Re: Issues with LED grid driving
- From: Jon Kirwan <jonk@xxxxxxxxxxxxxxxxxxx>
- Date: Wed, 09 Sep 2009 17:04:47 GMT
On Wed, 9 Sep 2009 09:20:27 -0700 (PDT), Bobby Joe
<bobbyjoe23928@xxxxxxxxx> wrote:
On Sep 8, 10:53 pm, Jon Kirwan <j...@xxxxxxxxxxxxxxxxxxx> wrote:
On Tue, 8 Sep 2009 17:10:50 -0700 (PDT), Bobby Joe
<bobbyjoe23...@xxxxxxxxx> 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).
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