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'[PIC]: [PDB] The PICLIST Development Project - Swi'
2002\09\12@055445 by Alan B. Pearce

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Next question for the Piclist Developers Board.

Switchmode regulator, to go from Wall Wart Power to 5V regulated.

I started looking at chips to select one, and the LTC1574-5 from Linear
Technology caught my eye. I don't see this as requiring the highest
efficiency regulator, so the possibility of the internal schottky diode and
switching transistor possibly not having the lowest possible losses I do not
see as an issue. However it looks like the maximum current available without
affecting regulation will be in the high 300mA range, which I think will be
OK, even with running a total of three processors, and LED displays. The
item that does concern me is the absolute maximum rating of Vin being 18.5
Volts. The operating range seems to suggest 16V max input in their example
circuits.

Has anyone any experience with this chip? Are there other chips more suited
to our purpose? I liked this one for it's lack of external components (2
caps, 1 inductor, plus a reverse protection diode), but we may have to go
with something with an external power transistor and catch diode.

Secondly, should we move this development discussion to the [EE]: tag, or
leave it under [PIC]: ? How do the admin folk feel about this?

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2002\09\12@061903 by Mircea Chiriciuc

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I would rather use the MC34063 not only for the price under 1$ but also for
the input voltage which can go up to 36V. It comes in a 8 pin DIP with power
transistor built in for currents up tu 1.5A. You will need oly 2caps, 3
resistors and an inductor to make thw power supply. This IC also featrures
current limiting and protection. I use this IC for a while now and I'm very
happy with it. I can provide schematics and PCB design if needed.

Mircea Chiriciu
Emco Invest

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2002\09\12@065327 by Ashley Roll

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Hi,

I've also used the MC34063, and while it isn't the most efficient of beasts,
it is good enough and even the local electronics hobby stores stock it.

In addition to the caps, resistors and inductor that Mircea mentions, you
will need a diode. It doesn't need to be anything special if efficiency
isn't that great of a concern. I've used a 1N914 (low current) and a UF4002
(ultra fast version of the 1N4002) but a 1N5819 1A schottky would be ideal..
All of these should be easy to find.

I'm no expert with SMPSs, but it was pretty easy to get going. I just
followed the datasheet/app note and had no problems.

I've also used a L5970 from ST, but this is a SMT SO8 package. However it is
a lot newer then the MC34063 and has a MOSFET switch and much higher
switching frequency (250KHz) which makes filtering a little easier. It also
has half the quiescent current of the MC34063.

Trouble is getting it.. for the PDB running from a plug pack (wall wart),
I'd go for the MC34063. Cheap, easy and available.

I'm also just used a LM2576 in a different product, but that is much higher
current (3A or so) and again, not really called for in the PDB

Cheers,
Ash.

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> {Original Message removed}

2002\09\12@074910 by Alan B. Pearce

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>I've also used the MC34063, and while it isn't the most
>efficient of beasts, it is good enough and even the local
>electronics hobby stores stock it.

OK thanks guys, I'll go investigate it. Certainly sounds like it should be a
go'er for this purpose.

Start with the RS catalogue, and I find it from 3 manufacturers, of which 2
do an SM version. :)

makes a real good starting point.

Thanks.
Alan.

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2002\09\12@120822 by Michael Rigby-Jones

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{Quote hidden}

What's wrong with a 7805?  Sure it might get warm, but they are cheap and
don't produce switching noise.  I've not been following all of the PBK
threads, but it seems that what started out as an affordable developement
kit is rapidly becomming a such complex solution that it will price itself
out of the market without some moderation.

Regards

Mike

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2002\09\12@122234 by Alan B. Pearce

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>but it seems that what started out as an affordable developement
>kit is rapidly becomming a such complex solution that it will price
>itself out of the market without some moderation.

Yeah I agree, and that is why I am trying to take things in little steps,
and chose the best solutions at each step. The concern I have with a 7805 is
that someone will inevitably short circuit something somewhere, and things
have the possibilty of getting real warm. I see a switching regulator as
being a reasonable compromise if it takes only 3 or 4 components around it.

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2002\09\12@123016 by Jim

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  > The concern I have with a 7805 is that someone will
  > inevitably short circuit something somewhere

Won't the wall-wart's output voltage drop off - thereby reducing
the P_subd somewhat (at least until the operator discovers his
or her error)?

IOW - make the design/heatsink/wall-wart-choice a "survivable design"
for 5 to 10 minutes ...

What about a simple scheme using a PNP xsistor and a I sample
resistor driving an LED to indicate a 'shorted' condition
(while using the 7805)?

RF Jim


{Original Message removed}

2002\09\12@124108 by Jim

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.. besides - the National 78xx spec sheet indicates:

  "Internal thermal overload protection"
  "Internal short circuit current limit"

No worries!

RF Jim

{Original Message removed}

2002\09\12@163725 by Peter L. Peres

picon face
Also examine the simple switchers from National and the ancient 78S40 and
other chips. F.ex. the venerable 494 can deliver 400mA but has no diode on
chip. Instead it has all the bells and whistles like scp and slow turn on
etc. These two venerable chips are second sourced and availability is not
a problem usually (haha).

