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8x8 RGB LED driver.

D

Daniel Pitts

Jan 1, 1970
0
TL;DR: I'd like some help with sourcing up-to 340ma from a 1-of-8
demuxer, and deciding on a 500ma power solution.

So, coming back to the LED driver design, which is at least a tad more
complicated than the newbie me thought :) Please pardon my brain-dump
here. I have some questions near the end.

My existing design has been posted elsewhere, but lets ignore that and
see if I can "start from scratch" so-to-speak, and get the right design.

I have this matrix:
<http://www.seeedstudio.com/depot/datasheet/2088RGBMatrix.pdf.>

If I'm reading that right, it is a common anode. I'm not sure about
whether the "maximum ratings" section is for the entire device, or for
each LED package. Given the math below, it seems likely to be per LED
triplet.

I'll want to change the current value for each color to achieve as pure
white as possible if all colors are on.

For now, I'll assume about half as much current for RED, but close to
equal current for GREEN and BLUE. Lets pick 10ma, 20ma, 20ma for now.

Given the 2.2v typical drop on Red, and the 3.3v on green/blue, that
gives me, at full "on", 22mw+66mw+66mw=154mw. Hmm, that is just above
the spec. Maybe I need to go to 8,16,16?

So, I can drive one column by a tlc5916 per color.
<http://www.ti.com/lit/ds/symlink/tlc5917-q1.pdf>

According to the "Adjusting Output Current" section in the 5917 spec, if
I want a given output current, Iout, I would use the following formula,
if the default power-on settings are used:

Iout = (1.25V/Rext) * 15

Solving for Rext, I get Rext=(1.25v/Iout) * 15

This means that for the 8ma I'd use 2343Ω for red and half that, 1.2K,
for the green/blue. Now, since I don't want to overdrive these, I'd
probably round up. I'd also probably add a rheostat so I could
calibrate them more accurately.

Now, on to the other "side" of the LEDs. There are 8 lines, each one is
the common anode of 24 LEDS. This means that I would need to source a
max of (8ma + 16ma + 16ma) * 8 = 320ma. Ouch, that's higher than I'd hoped.

This is where I need some real help. Are there any 3-to-8 decoder ICs
that can source that kind of current? If not, is there some sort of 8
bit buffer IC that I can use, where the input is my 74HC238 decoder, and
the output can source that current?

I guess the alternative is to get 8 transistors, and drive them from the
decoder. Would BJTs be the right device for this job? Would I use NPN,
or PNP? I guess if I use the 74x238, which is active high, then I'd
want NPN, 74x138 is actually easier to acquire, so maybe I should
switch to that and use PNP?

So, once that is answered, the next part is the power source.
Apparently I'll need well over 340ma to power this thing. It seems like
rounding to 500ma would be good enough. I could power it off of a 5v
500ma AC/DC adapter.

I was hoping for something more portable though. t looks like if I
wanted to power it off of AA's, I'd be looking at either 3 cells, and/or
using a DC-to-DC converter to get 5 volts. The device would run full
brightness for about 2-3 hours per battery (or 1-2 for rechargable).
Although, in reality the brightness will be far less than that on
average. Suggestions on this would be appreciated.

One of my goals is to prototype this cheaply. I'm not looking to sell
this design, its mostly just for the experience of designing and
building it. The digital circuitry is pretty easy and straight forward.
The analog aspect is still kind of kicking me in the pants.

Anyway, if you've made it this far, thanks for reading through my
stream-of-consciousness. Any suggestions (even criticisms) are highly
appreciated.

Thanks,
Daniel.
 
J

Jon Kirwan

Jan 1, 1970
0
Hi, Daniel. I'm still thinking about something similar, too.
Thoughts below, useful or not:

TL;DR: I'd like some help with sourcing up-to 340ma from a
1-of-8 demuxer, and deciding on a 500ma power solution.

I'm thinking that it _should_ be even worse than you imagine.
I think you are doing a x8 mux for the 8x24 (rgb) matrix, as
I gather you are using three of the 5916s, one for each
color.

Each LED is spec'd at 20mA. That's an average value. The
absolute max says no more than 70mA peak and 50mA average.
With a x8 mux, to achieve 20mA average you'd need to drive
160mA into each. Human intensity perception is logarithmic,
so shifting to 70mA/8 average from 20mA average will mean
about 82% brightness, perception-wise. Tolerable. But
shifting to 20mA/8 drops you to about 65% and 10mA/8 to about
58%. That's noticeable.

If you were to peak pulse them at 70mA, you are talking 560mA
for 8 or 1.68A for all 24 (pushing the red the same.) That's
a lot more than 340mA. That's assuming you push the red led
as much as the others, of course.
So, coming back to the LED driver design, which is at least
a tad more complicated than the newbie me thought :)
Please pardon my brain-dump here. I have some questions near
the end.
:)

My existing design has been posted elsewhere, but lets
ignore that and see if I can "start from scratch"
so-to-speak, and get the right design.

