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From: Bobby Joe on 8 Sep 2009 20:10
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.
From: linnix on 8 Sep 2009 20:46
On Sep 9, 8:10 am, 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).

1/8 cycle is already pushing it.

>
> 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).

1/24 duty cycle would be too dark for reasonable currents.

>
> 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.
>

Yes, those who shine so bright, live so short.

> 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.

From: Jon Kirwan on 8 Sep 2009 23:53
On Tue, 8 Sep 2009 17:10:50 -0700 (PDT), Bobby Joe
<bobbyjoe23928(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.

Jon
From: miso on 9 Sep 2009 00:36
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. 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.

Regarding power dissipation. it really is straight forward. You know
the current in the LED. You need to know the drop across the LED, as
well as the tempco of this voltage. Once you know the voltage across
the LED, then the remaining supply voltage is dropped across the chip.
P=VI. Do the worst case of all LED on. Next you need to know the theta
junction to ambient of the chip. It should be specified by the
manufacturer, though your layout effects it a bit.You need to pick the
highest ambient temperature you expect the chip to be in. This
generally isn't all that high since presumably a human being will be
viewing the display. Say 140 deg F.. [Few places on earth get to 120
deg F, so that give you some margin.] Convert 140 deg F to 60 deg C.
Pick a target for the die temperature and see if it fits the chip
limit. If not, then you need to reduce the target "viewing"
temperature or work out a heat sink scheme for the chips.

Unless the chip is in a special thermal package, most of the heat
comes out the pins. [The package might have a thermal slug in it,
which does release some heat from the package body. Generally a fat
bonding wire is used since the heat flow is proportional to the
crossectional area of the bond wire. ] You can marginally improve the
theta JA by putting more copper on the traces to the chip. Even metal
under the chip would help a bit.



From: Jon Kirwan on 9 Sep 2009 03:34
On Tue, 8 Sep 2009 21:36:19 -0700 (PDT), "miso(a)sushi.com"
<miso(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. 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(a)5mA/100% it
might be 2.6V(a)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
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