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From: Jan Panteltje on 9 Sep 2009 08:36
On a sunny day (Tue, 8 Sep 2009 21:36:19 -0700 (PDT)) it happened
"miso(a)sushi.com" <miso(a)sushi.com> wrote in
<7e3c5fba-edcd-4213-b393-90d4b48272f9(a)l9g2000yqi.googlegroups.com>:

>Regarding PWM, there are two schools of thought, both which have been
>discussed on SED. Some claim the eye retains the peak value, so as you
>PWM, it it does not look linear. Others say the eye averages
>perfectly. Who knows. An old HP app note claims the eye maintains the
>peak value.

My experience shows clearly that the eye does average.
That is why you can 'dim' a LED display with PWM.
Seem pretty linear to me too, but I added adjustable gamma correction just for fun:
http://panteltje.com/panteltje/pic/col_pic/
From: Bobby Joe on 9 Sep 2009 11:55
On Sep 8, 11:36 pm, "m...(a)sushi.com" <m...(a)sushi.com> wrote:
> On Sep 8, 5:10 pm, Bobby Joe <bobbyjoe23...(a)gmail.com> wrote:
>
>
>
> > Is anyone familiar with driving large RGB led grids. Such as 32x32
> > using cascaded LED drivers. Actually my specific grid is 24x19(each
> > point is one led and not an rgb). I have seen 24-ch led drivers along
> > with 16-ch x 8-com(for 128 total led's).
>
> > Think of the grid as a led matrix display panel as essentially it is
> > what it is. If I use 24-ch drivers then it requires 19 IC's. Some
> > chips have built in PWM, dot correction, and other nice features but
> > at a premium. I do not need error checking but thermal overload
> > shutdown would be nice.
>
> > Using a matrix would be much cheaper as I could use 1 24-ch driver and
> > 19 fets, one for each row. The main issue I am worry about here is the
> > duty cycle required for each led row and power requirements for the
> > driver(which I can split the rows up to reduce the power consumption).
>
> > If I require a nominal 10mA per led then this is 4.5A and
> > approximately 20W's total dissipation. I'm not quite sure how to
> > calculate the power dissipated by the IC. I would like to increase the
> > nominal current to 20mA if possible just for headroom in case it is
> > eventually required.
>
> > The only problem here is that it requires a duty cycle of 1/19 which
> > bumps up the peak current to approximately 200mA. Does this seem
> > pretty extreme? The peak current at 1/10 @ 1Khz is R=60mA, G=B=100mA.
> > So this seems to be pushing it assuming I can extrapolate linearly.
>
> > If it's too much I can split the grid into two or three but I'd like
> > to do it all at once.
>
> > What kinda of effect does using PWM have on the led optics? Does the
> > intensity and color end up changing or can I expect a fairly
> > consistent output over a wide range of duty cycles?
>
> > Are there issues with low current? I've heard of pre-charged fets but
> > not sure exactly what they do. I would like to operate the driving
> > chips for grayscale.
>
> > I guess the real question I'm asking is if running a 24x19 grid is
> > easily done off one or two drivers. My original thought was to use as
> > many drivers as needed and take advantage of the features they have
> > except it seems awful expensive just to drive the grid.
>
> You really sound like you are biting off more than you can chew.

