Re: Issues with LED grid driving
- From: "miso@xxxxxxxxx" <miso@xxxxxxxxx>
- Date: Wed, 9 Sep 2009 14:33:36 -0700 (PDT)
On Sep 9, 12:34 am, Jon Kirwan <j...@xxxxxxxxxxxxxxxxxxx> wrote:
On Tue, 8 Sep 2009 21:36:19 -0700 (PDT), "m...@xxxxxxxxx"
<m...@xxxxxxxxx> wrote:
On Sep 8, 5:10 pm, 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).
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. I
designed the MAX7219, but it doesn't sound applicable for your
application.
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.
<snip>
Broadly speaking, the eye averages when the rate is fast. I've tested
this and I have no question about it, anymore. (When it isn't fast,
other obvious things come into play -- namely, you can see the flicker
which pretty much changes the ball game, anyway.)
Anyone can purchase a copy of HP's "Optoelectronics: Fiber-Optics
Applications Manual," 2nd edition, through alibris or some other
bookseller outlet, and take a look at the quote in the last paragraph
on page 5.25, "The human eye is a time average detector..." That
quote is also from HP. In any case, it's clear enough through
experiment, too.
...
That said, the effect of pulsing is not entirely net-zero. There is a
suggestive curve on page 5.20 of the same book above, Figure 5.2.4-1,
"Relative Luminous Efficiency (Luminous Intensity Per Unit Current) vs
Peak Current Per Segment for a High-Efficiency Red Display." The
curve shows increased luminous efficiency when pulsing vs DC, using
the time-averaged current as the standard (until the LED junction
nears saturation, which it will do at some point.)
Their example note that pulsing a high-efficiency red LED with 50mA at
a 10% duty cycle (5mA time-averaged) yields a luminous intensity about
1.6 times as great as running the same LED at a constant 5mA. The
curve for this red LED basically flattens out at 1.6, so higher pulse
currents aren't helpful in this case.
Keep in mind that pulsing the LED with 50mA requires a higher drive
voltage than if the same LED were run with a DC current equal to the
time-averaged equivalent. For example, rather than 1.9V@5mA/100% it
might be 2.6V@50mA/10%; which is 9.5mW and 13mW average, respectively.
So although it may be 1.6 times brighter, it's also about 1.4 times
the power. Some gain, but nothing to write home about and certainly
not like some 10X brightness that a 'peak' response theory would
suggest.
(Higher temperatures also lower output, on the order of 1%/C,
roughly.)
On the other hand, if you have to drop voltage to control the current
anyway, you might as well hand that over to the LED and make something
out of it than just toss it all away in the regulation (or resistor)
if you can afford the reduced overhead.
Anyway, my experience is consistent with the comments from HP's book.
Since you designed the MAX7219, you must have seen enough of all this
on your own, by now. How is it that you remain ambivalent about the
question?
Jon
Market forces being what they are, you make a chip that multiplexes
and delivers the current suitable to the display elements because that
is what the customers already use. You just try to make a better
version. So it was a matter of "not my job" regarding if the eye sees
the peak or not. Now if I had to make a chip for LED video, where the
intensity linearity is important, that would be a different issue.
Note that the refresh rate is kept a bit higher than the eye requires.
This is due to the fact that some 7-segment displays are mounted on
machinery that vibrates. If the refresh rate is too low, the digits
are hard to read under those circumstances.
What I did do is add the feature to the objective spec to reduce the
number of digits driven so that the duty cycle could be boosted. You
do need to read the datasheet carefully since electromigration limits
mean the current has to be reduced when driving less than 4 digits.
There is the elevator market than just needed two digits, but also
needed higher voltage, so the chip would act more like a external
power fet driver than a LED driver. Money is not an issue to those
customers.
The only feature that I wanted that got nixed was the cursor. Freaking
marketing people are dense beyond belief. The cursor would put the
chip in more low volume instrumentation where LCD is not practical.
There is apparently something odd in the way I implemented the serial
interface that pisses off customers. They design around it. I'd need
to review the datasheet to recall the issue. The part was popular
enough that a low slew version was made for reduced EMI. It later got
a new layout for surface mount. It's been moved through a few
processes over the years, though given the digital nature of the
product, nobody really noticed. You would be shocked at the money the
chip makes since it is sold in low volume to many users, which is a
good formula to make money in the chip business Unless you are Intel,
volume means low profit. There is some Chinese copy out there, though
I don't know if it is still made.
I'm always surprised at these people calling Maxim chip unobtainium
since I saw the volume my stuff sold at. Even the lame stuff would do
a few hundred K per year. But I know some products had their issues.
.
- References:
- Issues with LED grid driving
- From: Bobby Joe
- Re: Issues with LED grid driving
- From: miso@xxxxxxxxx
- Re: Issues with LED grid driving
- From: Jon Kirwan
- Issues with LED grid driving
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