piclist 2002\10\28\071433a >
Thread: Son of simple step-up SMPS challenge
www.piclist.com/techref/index.htm?key=son+simple+step
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face BY : Russell McMahon email (remove spam text)



Let's have a look at some of the comments so far and see how well they match
the spec. As noted originally, the spec can be extended if people see fit.

Original spec was for a DC-DC step up from either single cell or a low
voltage rail to a LED, a processor supply, an op amp rail, a FET gate drive
or a programming supply. Current requirement would be from about 1 mA to 20
mA depending on the specific application.

I'll use Digikey prices unless otherwise noted. DK are not the cheapest
source but give a consistent reference point.

Decoupling caps on input and output not counted in parts counts.


               Russell McMahon


==========
Andy
> I use an LT1930 for these.  Pretty cheap, whole circuit fits in about 1cm
> square, VERY clean output.  2.5V to 15V input, 2.5 to 25V output, 1A input
> current limit.

Nice IC
Operation at 1 MHz+ allows small inductors.
Far too dear - $4 in 1's and $1.90/1k
High current compared to spec (1A+ switch)
EXTERNAL Schottky needed.
Vin 2.6v minimum (guaranteed)
36 volt switch so good for over 30v out

Needs 3 cells for operation at dead flat.
Not really suited to LED application.
OK for uP, FET, Vpp, OpAmp
Somewhat overkill powerwise in most apps.

1 IC, 1D, 2R, 1L
_______________
Marcelo
> Something like this could be a start:
> http://www.emanator.demon.co.uk/bigclive/joule.htm

This almost unbelieveable circuit is the "Joule Thief" single cell to White
LED driver also mentioned here a few weeks ago.
Main "problem" with this design is the non-standard two winding inductor. If
this is tolerated then a wide range of multi-winding designs become
possible.

This is about as low a parts count as you could get. (1 Tr, 1R, 1 x 2
winding L !!!)
Efficiency not known but will probably be poor due to hard to optimise
design. Adding parts still will produce a low parts count design.

IF efficiency is tolerable this may be a real winner for most of the
applications.

_____________________
Dave T
> I have a requirement to take the output of a Ni-MH battery pack that has a
> terminal voltage of 3-4 volts and convert this with good efficiency to 5V
> at up to 100 mA. I came up with a 2-transistor circuit that I posted on
> 10/1 (reproduced below). It's remarkably similar to your circuit, with the
> addition of output voltage regulation.

Discrete design with
2 TR, 5R, 1Z, 2C, 1L
Achieves step up and regulation with 2 transistors. Relies on beta
saturation or inductor saturation for startup but apparently starts well.
Efficiency below that of IC designs. Could be improved with extra work (and
probably another transistor).

____________
Dave T
> However, for really high efficiency and low parts count, it's hard to beat
> ICs like the Maxim MAX1678 ($1.89 in 100s from Digi-Key), which can
> maintain 80% efficiency all the way down to 1 mA of load current in this
> application.

Digikey $US1.90 / 1k according to me (may depend on version)

Another nice IC, but also with limitations.
Aimed at 1 cell or 2 cell opertation.
0.87v guaranteed startup (excellent!)
Inbult synchronous rectification and main switch.
Min confign uses 1 IC, 1L !!!
Add 2 R for voltage setting.
Add 2 R for optional power failure / low-battery input sense.
Add 1R for optional power fail out drive.

Vin max is sadly only 5.5V
Vout max is STUPIDLY only 5.5v making it useful for LED and procesor,
marginal for opamp and probably useless for FET and Vpp. A really nice
design made much less useful by an unnecessarily narrow spec.

Efficiencies up to 90% at 50 mA / 2.5v in / 3.3v out (due in large part to
synchronous rectification).

Worth a close look for battery to white LED drivers
eg 80% efficiency at 0.85v in, 3.3v out, 20 mA !!!!!!!!!!!!!!

____________________________
Peter
> I don't know about efficiency but you have to know that you are using a
> circuit that exists since times immemorial afaik.

"Very time honoured" was how I put it part way through the post. I 1st saw
it used as a mose code oscillator several decades ago. I've never seen it
with a "rung" inductor in the output.

> The one-transistor flyback converters that are used in flashes etc should
> serve as a better starting point for this kind of circuit imho..........
> Since these involve a transformer and this seems to cause ppl to avoid
> them here is an (untried) solution to implement the transformer using two
> (identical or not, but same sized) ready-made chokes (see attached
> drawing):

I tried coupling two chokes by placing them in proximity before now without
enough success. removing the platic coating to achieve ferrite to ferrite
coupling may help.
_____________________________________
Roman:
{Quote hidden}

Please supply. The more the merrier.

______________________
Dave D
I'm glad Dave decided to join the party after initial reservations :-)
Nice addition.
{Quote hidden}

The regulation takes 1 Z, 1R, 1 tr.
This could be dropped to 1 Z if you don't mind shunt regulating the output.
This destroys efficiency but may be an entirely viable solution for eg 5v to
Vpp, FET drive or Op amp supply applications.
D3 can almost certainly be omitted in most applications but the reason for
its inclusion is appreciated.

------------------------------------------
Someone (maybe several people?) mentioned MAX1522 / 1523 / 1524
$US1.70 / 1k est by Maxim (AFAIR)

Nice IC but parts count may even exceed discrete designs.
External diode AND main FET !!!
2.5v min Vin or 1.5v with bootstrap supply feedback.
Standard design needs 1 IC, 1L, 1 FET, 1 D, 2 R, 2C.
Bootstrap (1.5v start) version needs 1 IC, 1L, 1 FET, 1 D, 3 R, 3C.

Vin max 5.5v
Vout limited by external pass FET & diode (eg 28v easily achieved)
Power level set by external FET & diode
Low quiescent currents (25 uA typical)
Soft start and short-circuit output shutdown modes

Useful IC. While external parts add to parts count they allow user
selectable Vout and power levels.
____________________________

MC34063A, MC33063A
Mentioned by someone recently.
A very old design and suffers in several respects compared to modern ICs BUT
very very cheap compared and more flexible than many. Can be used for step
up or step down, quite a low voltage internal reference for older IC
allowing low voltage operation.

** $US0.30 ** approx even in small quantities (from the right place) !

Bipolar.
Emitter/Collector of driver available for flexibility.
Current sense in high side supply.
3v to 40v input
External diode, internal 1.5A switch or drives external switch.
8 pin dip/so-8, soeia-j8
100 kHz max so larger inductors than best modern practice.
Quiescent current in 1 to 4 mA range typically (depends on Vin)
The bipolar darlington output and modest saturation and beta specs will not
produce the ultimate efficiency designs.

Can produce eg 28v 150 mA using internal switch.

Unsuited to flat battery operation (3v min)
Suited to 3v or 5v in.
No good for LED drive
Opamp, FET, Vpp drive apps.

CHEAP !!!!!!!!!!!!!!!!!!!!!!!!
(Maybe not as cheap parts wise as bottom end discrete, but close)

Min step up cct    1 IC, 1 L, 3 R, 1 D, 1 C
Current limit add 1 R

Min step down cct    1 IC, 1 L, 2 R, 1 D, 1 C

____________________________

LT1937
Dedicated white LED converter
Nobody mentioned these (yet)

$US2.50/1, $US.130/1k


LT1937 is designed to drive 2, 3 or 4 LEDs in series with constant current
feed.
1.2 MHz for small inductor size.
Targetted at Li-Ion supply.
2.5v to 10 v vin

Could be used for other apps.

__________________________

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