Does that make you feel more superior? How do you have any idea if
it's more than I can handle or not? Because I asked some questions? Or
maybe you believe it is difficult and hence can't see any lower life
forms being able to do it? Or maybe your just taking after your
messiah Obama?
From: Bobby Joe on 9 Sep 2009 12:20
On Sep 8, 10:53 pm, Jon Kirwan <j...(a)infinitefactors.org> wrote:
> On Tue, 8 Sep 2009 17:10:50 -0700 (PDT), Bobby Joe
>
> <bobbyjoe23...(a)gmail.com> wrote:
> >Is anyone familiar with driving large RGB led grids. Such as 32x32
> >using cascaded LED drivers. Actually my specific grid is 24x19(each
> >point is one led and not an rgb). I have seen 24-ch led drivers along
> >with 16-ch x 8-com(for 128 total led's).
>
> I have experience _using_ them.  Not designing them.  Electronics is a
> hobby of mine, not a profession.
>
> >Think of the grid as a led matrix display panel as essentially it is
> >what it is. If I use 24-ch drivers then it requires 19 IC's. Some
> >chips have built in PWM, dot correction, and other nice features but
> >at a premium. I do not need error checking but thermal overload
> >shutdown would be nice.
>
> The chips I've used do use PWM and other 'nice features.'  They were
> arranged as 8x16 drivers (1/8th period .  The ones I used were in a
> 16x16 module and they used 6 ICs, two to make up a 16x16 of one color
> and three sets of these pairs for the tri-color LED system.  Separate
> power supply rails for each color, to reduce power consumption.  Each
> 16x8 graphics module IC included RAM, an address decoder, the mux
> circuitry, and constant current drivers with their 7-bit current value
> stored in non- volatile memory, including column staggering to reduce
> EMI, interdigit blanking time, etc.  The constant current drivers set
> the maximum current value and the PWM was used to reduce the intensity
> from there.  It included over-temp shutdown and also a kind of deadman
> thing where if the external clock wasn't present for 30ms, it would
> also shut down.  The ICs were custom, but the whole 16x16x3 module,
> with heatsink and 6 ICs built into it was about $80 to the customer,
> years ago.
>
> >Using a matrix would be much cheaper as I could use 1 24-ch driver and
> >19 fets, one for each row. The main issue I am worry about here is the
> >duty cycle required for each led row and power requirements for the
> >driver(which I can split the rows up to reduce the power consumption).
>
> Power requirements were nasty.  It supported up to about 2.5A per
> color, for a total of 7.5A.  The red supply (typical) was 4V, the blue
> and green were 5.75V.  The dissipation for the 16x16 was, as you can
> see, nearing 40W.  (That's all 6 ICs.)  The actual, considering that
> not LEDs were on all the time or at full brightness, was less than
> half that.  But it had a heat sink of its own that was intended to be
> bolted into something else to help out.  And sometimes you wanted
> everything ON, so it had to handle worst case -- at least for some
> time.
>
> >If I require a nominal 10mA per led then this is 4.5A and
> >approximately 20W's total dissipation. I'm not quite sure how to
> >calculate the power dissipated by the IC. I would like to increase the
> >nominal current to 20mA if possible just for headroom in case it is
> >eventually required.
>
> >The only problem here is that it requires a duty cycle of 1/19 which
> >bumps up the peak current to approximately 200mA.  Does this seem
> >pretty extreme?
>
> Yes.  It's pretty extreme.  I thought x8 was pushing things.  Worked
> okay, I admit.  But I'd probably not push things harder than that
> without good experimentation to support more, first.  You actually
> lose something in the process, too.  LEDs do gain a little in
> brightness, keeping average current a constant, if you raise the peak
> current and reduce down from 100% duty.  But only up to a small bit.
> Maybe 50% duty and twice the average?  Something like that.  After
> that, it goes back downhill again.  For some LEDs, anyway.
>
> >The peak current at 1/10 @ 1Khz is R=60mA, G=B=100mA.
> >So this seems to be pushing it assuming I can extrapolate linearly.
>
> >If it's too much I can split the grid into two or three but I'd like
> >to do it all at once.
>
> Split the grid.  Use identical drivers, chained up together.  Make
> them yourself.
>
> >What kinda of effect does using PWM have on the led optics? Does the
> >intensity and color end up changing or can I expect a fairly
> >consistent output over a wide range of duty cycles?
>
> With 1/8th (8 by), you might consider 32 PWM intensity steps as
> adequate.  I don't know your application, though.  The choice of what
> those steps should be... well, that's up to you.  And no, don't expect
> consistent output from different LEDs, even if they are from the same
> manufacturer and same batch.  (Unless they tell you that they bin
> them, first.)  They generally won't look the same side-by-side at the
> same current and same duty cycle.  At least, not to me.  I had to bin
> the damned things, myself, on both color and intensity.
>
> >Are there issues with low current? I've heard of pre-charged fets but
> >not sure exactly what they do. I would like to operate the driving
> >chips for grayscale.
>
> >I guess the real question I'm asking is if running a 24x19 grid is
> >easily done off one or two drivers. My original thought was to use as
> >many drivers as needed and take advantage of the features they have
> >except it seems awful expensive just to drive the grid.
>
> What seems simple to imagine at first can get hairy fast.
>

The devil is in the details. But this is a somewhat common application
and is not technically challenging. The biggest problem seems to be
dot correction but it is not something difficult since I can simply
compensate for led variance and even IC variance in software if needed
(assuming I have some enough PWM steps). I do think that dot
correction will not be an issue for my application in any event.

My main issue is simply one of economy. I have laid out the matrix
using one channel per LED but this requires and ton lot of drivers. If
I go with a fully featured driver the cost is somewhat astronomical.
If I use simple drivers such as the http://focus.ti.com/docs/prod/folders/print/tlc59025.html
which is bare bones it is about 6 times cheaper(from ~80$ to ~15).