I have this matrix:
<http://www.seeedstudio.com/depot/datasheet/2088RGBMatrix.pdf.>

If I'm reading that right, it is a common anode. I'm not
sure about whether the "maximum ratings" section is for the
entire device, or for each LED package. Given the math
below, it seems likely to be per LED triplet.

I read the absolute max specifications as being "per LED" not
"per RGB triplet." The 8V reverse spec seems normal on a per
LED basis. The continuous forward current of 50mA would also
be "per LED," I think. The peak forward current of 70mA would
also be "per LED," I think. Because those are quite normal
for individual LEDs. The power dissipation of 150mW (average,
of course) makes sense primarily because of the temperature
range spec. They are allowing for operation of no more than
60C over ambient. With 150mW, this suggests 400W/C which is
about twice that of a TO92 BJT (worse than, in short.) Which
makes sense to me, again on a per LED basis. But only because
they are telling you that you "could" do that if you only
operate one of the LEDs. I think that if you run all three of
them, and they are certainly nearby each other, I'd bet the
150mW spec would be applied to the triplet, too. So that one,
I think I'd take as a triplet spec if operating all three.

Let's say you drive all three at 70mA peak, using a x8 mux
scheme. Using typical Vf, that's 2.2V@70mA/8 + 3.3V@70mA/8 +
3.3V@70mA/8 or 77mW. Even allowing that the voltage is
"typical" and permitting for another .4V headroom on each
LED, it's still under 88mW for the triplet. Average. Of
course, during the 1/8th cycle, the power will be 8 times
that or slightly over 600mW. But since that is a short time
and there is a thermal filtering response, it's likely you
can do that safely. The only concern I'd have is thermal
flexing of the leds on their mountings within embedded epoxy.
But I'd go with that for hobby use, anyway.

Another take on that: They say that the LEDs can support 50mA
continuous. I think that's "per LED." But... only one of them
in that case. At 50mA and a typical of 3.3V, you'd see 165mW.
Which is close to their power dissipation spec.

The sheet doesn't do a complete analysis for you. It doesn't
even say much. Nothing about the C/W thermal resistance, for
example. So it's low on specs and you have to make some
reasoned guesses. I'm just a hobbyist too, but those are my
guesses looking at the sheet.

I'm guessing they are telling you not to drive any LED at
more than 70mA peak and 50mA average. The operational
temperature of 85C max is probably the most any particular
LED should be at. Also, no single LED should dissipate more
than 150mW on average.
I'll want to change the current value for each color to
achieve as pure white as possible if all colors are on.

The rough equivalent for what we did at OSRAM would be to use
three TLC5916's and use three pots to set the max current.
Then set up all the LEDs at 25% PWM, turn the whole panel on
at once, and adjust the three pots until the display read a
specific (D60) CIE white point using the spectro (and
software.) Once the pots are set, you are golden. PWM from
there. The results were quite good enough.

What I'm tentatively thinking about doing right now is to use
the TLC5916 on the low side to drive a current mirror on the
high side... but where I set up the TLC5916 for about 1/10th
the current. The current mirror will multiply by 10 to
achieve the actual drive current, using a resistor in one
leg. That way I can select BJTs quite capable of a
significant voltage drop AND current and be able to dissipate
the power without problems. That adds two BJTs per TLC5916
current sink pin. But that's the price I pay to move the
dissipation elsewhere while retaining the convenience of a
shift register and single resistor for setting each current
sink value.

But I'll require common cathode arrangement for this, as the
switches will be on the cathode side, since the current
mirrors will have to be on the high side.

I want to pulse as much as 160mA (or a little more) and
support a voltage drop of more than 2V on the BJT. That's
20mA = 160mA * 1/8th duty. Can't do that with the 5916 under
any circumstances.
For now, I'll assume about half as much current for RED, but
close to equal current for GREEN and BLUE. Lets pick 10ma,
20ma, 20ma for now.

That's not going to be white. But you know your experience
better than I do. Mine says... no guessing... use pots to set
the white balance. And even then, you may need to calibrate
each pixel individually. LED manufacturing, 10 years ago at
least, wasn't up the ability to produce consistent LED
performance -- even when the LED dies were cut from the exact
same wafer. In fact, I worked on programming (and some
optical design) for machines that a big company (Siemens and
OSRAM) required so they could "bin" their parts before
selling them, or using them in composite displays. The extra
expense was not optional. It was required. (Unless the
customers didn't care and could tolerate "slightly more
orangish red leds" sitting side by side each other.)

By the way, red LEDs do not emit much in the green where
human intensity is perceived. (Only one of the three color
rods is involved -- usually denoted Y.) I'm not sure why
you've arbitrarily decided your numbers. But I assume it is
from personal experience with those matrices. So I just
caution you, but don't know better than you about them.
Given the 2.2v typical drop on Red, and the 3.3v on green/blue, that
gives me, at full "on", 22mw+66mw+66mw=154mw. Hmm, that is just above
the spec. Maybe I need to go to 8,16,16?

