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'[PIC:] [EE:] Driving a 15W LED?'
2002\09\06@133123 by Harold M Hallikainen

picon face
    I've got an application where we need to drive a high power LED.
It's rated at 700mA at about 6.5V, but with adequate heat sinking, it's
been run up to about 2A with 7V across it. Back when we were looking at
700mA, I was considering the LT1618 boost converter with a 4.8V battery.
But, now that they want the higher current, the switch in this part can't
handle it. Also, we're now looking at moving from 4 nickle metal hydride
batteries down to a single lithium ion, reducing the voltage from 4.8 to
3.7. The great thing about the LT1618 is that it has a current sense
amplifier in it so it can regulate based on load current instead of load
voltage, which is what we want here. I have not seen any other switchers
with a current regulator like this. Anyone know of one?
       Any thoughts on using the PIC (which will also be in the circuit) for
regulating the LED current? Since the LED can float, I could put a small
current sense resistor between the bottom of the LED and ground and
perhaps have an analog input on the PIC sense the current. The LT part
has an oscillator that turns on the switch every 750ns or so. A current
sensor turns the switch off when the switch current (and inductor
current) hits some limit. This limit is adjusted to produce the desired
load voltage or load current with there being an absolute limit on the
switch current (about 2A). It seems that the PIC that's handling the rest
of the project could also perhaps measure the switch current and the load
current and drive the external FET switch to do something like this.
However, it seems that I'd need a couple external amplifiers so the
current sense resistors could be small. My parts count is going up again!
       If I go with a standard boost converter chip (or the clever two
transistor circuit recently posted), I'd need to convert the load current
sample to a feedback voltage to drive the converter chip. Again, parts
count goes up. But, I may have no other choice.
       Any ideas out there? What'd be the simplest way to get a regulated 2A
through a 7V LED from a 3.7V battery?

Thanks!

Harold



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2002\09\06@144715 by Roman Black

flavicon
face
Harold M Hallikainen wrote:
>
>      I've got an application where we need to drive a high power LED.
> It's rated at 700mA at about 6.5V, but with adequate heat sinking, it's
> been run up to about 2A with 7V across it.
>         Any thoughts on using the PIC (which will also be in the circuit) for
> regulating the LED current? Since the LED can float, I could put a small
> current sense resistor between the bottom of the LED and ground and
> perhaps have an analog input on the PIC sense the current.
>         Any ideas out there? What'd be the simplest way to get a regulated 2A
> through a 7V LED from a 3.7V battery?


Hi Harold, does your PIC have a comparator?
You could set the comparator vref to the low
setting of 0.2v I think, and use a current sense
resistor of 0.1 ohms. Then a simple boost
converter consisting of one mosfet/inductor/diode
and the PIC controlling duty cycle.

Having the luxury of a PIC brain and nice square
wave output to drive the fet you should be able to
get it efficient with the minimum of parts. :o)
-Roman

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2002\09\06@150456 by Harold M Hallikainen

picon face
On Sat, 7 Sep 2002 04:42:04 +1000 Roman Black <spam_OUTfastvidTakeThisOuTspamEZY.NET.AU>
writes:
> Hi Harold, does your PIC have a comparator?
> You could set the comparator vref to the low
> setting of 0.2v I think, and use a current sense
> resistor of 0.1 ohms. Then a simple boost
> converter consisting of one mosfet/inductor/diode
> and the PIC controlling duty cycle.
>
> Having the luxury of a PIC brain and nice square
> wave output to drive the fet you should be able to
> get it efficient with the minimum of parts. :o)
> -Roman
>

       THANKS Roman! I've been reviewing your simple SMPS page. Nice! Are you
suggesting using the comparator (and sense resistor) to make sure we
don't go over a maximum switch current, but regulating the output using
the PIC's PWM output? Relying purely on the PWM could result in the
switch current taking off if something goes wrong, so limiting duty cycle
based on a maximum measured switch current seems appropriate. I still
need to adjust the switch duty cycle based on the LED current. I'm
thinking A/D there, but are you suggesting use of a comparator there?
Perhaps ramp up duty cycle until the comparator trips, then ramp down
until it "untrips"?
       Anyone know of a simple current to voltage converter chip with an
accurate built-in shunt? Maxim (I think it is) has some that are +/-7A
with PWM output. Interesting parts.