By reducing the number of IC's means the complexity goes up. After
all, I have to implement the PWM myself if I want to use a matrix
based design because I can't pwm through multiple rows. I then also
have to deal with finding the right way to divide the matrix so that I
reduce the duty cycle and peak current to those that will work(which
seems like I might have to do some testing to find the best way).

The refresh rate is only an issue in that the faster it is requires
communicating faster with the ic's. It also changes the dynamics of
driving. Is 100mA 10%@1kHz the same as 100mA 10%@100kHz? I've read
that actually the faster you pwm the better because of thermal
resistance. How much? I have no idea ;/ Last thing I want to do is
create a system that burns up a 100$ worth of led's.

I've noticed with my led's that with low current but all LED's on that
I can see the individual colors. They do not mix well to form white.
But with higher currents I can a much better mix. I'm not sure if this
is a defect in the specific led's or one of all RGB led's. It
obviously has to do with how close the individual colors.

I guess the only thing for me to really do is run some tests. Was
hoping someone else already did this(I'm sure someone has).


From: Jon Kirwan on 9 Sep 2009 13:04
On Wed, 9 Sep 2009 09:20:27 -0700 (PDT), Bobby Joe
<bobbyjoe23928(a)gmail.com> wrote:

>On Sep 8, 10:53�pm, Jon Kirwan <j...(a)infinitefactors.org> wrote:
>> On Tue, 8 Sep 2009 17:10:50 -0700 (PDT), Bobby Joe
>>
>> <bobbyjoe23...(a)gmail.com> wrote:
>> >Is anyone familiar with driving large RGB led grids. Such as 32x32
>> >using cascaded LED drivers. Actually my specific grid is 24x19(each
>> >point is one led and not an rgb). I have seen 24-ch led drivers along
>> >with 16-ch x 8-com(for 128 total led's).
>>
>> I have experience _using_ them. �Not designing them. �Electronics is a
>> hobby of mine, not a profession.
>>
>> >Think of the grid as a led matrix display panel as essentially it is
>> >what it is. If I use 24-ch drivers then it requires 19 IC's. Some
>> >chips have built in PWM, dot correction, and other nice features but
>> >at a premium. I do not need error checking but thermal overload
>> >shutdown would be nice.
>>
>> The chips I've used do use PWM and other 'nice features.' �They were
>> arranged as 8x16 drivers (1/8th period . �The ones I used were in a
>> 16x16 module and they used 6 ICs, two to make up a 16x16 of one color
>> and three sets of these pairs for the tri-color LED system. �Separate
>> power supply rails for each color, to reduce power consumption. �Each
>> 16x8 graphics module IC included RAM, an address decoder, the mux
>> circuitry, and constant current drivers with their 7-bit current value
>> stored in non- volatile memory, including column staggering to reduce
>> EMI, interdigit blanking time, etc. �The constant current drivers set
>> the maximum current value and the PWM was used to reduce the intensity
>> from there. �It included over-temp shutdown and also a kind of deadman
>> thing where if the external clock wasn't present for 30ms, it would
>> also shut down. �The ICs were custom, but the whole 16x16x3 module,
>> with heatsink and 6 ICs built into it was about $80 to the customer,
>> years ago.
>>
>> >Using a matrix would be much cheaper as I could use 1 24-ch driver and
>> >19 fets, one for each row. The main issue I am worry about here is the
>> >duty cycle required for each led row and power requirements for the
>> >driver(which I can split the rows up to reduce the power consumption).
>>
>> Power requirements were nasty. �It supported up to about 2.5A per
>> color, for a total of 7.5A. �The red supply (typical) was 4V, the blue
>> and green were 5.75V. �The dissipation for the 16x16 was, as you can
>> see, nearing 40W. �(That's all 6 ICs.) �The actual, considering that
>> not LEDs were on all the time or at full brightness, was less than
>> half that. �But it had a heat sink of its own that was intended to be
>> bolted into something else to help out. �And sometimes you wanted
>> everything ON, so it had to handle worst case -- at least for some
>> time.
>>
>> >If I require a nominal 10mA per led then this is 4.5A and
>> >approximately 20W's total dissipation. I'm not quite sure how to
>> >calculate the power dissipated by the IC. I would like to increase the
>> >nominal current to 20mA if possible just for headroom in case it is
>> >eventually required.
>>
>> >The only problem here is that it requires a duty cycle of 1/19 which
>> >bumps up the peak current to approximately 200mA. �Does this seem
>> >pretty extreme?
>>
>> Yes. �It's pretty extreme. �I thought x8 was pushing things. �Worked
>> okay, I admit. �But I'd probably not push things harder than that
>> without good experimentation to support more, first. �You actually
>> lose something in the process, too. �LEDs do gain a little in
>> brightness, keeping average current a constant, if you raise the peak
>> current and reduce down from 100% duty. �But only up to a small bit.
>> Maybe 50% duty and twice the average? �Something like that. �After
>> that, it goes back downhill again. �For some LEDs, anyway.
>>
>> >The peak current at 1/10 @ 1Khz is R=60mA, G=B=100mA.
>> >So this seems to be pushing it assuming I can extrapolate linearly.
>>
>> >If it's too much I can split the grid into two or three but I'd like
>> >to do it all at once.
>>
>> Split the grid. �Use identical drivers, chained up together. �Make
>> them yourself.
>>
>> >What kinda of effect does using PWM have on the led optics? Does the
>> >intensity and color end up changing or can I expect a fairly
>> >consistent output over a wide range of duty cycles?
>>
>> With 1/8th (8 by), you might consider 32 PWM intensity steps as
>> adequate. �I don't know your application, though. �The choice of what
>> those steps should be... well, that's up to you. �And no, don't expect
>> consistent output from different LEDs, even if they are from the same
>> manufacturer and same batch. �(Unless they tell you that they bin
>> them, first.) �They generally won't look the same side-by-side at the
>> same current and same duty cycle. �At least, not to me. �I had to bin
>> the damned things, myself, on both color and intensity.
>>
>> >Are there issues with low current? I've heard of pre-charged fets but
>> >not sure exactly what they do. I would like to operate the driving
>> >chips for grayscale.
>>
>> >I guess the real question I'm asking is if running a 24x19 grid is
>> >easily done off one or two drivers. My original thought was to use as
>> >many drivers as needed and take advantage of the features they have
>> >except it seems awful expensive just to drive the grid.
>>
>> What seems simple to imagine at first can get hairy fast.
>
>The devil is in the details.