No. Power is AVERAGE. The peak power (ramming Joules into
something for a short time) does cause some thermal flexing,
though. And it also creates some excess voltage drops in the
diodes that doesn't translate directly into light emission.
So you don't get exactly X times as much light out for X
times the power in. But the specification is always an
average specification. If you are pulsing fast, I think the
thermal response will be very sluggish by contrast and will
do a good job of "averaging" the pulsed power.
So, I can drive one column by a tlc5916 per color.
<http://www.ti.com/lit/ds/symlink/tlc5917-q1.pdf>

According to the "Adjusting Output Current" section in the 5917 spec, if
I want a given output current, Iout, I would use the following formula,
if the default power-on settings are used:

Iout = (1.25V/Rext) * 15

Solving for Rext, I get Rext=(1.25v/Iout) * 15

This means that for the 8ma I'd use 2343? for red and half that, 1.2K,
for the green/blue. Now, since I don't want to overdrive these, I'd
probably round up. I'd also probably add a rheostat so I could
calibrate them more accurately.

Yes, I'd definitely use a pot for each of the 5916s.
Now, on to the other "side" of the LEDs. There are 8 lines, each one is
the common anode of 24 LEDS. This means that I would need to source a
max of (8ma + 16ma + 16ma) * 8 = 320ma. Ouch, that's higher than I'd hoped.

As I wrote above, I think it's even worse than you calculate
if you really want to push these to produce brightness near
their capacity. Obviously, less works too. But I'd be pushing
them to do what they can and then use PWM for dimming (and
geometrically, not linearly.)

....

I'm shooting for 20mA average (I'll never use that much in
practice), but this means 160mA peak at a x8 mux. In my case,
it's x5. So just 100mA.

But I'm also facing limitations in the 5916, in terms of
dissipation. Aside from the LEDs themselves, all of the
remaining dissipation takes place in the current sinks. The
high side switches are ... switches. They don't drop much by
way of volts. Just a lot of current. And they are external
BJTs (in my case, anyway.) So they can handle it fine. The
real problem is with the 5916, which is pathetic at 600mW max
total. In using all 8 outputs as in your case, this means
75mW per sink. If I tried to sink 100mA (within spec), that
would be 750mV. That's all?

So let's say my LED power rail is 5V and my RED LEDs drop
2.2V and my high side switch drops 200mV. That leaves 2.6V
for the sink. Blows that 750mV out of the water! Not even
close. I'd need a RED LED rail of 3.15V worst case to meet
the max spec. All because the current sink MUST pick up all
the excess voltage at the desired current and dissipate it.
That's what the sink does.

I could use an external BJT, in common base config, to pick
up the slack. But then I'd need another power rail at, say
1.5V to attach to the BJT base. That would move the
dissipation outside the package and into the BJT. But then
I'd waste a lot of unnecessary power and voltage headroom
doing that. Aside from the extra rail.

I'm considering the idea of a high side current mirror with
gain (resistor in one leg) and setting up the currents at the
5916 much lower than I want for the LEDs. The problem there
is that one of the BJTs will heat up a lot more than the
other and with Vbe shifting about 2mV/C and perhaps 30C
difference in temps.. it would be very hard to operate
correctly without adding feedback on the low side switch so
that the software could "observe" and correct by changing the
config registers in the 5916, dynamically.
This is where I need some real help. Are there any 3-to-8 decoder ICs
that can source that kind of current? If not, is there some sort of 8
bit buffer IC that I can use, where the input is my 74HC238 decoder, and
the output can source that current?

So you are looking for high side x8 driver ICs. I see the
Mitsubishi M54561P, but it's only x7. Not x8. Allegro Micro
is the site I'd examine.

http://www.allegromicro.com/en/Prod...-Interface-ICs/High-and-Low-Side-Drivers.aspx
http://www.allegromicro.com/en/Prod...ICs/High-and-Low-Side-Drivers/UDN2987x-6.aspx

Their A2982, 2985, and 2987 may work? No experience with
them.
I guess the alternative is to get 8 transistors, and drive them from the
decoder. Would BJTs be the right device for this job? Would I use NPN,
or PNP? I guess if I use the 74x238, which is active high, then I'd
want NPN, 74x138 is actually easier to acquire, so maybe I should
switch to that and use PNP?

Your 5916's are stuck SINKING current. So you need high side
switches. Even if you drive them with a shift register.
There's no choice there. You can't use NPN switches with a
sinking IC.
So, once that is answered, the next part is the power source.
Apparently I'll need well over 340ma to power this thing. It seems like
rounding to 500ma would be good enough. I could power it off of a 5v
500ma AC/DC adapter.

Could be even worse, as I indicated earlier. Need to hear
your response to that, first.
I was hoping for something more portable though. t looks like if I
wanted to power it off of AA's, I'd be looking at either 3 cells, and/or
using a DC-to-DC converter to get 5 volts. The device would run full
brightness for about 2-3 hours per battery (or 1-2 for rechargable).
Although, in reality the brightness will be far less than that on
average. Suggestions on this would be appreciated.