Peter

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2002\09\13@111349 by Alan B. Pearce

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>Also examine the simple switchers from National and the
>ancient 78S40 and other chips.


The MC34063 seems to be a cut down version of the 78S40 so it fits in an 8
pin package. It is also multi-sourced, so should be readily available at a
good price. In the UK RS Components has them for 56p 1 off, and a tube of 98
set you back under GBP30. These prices are for the SM version. This is not
that different in price to a 7805 1A regulator in TO220 package, which would
then need to have legs bent and be screwed down to mount on board.

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2002\09\14@064501 by Peter L. Peres

picon face
On Fri, 13 Sep 2002, Alan B. Pearce wrote:

>>Also examine the simple switchers from National and the
>>ancient 78S40 and other chips.
>
>
>The MC34063 seems to be a cut down version of the 78S40 so it fits in an 8
>pin package. It is also multi-sourced, so should be readily available at a
>good price. In the UK RS Components has them for 56p 1 off, and a tube of 98
>set you back under GBP30. These prices are for the SM version. This is not
>that different in price to a 7805 1A regulator in TO220 package, which would
>then need to have legs bent and be screwed down to mount on board.

And a heatsink which may be more of a pita in a small device that cannot
get hot. There remains to be seen how this works out noise-wise. I have
mixed experiences with 78S40. Unless you guarantee a minimum load it will
be 'chaotic' <g> in operation.

Peter

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2002\09\14@090846 by Alan B. Pearce

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>Unless you guarantee a minimum load it will
>be 'chaotic' <g> in operation.

I understand that. My reckoning is that the minimum load will be a 16F877(A)
running at 3.6MHz, and miscellaneous 74HC logic, along with a MAX202, an
FTDI FT232B and maybe a power LED, so I figured a minimum load should not be
too much trouble.

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2002\09\14@112616 by Peter L. Peres

picon face
On Sat, 14 Sep 2002, Alan B. Pearce wrote:

>>Unless you guarantee a minimum load it will
>>be 'chaotic' <g> in operation.
>
>I understand that. My reckoning is that the minimum load will be a 16F877(A)
>running at 3.6MHz, and miscellaneous 74HC logic, along with a MAX202, an
>FTDI FT232B and maybe a power LED, so I figured a minimum load should not be
>too much trouble.

I usually ensure 5 to 10% of full load for 78S40. Sometimes I have to use
a resistor across the output to achieve this. The problems occur only in
RFI/radioation, not in ripple and output voltage.

Peter

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2002\09\14@215308 by Mike Singer

picon face
Peter L. Peres wrote:
> .I have mixed experiences with 78S40. Unless you
> guarantee a minimum load it will be 'chaotic' <g> in operation.

Why not use some Microchip stuff:
TC115 or TC120 PFM/PWM Step-Up DC/DC Controllers?

From 21361b.pdf, 21365b.pdf:
" Unlike conventional PWM step-up converters, the TC115
(TC120) automatically shifts to pulse frequency modulation
(PFM) at low loads, resulting in reduced supply current and
improved efficiency."

              >The chaos is elsewhere.<
Mike.
-
-
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2002\09\15@044038 by Peter L. Peres

picon face
On Sun, 15 Sep 2002, Mike Singer wrote:

>Peter L. Peres wrote:
>> .I have mixed experiences with 78S40. Unless you
>> guarantee a minimum load it will be 'chaotic' <g> in operation.
>
> Why not use some Microchip stuff:
>TC115 or TC120 PFM/PWM Step-Up DC/DC Controllers?

Please quote a second source for those parts, and a shop that sells these
in small quantity (<20 pcs) without steep markup.

Otherwise I aggree, they are good parts. Just not suitable for small runs
and prototyping w/o large buy commitment.

Peter

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2002\09\15@062252 by Mike Singer

picon face
Peter L. Peres wrote:
> >> .I have mixed experiences with 78S40. Unless you
> >> guarantee a minimum load it will be 'chaotic' <g> in operation.
> >
> > Why not use some Microchip stuff:
> >TC115 or TC120 PFM/PWM Step-Up DC/DC Controllers?
>
> Please quote a second source for those parts, and a shop that
> sells these in small quantity (<20 pcs) without steep markup.
> Otherwise I aggree, they are good parts. Just not suitable for
> small runs and prototyping w/o large buy commitment.

In Ukraine, Dnepropetrovsk:
http://www.microchip.com.ua/sklad/price.zip (Russian)
TC115501EMTTR - $1,22 (ones)

Your country has Consulate there. A lot of people go
through it to your country each day still. You may ask
their relatives to arrange chips' delivery. Sorta business.
I can send further information privately (assuming all my
postings are likely to be red by "our friends ." [|l:-(

(Please, do not download this file without need. They
can't sell abroad, I think. They've closed Home Page
already may be due to a traffic overload. If they blacklist
me, I'll be forced to shift to AVR, more common and
cheaper here.)

Mike   c(|:-)

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