switching current supply
RST Engineering - JIm
regulator and how to use an upside-down PNP and a couple of silicon diodes
as a relatively constant current source. No big deal. However, I'm a
little unsure exactly how to translate this knowledge into making a
switching current regulator. I don't really care if the current varies 10%
or so over the input voltage range as it is only going to be used to light
LEDs. What I'm really trying to do is save as much battery life as
possible.
Thoughts or pointers to articles would be appreciated.
Jim
BobW
"RST Engineering - JIm" <jwe...@gmail.com> wrote in message
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Typically, in a switching regulator, there's a feedback pin that
(eventually) connects to the node that you're trying to maintain at a
constant voltage (with respect to your circuit common (aka GND)). This
feedback pin usually connects through a simple resitor divider because the
output supply voltage is usually larger than what this feedback pin wants.
So, rather than using a resistor divider to connect this feedback pin, if
you connect it directly to a low value resistor (Vfb/Iout) and the other
side of the resistor to GND, and then tie the low side of your load to this
feedback point, too, then you'll convert the constant voltage regulator into
a constant current regulator.
I've done this and it works.
Bob
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Jamie
Something like this?
http://www.maxim-ic.com/appnotes.cfm/an_pk/113
The MAX1640 lets you use a wide supply range and has current
mode options which are very simple.. Just use the LED string
in place of the batter shown as the example..
http://www.digchip.com/datasheets/parts/datasheet/280/MAX1640.php
mi...@sushi.com
wrote:
> "RST Engineering - JIm" <jwei...@gmail.com> wrote in messagenews:ydudnc6-sOOhixrX...@supernews.com...
Your scheme works, though it is pretty inefficient if the voltage at
the feedback pin is a full bandgap, unless the LED string is pretty
long.
gearhead
You can use current sense with hysteresis to turn your led's on and
off.
Jim Thompson has a simple two-transistor circuit on his website that
fits the bill
http://www.analog-innovations.com/SED/CurrentSense.pdf
It's high side but you can flip everything. You'll have to fiddle
with the values, natch.
It's a very interesting circuit. Here's a link to a bit of analysis
http://groups.google.com/group/sci.electronics.basics/browse_thread/thread/905d2e092ea51193/edf7ca0455963b3?q=script+group:sci.electronics.basics+author:gearhead#
You can make the voltage on the current sense resistor a tenth of a
volt if you like.
Paul E. Schoen
"RST Engineering - JIm" <jwe...@gmail.com> wrote in message
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You might want to check the Joule Thief thread for some ideas. I made a
current regulated version that holds the output current to 11.25 mA +/- 2%
at 75% efficiency with input voltage of 1 to 5 VDC, and also a more
powerful version of a blocking oscillator with a MOSFET that drives a
string of 7 white LEDs at up to 30 watts at 80% efficiency with a nominal
12 VDC supply.
I made a more complex circuit with a PIC and PWM which has up to 90%
efficiency at 12-25 watts. It uses an amplifier from the 0.1 ohm current
sense resistor so it does not affect efficiency.
For maximum battery life there are LED drivers that work at up to 98%
efficiency if you have a voltage source greater than the LED string. You
can probably make a boost voltage switcher that is at least 90% efficient.
There are different design considerations if you have rechargeable
batteries that should not be discharged beyond a certain point, or
throw-away batteries that can be milked for the last drop of energy like a
Joule Thief.
Paul
Tim Williams
It's a current mode boost/flyback converter, so you'll get whatever maximum
current the inductor runs. If the inductance is fairly high, current will
be continuous and therefore fairly well regulated without regard for supply
variation.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
"RST Engineering - JIm" <jwe...@gmail.com> wrote in message
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Vladimir Vassilevsky
RST Engineering - JIm wrote:
> I know how to make a linear voltage regulator into a linear current
> regulator and how to use an upside-down PNP and a couple of silicon diodes
> as a relatively constant current source. No big deal. However, I'm a
> little unsure exactly how to translate this knowledge into making a
> switching current regulator. I don't really care if the current varies 10%
> or so over the input voltage range as it is only going to be used to light
> LEDs. What I'm really trying to do is save as much battery life as
> possible.
The "backward" switchers (flyback, sepic...) are the sources of current
by nature of their operation.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Michael Robinson
"gearhead" <nos...@billburg.com> wrote in message
news:83a9beff-49c6-4282...@d32g2000yqh.googlegroups.com...
Hmm, try putting a cap across the sense resistor in that circuit, and use
logicout to drive the led's.
then you'll get a sawtooth and the current will run up and down in the
window between the two trip points. I think.
qrk
Linear Technology, and others, have a multitude of switching power
supplies for LEDs. Check out data sheets and app notes for ideas.
--
Mark
Rich Grise
Search on "joule thief".
Good Luck!
Rich
whit3rd
> I know how to make a linear voltage regulator into a linear current
> regulator and how to use an upside-down PNP and a couple of silicon diodes
> LEDs. What I'm really trying to do is save as much battery life as
> possible.
The LM317-and-a-resistor current source wastes 2V to bias the
regulator,
and 1.25V drop across the resistor, so lighting a 2V LED consumes
current
from a 2+2+1.25 = 5.25V (minimum) source. It's not power-efficient
to proceed in that way, of course.
So, you want an LED and a current sense resistor (that maybe steals
50 mV or so), and your switching regulator has to regulate that 50 mV
value. That means you need a reference voltage in the 50 mV
range (not convenient: 1.25V is the state-of-the-art for silicon ICs).
You also need a (I presume) battery that drives the reference and an
amplifier, in addition to standard SMPS circuitry. One battery cell
can be 1V (NiCd end-of-life) to 3.6V (lithium); easy recharging
can be had for pairs of NiMH, 2 to 2.8V if you put 'em in series.
So, your problem is to do current sensing with op amps and switchmode
power switching with lower-than-convenient voltages. One solution
is to use more cells (four AA NiMH?).
RST Engineering - JIm
charge voltage, should I need it, is on the order of 12 to 20 volts variable
as a function of sun angle.
I'll post the final article in ABSE.
Jim
"whit3rd" <whi...@gmail.com> wrote in message
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