Keep in mind where all the dissipation takes place. The rail
voltage(s) are crucial. With a difference between 2.2V and
3.3V for your LEDs, if you use a SINGLE RAIL for all of them
then you KNOW in advance that you have to drop an extra 1.1V
for the RED LEDs and waste that power somewhere. MUCH better
to use a separate rail there with a switcher supply and not
have to burn unnecessary power, which only drains your
batteries that much quicker.
One of my goals is to prototype this cheaply. I'm not looking to sell
this design, its mostly just for the experience of designing and
building it. The digital circuitry is pretty easy and straight forward.
The analog aspect is still kind of kicking me in the pants.

Yeah. It's frustrating. I'm seeing more clearly just how much
is really involved in a "good design" of an LED matrix driver
system. Now I apprehend better why OSRAM used three separate
external supplies for their modules (switchers were used
there.) And I also better apprehend why there were 6 ICs in
there to run a 16x16 grid of RGBs.
Anyway, if you've made it this far, thanks for reading through my
stream-of-consciousness. Any suggestions (even criticisms) are highly
appreciated.

Well, that's my thoughts so far.

Jon
 
D

Daniel Pitts

Jan 1, 1970
0
Hi, Daniel. I'm still thinking about something similar, too.
Thoughts below, useful or not: They are very useful, thanks.



I'm thinking that it _should_ be even worse than you imagine.
I think you are doing a x8 mux for the 8x24 (rgb) matrix, as
I gather you are using three of the 5916s, one for each
color.

Each LED is spec'd at 20mA. That's an average value. The
absolute max says no more than 70mA peak and 50mA average.
With a x8 mux, to achieve 20mA average you'd need to drive
160mA into each. Human intensity perception is logarithmic,
so shifting to 70mA/8 average from 20mA average will mean
about 82% brightness, perception-wise. Tolerable. But
shifting to 20mA/8 drops you to about 65% and 10mA/8 to about
58%. That's noticeable.
Well, that's assuming I want to drive them hard. My experience is that
they don't need to be near the brightness. If I wanted to make this
into an out-door direct sunlight display, that'd be different. This is
more like a desk-top blinky-thing.
If you were to peak pulse them at 70mA, you are talking 560mA
for 8 or 1.68A for all 24 (pushing the red the same.) That's
a lot more than 340mA. That's assuming you push the red led
as much as the others, of course.
Interesting. I might have to set up a simple circuit to try to drive
these at their peak pulse with a 1/8th duty cycle at 3.7KHz (which is
the minimum my application would call for, using 30 FPS). I can afford
to burn out a matrix for testing purposes. I wouldn't drive it near
that hard on the real product, since for "debugging" purposes I
sometimes lower the frequency downward to 1Hz.
I read the absolute max specifications as being "per LED" not
"per RGB triplet." The 8V reverse spec seems normal on a per
LED basis. The continuous forward current of 50mA would also
be "per LED," I think. The peak forward current of 70mA would
also be "per LED," I think. Because those are quite normal
for individual LEDs. The power dissipation of 150mW (average,
of course) makes sense primarily because of the temperature
range spec. They are allowing for operation of no more than
60C over ambient. With 150mW, this suggests 400W/C which is
about twice that of a TO92 BJT (worse than, in short.) Which
makes sense to me, again on a per LED basis. But only because
they are telling you that you "could" do that if you only
operate one of the LEDs. I think that if you run all three of
them, and they are certainly nearby each other, I'd bet the
150mW spec would be applied to the triplet, too. So that one,
I think I'd take as a triplet spec if operating all three.
Each triplet, AFAICT, is a single SMD chip. I would think the heat
dissipation is per triplet too. I wasn't thinking average, as I should
have been. This means I could in fact drive this thing a lot harder
than I was thinking, if I need/want to. It's worth an experiment, when
I have more time.
Let's say you drive all three at 70mA peak, using a x8 mux
scheme. Using typical Vf, that's 2.2V@70mA/8 + 3.3V@70mA/8 +
3.3V@70mA/8 or 77mW. Even allowing that the voltage is
"typical" and permitting for another .4V headroom on each
LED, it's still under 88mW for the triplet. Average. Of
course, during the 1/8th cycle, the power will be 8 times
that or slightly over 600mW. But since that is a short time
and there is a thermal filtering response, it's likely you
can do that safely. The only concern I'd have is thermal
flexing of the leds on their mountings within embedded epoxy.
But I'd go with that for hobby use, anyway.

Another take on that: They say that the LEDs can support 50mA
continuous. I think that's "per LED." But... only one of them
in that case. At 50mA and a typical of 3.3V, you'd see 165mW.
Which is close to their power dissipation spec.