Harold


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Reach broadcasters, engineers, manufacturers, compliance labs, and
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2002\09\06@183827 by Brent Brown

picon face
On 6 Sep 2002 at 10:20, Harold M Hallikainen wrote:

>      I've got an application where we need to drive a high power LED.
> It's rated at 700mA at about 6.5V, but with adequate heat sinking,
> it's been run up to about 2A with 7V across it.

Hi Harold,

I'm interested in really high power LED's, care to share which ones
you're using?

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2002\09\06@193727 by Olin Lathrop

face picon face
part 1 3965 bytes content-type:text/plain; (decoded 7bit)

>         Any thoughts on using the PIC (which will also be in the circuit)
for
> regulating the LED current?

Yup, done that.  See the schematic at http://www.embedinc.com/pic/light.pdf.
This was something I whipped up quickly the day before I was heading off to
the woods of Maine for a week, so the design was limited to parts on hand.

The purpose of this circuit is to run a constant 20mA thru the string of 4
LEDs D2 - D5.  These are bright white, and need a bit over 12V total.  The
battery is a rechargeable NiMH pack with a useful voltage range from 4V down
to about 2.5V.  The PIC is a 16LF628 because that was the smallest L part I
had around.

The basic algorithm is to do a single fixed pulse if the current is too low,
the voltage not too high, and the previous pulse is done.  The port B
passive pullups are enabled.  The B-E voltage of Q2 is used as a cheap
reference with R4 the current sense resistor.  Q2 pulls RB1 low if the LED
current is above the regulation threshold.  Q5 works similarly except that
it senses the LED power voltage.  The purpose of this is to keep the circuit
from destoying itself in case there is ever a break in the LED current path.
Q5 triggers at about 15V, which is above the 13V or so needed to run the
LEDs, but low enough to keep things from getting fried.  Therefore RB1 high
means another pulse is desired.

Q3 turns off during the flyback phase of a pulse, allowing RB0 to go high.
RB0 is set to catch rising edges, so the INTF bit gets set when the previous
pulse is over.  However, this mechanism only works when the LED voltage is
700mV or more higher than the battery voltage, so it is not reliable during
startup.  The firmware therefore also uses a timeout to decide that the
previous pulse is over, which should only kick in during initial power up.

When all the conditions are met, a pulse is started by raising RB7 for a
fixed time.  This is directly tied to the gate of Q1.  These are amazing
little FETs that have only 23mOhm on resistance with 2.8V gate drive, even
better with more.  Inductor L1 charges while Q1 is on, and dumps its energy
onto the LED supply thru D1 when Q1 is turned off.  The firmware then waits
for INTF to get set indicating the pulse is over, then waits for RB1 to be
high to do another pulse.

Efficiency was only 74%, but there are a number of improvements I didn't
have time to make or didn't have parts in stock.  I think in this case I
made the Q1 pulses too long, thereby increasing I*R losses in the inductor
and increasing the losses in D1.

The thing worked great, see the attached picture.  I built the circuit
one-off on a piece of perfboard.  Unfortunately you can't see the circuit
itself because it's all wrapped up in electrical tape.  I also used the tape
to make a pocket for the battery.  That's the green thing with the wire
coming from it.  I epoxied the LEDs to the perfboard you see and rigged up a
mount with two paper clips to clip it to the visor of a hat.  A used
margarine container gave its life for the white bracket.  For now the cord
runs thru another paper clip behind the right ear and then down into the
shirt to a pocket somewhere.  The circuit is small and light enough that I
think it could be mounted on the back of the hat somehow.  I'd like to make
a few real circuit boards, but haven't found (haven't looke real hard either
yet) a nice way of holding the battery in place.  Anyway, it seemed to work
at least as well as any other hiker's headlamp.  I had no problem walking
around the woods with this thing at night.  One fully charged battery pack
lasts for about 4 hours.  These things are small and light (about 3 AAA), so
it's easy to carry several of them.

