old 17th August 2006, 02:13 PM   #8
jarthel is offline jarthel  Australia
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Can you share a pcb design you used? thank you

hopefully I can print it on a transparency then use the old method of using uv.
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Old 17th August 2006, 02:16 PM   #9
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all of my pcb's are hand-done, but I will take a pic of one and post it. I did a board a few years ago, and tinned it with solder, but never populated it. It was designed for an LM2596 (150kHz, 3.0A). I am at work now , but I will post it when I get home.
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Old 17th August 2006, 02:19 PM   #10
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I guess RS getting out if the parts business is just a sign o' the times, that I need to update my manufacturing process, and go digital, using a pc board house to make nice professional boards, since even in small qtys, the prices are beginning to get reasonable.
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