The sheet doesn't do a complete analysis for you. It doesn't
even say much. Nothing about the C/W thermal resistance, for
example. So it's low on specs and you have to make some
reasoned guesses. I'm just a hobbyist too, but those are my
guesses looking at the sheet.
The spec even says "Corol" instead of "Color" and a few other Engrishisms.
I'm guessing they are telling you not to drive any LED at
more than 70mA peak and 50mA average. The operational
temperature of 85C max is probably the most any particular
LED should be at. Also, no single LED should dissipate more
than 150mW on average.
I wonder if this would be an issue for outdoor displays in direct
sunlight. I think it unlikely for my case to get anywhere near those
extremes.
The rough equivalent for what we did at OSRAM would be to use
three TLC5916's and use three pots to set the max current.
Then set up all the LEDs at 25% PWM, turn the whole panel on
at once, and adjust the three pots until the display read a
specific (D60) CIE white point using the spectro (and
software.) Once the pots are set, you are golden. PWM from
there. The results were quite good enough.
Yup, that's kind of what I was thinking, though I'd just eyeball it.
What I'm tentatively thinking about doing right now is to use
the TLC5916 on the low side to drive a current mirror on the
high side... but where I set up the TLC5916 for about 1/10th
the current. The current mirror will multiply by 10 to
achieve the actual drive current, using a resistor in one
leg. That way I can select BJTs quite capable of a
significant voltage drop AND current and be able to dissipate
the power without problems. That adds two BJTs per TLC5916
current sink pin. But that's the price I pay to move the
dissipation elsewhere while retaining the convenience of a
shift register and single resistor for setting each current
sink value.
You can combine the outputs pins to aggregate the current, says so in
the spec.
But I'll require common cathode arrangement for this, as the
switches will be on the cathode side, since the current
mirrors will have to be on the high side.

I want to pulse as much as 160mA (or a little more) and
support a voltage drop of more than 2V on the BJT. That's
20mA = 160mA * 1/8th duty. Can't do that with the 5916 under
any circumstances.
You can if you use two pins instead of 1. You'd have to use twice as
many 5916s though. Might still be easier than a current mirror. YMMV.
That's not going to be white. But you know your experience
better than I do. Mine says... no guessing... use pots to set
the white balance.
Meh, close enough for my needs. Anyway, I was mostly picking some
relative for analysis.
And even then, you may need to calibrate
each pixel individually. LED manufacturing, 10 years ago at
least, wasn't up the ability to produce consistent LED
performance -- even when the LED dies were cut from the exact
same wafer. In fact, I worked on programming (and some
optical design) for machines that a big company (Siemens and
OSRAM) required so they could "bin" their parts before
selling them, or using them in composite displays. The extra
expense was not optional. It was required. (Unless the
customers didn't care and could tolerate "slightly more
orangish red leds" sitting side by side each other.)
The pixels in this matrix seems pretty consistent with each other. At
least, to my eyes.
By the way, red LEDs do not emit much in the green where
human intensity is perceived. (Only one of the three color
rods is involved -- usually denoted Y.) I'm not sure why
you've arbitrarily decided your numbers. But I assume it is
from personal experience with those matrices. So I just
caution you, but don't know better than you about them.
Thanks for the tip. It will have to be calibrated more exactly at some
point, but just trying to do simple analysis.
No. Power is AVERAGE. The peak power (ramming Joules into
something for a short time) does cause some thermal flexing,
though. And it also creates some excess voltage drops in the
diodes that doesn't translate directly into light emission.
So you don't get exactly X times as much light out for X
times the power in. But the specification is always an
average specification. If you are pulsing fast, I think the
thermal response will be very sluggish by contrast and will
do a good job of "averaging" the pulsed power.
Yes, this is true. I wasn't thinking about this, but I'd be willing to
risk burning out one of these $8 modules to find out you're wrong :)
Yes, I'd definitely use a pot for each of the 5916s.
The other alternative is that the scaling can be set in "software" by
sending specific commands to the 5916. That may be better in the end.
My current circuit just doesn't have any kind of communication to update
those values without a complete reprogramming, so a physical variable
resistor is the easiest addition.
As I wrote above, I think it's even worse than you calculate
if you really want to push these to produce brightness near
their capacity. Obviously, less works too. But I'd be pushing
them to do what they can and then use PWM for dimming (and
geometrically, not linearly.)
If I wanted true image reproduction certainly. Again, linearly is good
enough for my purposes.
...

I'm shooting for 20mA average (I'll never use that much in
practice), but this means 160mA peak at a x8 mux. In my case,
it's x5. So just 100mA.
Interesting, so you're scanning the segments, and shifting the digits? I
would have thought you'd mux 7 and shift 5. Or am getting these backwards.
But I'm also facing limitations in the 5916, in terms of
dissipation. Aside from the LEDs themselves, all of the
remaining dissipation takes place in the current sinks. The
high side switches are ... switches. They don't drop much by
way of volts. Just a lot of current. And they are external
BJTs (in my case, anyway.) So they can handle it fine. The
real problem is with the 5916, which is pathetic at 600mW max
total. In using all 8 outputs as in your case, this means
75mW per sink. If I tried to sink 100mA (within spec), that
would be 750mV. That's all?
Hmm, that doesn't quite add up to me. It looks like that max is if
you're running the thing at ambient 85°C, without heat sinks, where the
junctions get to be 125°C. I've definitely run it at more than 750mV,
and it isn't getting even warm. Something isn't quite right with that 750mv.
So let's say my LED power rail is 5V and my RED LEDs drop
2.2V and my high side switch drops 200mV. That leaves 2.6V
for the sink. Blows that 750mV out of the water! Not even
close. I'd need a RED LED rail of 3.15V worst case to meet
the max spec. All because the current sink MUST pick up all
the excess voltage at the desired current and dissipate it.
That's what the sink does.