I've been thinking of similar circuits where a little more accuracy is
needed.  I haven't done this yet, but I'm thinking of using a 12F629 and
using its built in comparator and an external reference (LM385-1.2, 1.23V @
20uA).


part 2 17217 bytes content-type:image/jpeg; (decode)


part 3 305 bytes

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2002\09\07@021327 by Code

flavicon
face
I'm wondering why are you using an external 4Mhz crystal instead of
16LF628's internal oscillator? Or the LF version doesn't have one? That
could save some power and reduce circuit complexity.

{Original Message removed}

2002\09\07@021555 by Code

flavicon
face
I'm wondering why are you using an external 4Mhz crystal instead of
16LF628's internal oscillator? Or the LF version doesn't have one? That
could save some power and reduce circuit complexity.

Tan C C

{Original Message removed}

2002\09\07@080342 by Roman Black

flavicon
face
Harold M Hallikainen wrote:

> > Hi Harold, does your PIC have a comparator?
> > You could set the comparator vref to the low
> > setting of 0.2v I think, and use a current sense
> > resistor of 0.1 ohms. Then a simple boost
> > converter consisting of one mosfet/inductor/diode
> > and the PIC controlling duty cycle.

>         THANKS Roman! I've been reviewing your simple SMPS page. Nice! Are you
> suggesting using the comparator (and sense resistor) to make sure we
> don't go over a maximum switch current, but regulating the output using
> the PIC's PWM output?

Yes. The current will not be 100% smooth, it
will modulate a tiny percentage (like 5%) around
the average desired current. This system is used
in chopper stepper motor drives, all you need is
a comparator which triggers at your desired current
through the sense resistor.

You need a boost regulator (not buck) so it is
slightly different but still quite do-able.

> Relying purely on the PWM could result in the
> switch current taking off if something goes wrong, so limiting duty cycle
> based on a maximum measured switch current seems appropriate. I still
> need to adjust the switch duty cycle based on the LED current. I'm
> thinking A/D there, but are you suggesting use of a comparator there?
> Perhaps ramp up duty cycle until the comparator trips, then ramp down
> until it "untrips"?

No. :o) Like in a chopper drive the comparator will
trigger on the peak of every output pulse, as the
nature of the inductor will cause surges of current
to the output. If current is too high the comparator
will be on for a LONG pulse, if current is too low
the comparator will only trigger for a short pulse
or maybe not at all. You can synchronise testing of
the comparator pulse with your PWM cycle, at the
same frequency. And obviously you have a software
"safe maximum" pulse width!

If you require the highest reliability, I would change
to a variable frequency system. The PIC is configured
to make a PULSE when it changes output, via a simple
capacitor differentiator. That way even if the PIC locks
up the MOSFET can not stay on, as it can only be ON
for a standard length pulse when the PIC pin goes hi.
The extra cap, resistor and diode may be worth it.
Variable freq fixed pulse length system can be good
for a boost converter as the per-pulse energy size can
be optimised for your magnetics.

Again set it to default-oscillate at about 80% of
your desired current, and the PIC can then increase
the pulsing freq slightly to increase the current up
to the desired average.
-Roman

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2002\09\07@092831 by Olin Lathrop

face picon face
> I'm wondering why are you using an external 4Mhz crystal instead of
> 16LF628's internal oscillator?

Mostly because it was Friday afternoon and I knew the calibration value had
long since been lost (or does 16F628 even have a calibration value like
12C508?), so I went with something I'd done many times before and didn't
require any looking up in the manual.  You are right, the internal RC
oscillator would probably be the right answer, after checking a few things
in the manual.

> Or the LF version doesn't have one?

It does.  F and LF parts are the same silicon.  The LF parts are the ones
found to work at lower voltages.  Sometimes regular F parts could have been
labeled as LF but weren't because of stocking and demand issues.

> That could save some power and reduce circuit complexity.

Complexity yes, but power isn't an issue.  Either way the PIC draws less
than 1mA, which is dwarfed by the 100mA required from the battery to power
the LEDs.




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