It is. Power is a big issue, for example. Distribution as well as
dissipation. Even though it remains a broadly simple concept.

You asked earlier how to tell about power consumption other than with
the LEDs. I hope you have the means to estimate that, now. It's not
hard to estimate, but it is important.

>But this is a somewhat common application and is not technically
>challenging.

It's challenging enough so that you ask some reasoned questions,
though.

>The biggest problem seems to be
>dot correction but it is not something difficult since I can simply
>compensate for led variance and even IC variance in software if needed
>(assuming I have some enough PWM steps). I do think that dot
>correction will not be an issue for my application in any event.
>
>My main issue is simply one of economy. I have laid out the matrix
>using one channel per LED but this requires and ton lot of drivers. If
>I go with a fully featured driver the cost is somewhat astronomical.
>If I use simple drivers such as the http://focus.ti.com/docs/prod/folders/print/tlc59025.html
>which is bare bones it is about 6 times cheaper(from ~80$ to ~15).
>
>By reducing the number of IC's means the complexity goes up. After
>all, I have to implement the PWM myself if I want to use a matrix
>based design because I can't pwm through multiple rows. I then also
>have to deal with finding the right way to divide the matrix so that I
>reduce the duty cycle and peak current to those that will work(which
>seems like I might have to do some testing to find the best way).
>
>The refresh rate is only an issue in that the faster it is requires
>communicating faster with the ic's. It also changes the dynamics of
>driving. Is 100mA 10%@1kHz the same as 100mA 10%@100kHz? I've read
>that actually the faster you pwm the better because of thermal
>resistance. How much? I have no idea ;/ Last thing I want to do is
>create a system that burns up a 100$ worth of led's.
>
>I've noticed with my led's that with low current but all LED's on that
>I can see the individual colors. They do not mix well to form white.
>But with higher currents I can a much better mix. I'm not sure if this
>is a defect in the specific led's or one of all RGB led's. It
>obviously has to do with how close the individual colors.
>
>I guess the only thing for me to really do is run some tests. Was
>hoping someone else already did this(I'm sure someone has).
>
From: Jon Kirwan on 9 Sep 2009 13:06
On Wed, 09 Sep 2009 17:04:47 GMT, I wrote:

>><snip>
>>
>>The devil is in the details.
>
>It is. Power is a big issue, for example. Distribution as well as
>dissipation. Even though it remains a broadly simple concept.
>
>You asked earlier how to tell about power consumption other than with
>the LEDs. I hope you have the means to estimate that, now. It's not
>hard to estimate, but it is important.
>
>>But this is a somewhat common application and is not technically
>>challenging.
>
>It's challenging enough so that you ask some reasoned questions,
>though.
><snip>

I didn't finish. Sorry about sending this a little too quickly. More
thoughts perhaps, later on.

Jon
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