I could use an external BJT, in common base config, to pick
up the slack. But then I'd need another power rail at, say
1.5V to attach to the BJT base. That would move the
dissipation outside the package and into the BJT. But then
I'd waste a lot of unnecessary power and voltage headroom
doing that. Aside from the extra rail.

I'm considering the idea of a high side current mirror with
gain (resistor in one leg) and setting up the currents at the
5916 much lower than I want for the LEDs. The problem there
is that one of the BJTs will heat up a lot more than the
other and with Vbe shifting about 2mV/C and perhaps 30C
difference in temps.. it would be very hard to operate
correctly without adding feedback on the low side switch so
that the software could "observe" and correct by changing the
config registers in the 5916, dynamically.
I think you're underestimating the 5916. I can't figure out where the
flaw is in your reasoning, but it seems to me that heat caused by this
device will be less than your reasoning leads to.
So you are looking for high side x8 driver ICs. I see the
Mitsubishi M54561P, but it's only x7. Not x8. Allegro Micro
is the site I'd examine.

http://www.allegromicro.com/en/Prod...-Interface-ICs/High-and-Low-Side-Drivers.aspx
http://www.allegromicro.com/en/Prod...ICs/High-and-Low-Side-Drivers/UDN2987x-6.aspx

Their A2982, 2985, and 2987 may work? No experience with
them.


Your 5916's are stuck SINKING current. So you need high side
switches. Even if you drive them with a shift register.
There's no choice there. You can't use NPN switches with a
sinking IC.
It depends on orientation doesn't it? As long as I don't exceed the
maximums on the reverse base/emitter bias.
Could be even worse, as I indicated earlier. Need to hear
your response to that, first.


Keep in mind where all the dissipation takes place. The rail
voltage(s) are crucial. With a difference between 2.2V and
3.3V for your LEDs, if you use a SINGLE RAIL for all of them
then you KNOW in advance that you have to drop an extra 1.1V
for the RED LEDs and waste that power somewhere. MUCH better
to use a separate rail there with a switcher supply and not
have to burn unnecessary power, which only drains your
batteries that much quicker.
Hmm, good point. I'm not yet knowledgeable enough to design such a
circuit. Hopefully by the end of this Linear Circuits class ;-)
Yeah. It's frustrating. I'm seeing more clearly just how much
is really involved in a "good design" of an LED matrix driver
system. Now I apprehend better why OSRAM used three separate
external supplies for their modules (switchers were used
there.) And I also better apprehend why there were 6 ICs in
there to run a 16x16 grid of RGBs.
This is a common Engineers hubris. "This field I've never studied is
probably easier than everyone else makes it out to be." :) I fall for
it from time to time.
Well, that's my thoughts so far.

Thanks, this does help me set the parameters for my system a little
better. The 6 ICs figure in the OSRAM module sounds about right,
assuming they have more output channels than the 8bit ICs I'm using.

I'm actually considering a non-MCU driven design. I would replace the
MCU with 1 timer, a counter, 2 RAM chips (for one is the active display,
the other is the writable buffer) and a some logic ICs to compare the
values in the RAM to some values in the counter, to handle the PWM aspect.

After all that, it *might* still be cheaper/easier to go with a cheap
MCU, but maybe not. Still gotta spec it out a little more.
 
J

Jon Kirwan

Jan 1, 1970
0
Well, that's assuming I want to drive them hard. My experience is that
they don't need to be near the brightness. If I wanted to make this
into an out-door direct sunlight display, that'd be different. This is
more like a desk-top blinky-thing.

Yeah, I understand. I want to design for future flexibility
so that I don't have to revisit the design every time I
change something, though. But I also have to find the "sweet
spot," too. Overdesigning to a point where I'll never care is
going too far, too.
Interesting. I might have to set up a simple circuit to try to drive
these at their peak pulse with a 1/8th duty cycle at 3.7KHz (which is
the minimum my application would call for, using 30 FPS). I can afford
to burn out a matrix for testing purposes. I wouldn't drive it near
that hard on the real product, since for "debugging" purposes I
sometimes lower the frequency downward to 1Hz.

I don't think you'll lose it. If you pulse fast, the
averaging should work out well.
Each triplet, AFAICT, is a single SMD chip. I would think the heat
dissipation is per triplet too. I wasn't thinking average, as I should
have been. This means I could in fact drive this thing a lot harder
than I was thinking, if I need/want to. It's worth an experiment, when
I have more time.

Let me know what you find out. I think it'll be fine. But
yeah, I also think the 150mW average has to apply to the
entire RGB as a unit since that is the basic dissipation
unit, physically.
The spec even says "Corol" instead of "Color" and a few other Engrishisms.
:)
I wonder if this would be an issue for outdoor displays in direct
sunlight. I think it unlikely for my case to get anywhere near those
extremes.

The units I worked on from OSRAM burned as much as 100W per
module for the 16x16. That's 390mW per RGB or 130mW per LED.
They were professionally designed for outdoor use in large,
multi-kilowatt displays. (About 20kW, or so.)

I'm kind of thinking of running mine at the equivalent of
about half that, and often much less, per LED. Aggressive,
but not nearly as aggressive as they were doing. A 5x7, all
lit up, would be less than 2W. And in normal full brightness
about a watt. If I dim it down to 25% via PWM (likely, for
typical use) it's about 1/4 watt average per 5x7. But I'd
like to design so that I _can_ use them at full brightness in
certain cases.
Yup, that's kind of what I was thinking, though I'd just eyeball it.

Okay. I'll be using a cobbled up spectro from a DVD-RW and a
little cardboard box it goes into, with a standard (cheap)
$10 digital 1Mpixel camera and some software I've already
written. I'll still have problems because I don't have a good
way to calibrate pixel intensities over wavelength without
spending too much money on it. But I'll get by using a cheap
incandescent (black body radiator) bulb and a standard data
chart on its emissions to take a hack at it. That is... if I
ever bother with RGB for anything more than toy use. I may
not.
You can combine the outputs pins to aggregate the current, says so in
the spec.

That's true of any current source or sink worth its salt. I'd
assume it, even if I didn't read it.
You can if you use two pins instead of 1. You'd have to use twice as
many 5916s though. Might still be easier than a current mirror. YMMV.

No, that's getting spendy just to increase dissipation. I
might prefer to bond something to the package, instead,
though. Or just avoid depending on them for power.
Meh, close enough for my needs. Anyway, I was mostly picking some
relative for analysis.
Okay.
The pixels in this matrix seems pretty consistent with each other. At
least, to my eyes.

Yeah. But then let's say you are selling these to an aircraft
instrumentation maker. They buy a bunch of 7-seg displays to
use from you. They put them side by side to make up the
display and someone pays $10,000 for the instrument. It's
dark at night, while flying, and all the pilot has to do is
keep staring at the display in the dark. And they see that
the segments are different colors and brightness and think
that the instrument maker is putting out cheap stuff and that
they should pay for a better display. So they don't recommend
it to their friends. Etc.

The manufacturers actually DO spend money and time to bin
their displays because they have to. Not because they want
to.

Human perception of shifts in wavelength aren't so good in
the red -- lousy, actually. But elsewhere our eyes may
discern even a fraction of a nanometer difference, if next to
each other. Particularly women, whose wavelength
discrimination is much better as a rule than men.
Thanks for the tip. It will have to be calibrated more exactly at some
point, but just trying to do simple analysis.

Got it.
Yes, this is true. I wasn't thinking about this, but I'd be willing to
risk burning out one of these $8 modules to find out you're wrong :)

Test away. I'm interested!
The other alternative is that the scaling can be set in "software" by
sending specific commands to the 5916. That may be better in the end.
My current circuit just doesn't have any kind of communication to update
those values without a complete reprogramming, so a physical variable
resistor is the easiest addition.

The pots allow me to calibrate the white point externally so
that the software doesn't have to be recompiled for every
single display. That's a REAL PAIN. Doing it with the pots
makes the displays consistent, unit to unit.

Can you imagine what it would be like to have to carry
individual calibration information for each and every display
and to adjust your PWM all over the place to keep track of
that?
If I wanted true image reproduction certainly. Again, linearly is good
enough for my purposes.
Okay.
Interesting, so you're scanning the segments, and shifting the digits? I
would have thought you'd mux 7 and shift 5. Or am getting these backwards.

I want to use the x8 5916 with its ability to individually
turn on and off specific sinks. That's a nice feature. But
this means the high side is what is scanned. Has to be.

My original post about this was doing it the other way. I put
all the LEDs onto a shared current sink and then turned on
individual high side switches. But I was forced to use a
separate BJT for each LED then. Which is what brought on a
short discussion and led me to look at the 5916, too.
Hmm, that doesn't quite add up to me. It looks like that max is if
you're running the thing at ambient 85°C, without heat sinks, where the
junctions get to be 125°C. I've definitely run it at more than 750mV,
and it isn't getting even warm. Something isn't quite right with that 750mv.

Now, that's easy to compute. 600mW for the whole package.
Divide that by 8, you get 75mW. Divide that by the current
(I'm considering 100mA) and you get 750mV. It's straight
forward. But you aren't using 100mA. Remember? That's why you
are able to handle more of a drop in the package. At 20mA,
for example, it's 5 times more, or 3.75V. So of course you
don't notice a problem.
I think you're underestimating the 5916. I can't figure out where the
flaw is in your reasoning, but it seems to me that heat caused by this
device will be less than your reasoning leads to.

No, my calcs are right, I think.
It depends on orientation doesn't it? As long as I don't exceed the
maximums on the reverse base/emitter bias.

Not if I understand your discussion. The 5916 sinks. That's
what it does. Your matrix has its cathodes worked out so that
works really well, in fact, using three 5916's there. The
common anode side is what you need to switch. And you can't
do that with NPNs (without a still higher voltage rail,
anyway.) You can drive the PNPs from the low side, of course,
using an NPN together with each.

One thing to also keep in mind (or at least I am) is that the
high side rail might be higher than the microcontroller's.
For me, this means I can't directly drive a PNP from a micro
output, since the output of the micro can't "reach" high
enough to turn the PNP off. In your case, that may not be an
issue.
Hmm, good point. I'm not yet knowledgeable enough to design such a
circuit. Hopefully by the end of this Linear Circuits class ;-)

Linear (and others) have boilerplate programs that will
design them for you. Just plug in what you want and they tell
you all the parts you need to use.
This is a common Engineers hubris. "This field I've never studied is
probably easier than everyone else makes it out to be." :) I fall for
it from time to time.

Yeah. But you know? It's just an LED!! I mean... cripes. Just
stick a resistor and you are good, right?

Then you start thinking about efficiency, parts count, costs,
power distribution, power dissipation, battery life, size and
the ability to stack vertically or horizontally, utility and
cost/benefit to the consumer, brightness, color rendition,
speed, and specialized software features.... and ...

Wow. You start looking for an LCD, again. They have fancy
controller all built in, don't need much power, cost almost
nothing....

Of course, there's a reason for LED, too. In my case, LCD
won't work.
Thanks, this does help me set the parameters for my system a little
better. The 6 ICs figure in the OSRAM module sounds about right,
assuming they have more output channels than the 8bit ICs I'm using.

I'm actually considering a non-MCU driven design. I would replace the
MCU with 1 timer, a counter, 2 RAM chips (for one is the active display,
the other is the writable buffer) and a some logic ICs to compare the
values in the RAM to some values in the counter, to handle the PWM aspect.

After all that, it *might* still be cheaper/easier to go with a cheap
MCU, but maybe not. Still gotta spec it out a little more.

Hehe. I'll be watching, because frankly I'm interested in
your thoughts, too.

Jon
 
G

George Herold

Jan 1, 1970
0
Hi, Daniel. I'm still thinking about something similar, too.
Thoughts below, useful or not:


<BIG SNIP... of the long winded Jon K.)
(or is that long fingered? :^)
But I'm also facing limitations in the 5916, in terms of
dissipation. Aside from the LEDs themselves, all of the
remaining dissipation takes place in the current sinks. The
high side switches are ... switches. They don't drop much by
way of volts. Just a lot of current. And they are external
BJTs (in my case, anyway.) So they can handle it fine. The
real problem is with the 5916, which is pathetic at 600mW max
total. In using all 8 outputs as in your case, this means
75mW per sink. If I tried to sink 100mA (within spec), that
would be 750mV. That's all?

I'm not sure it's quite that bad Jon. The 0.6W spec is for an ambient
temperture of 85C... I assume you can keep it cooler that that.
So let's say my LED power rail is 5V and my RED LEDs drop
2.2V and my high side switch drops 200mV. That leaves 2.6V
for the sink. Blows that 750mV out of the water! Not even
close. I'd need a RED LED rail of 3.15V worst case to meet
the max spec. All because the current sink MUST pick up all
the excess voltage at the desired current and dissipate it.
That's what the sink does.

I could use an external BJT, in common base config, to pick
up the slack. But then I'd need another power rail at, say
1.5V to attach to the BJT base. That would move the
dissipation outside the package and into the BJT. But then
I'd waste a lot of unnecessary power and voltage headroom
doing that. Aside from the extra rail.

Maybe some series PN diodes to move the power dissipation outside of
the chip? I assume the spec sheet lists the minimum head room voltage
that it needs to operate the current source. (But I didn't read
carefully.)

I'm considering the idea of a high side current mirror with
gain (resistor in one leg) and setting up the currents at the
5916 much lower than I want for the LEDs. The problem there
is that one of the BJTs will heat up a lot more than the
other and with Vbe shifting about 2mV/C and perhaps 30C
difference in temps.. it would be very hard to operate
correctly without adding feedback on the low side switch so
that the software could "observe" and correct by changing the
config registers in the 5916, dynamically.


So you are looking for high side x8 driver ICs. I see the
Mitsubishi M54561P, but it's only x7. Not x8. Allegro Micro
is the site I'd examine.

http://www.allegromicro.com/en/Prod...com/en/Products/Motor-Driver-And-Interface-IC...

Their A2982, 2985, and 2987 may work? No experience with
them.


Your 5916's are stuck SINKING current. So you need high side
switches. Even if you drive them with a shift register.
There's no choice there. You can't use NPN switches with a
sinking IC.

<more snipping>

George H.
 
J

Jon Kirwan

Jan 1, 1970
0
<BIG SNIP... of the long winded Jon K.)
(or is that long fingered? :^)


I'm not sure it's quite that bad Jon. The 0.6W spec is for an ambient
temperture of 85C... I assume you can keep it cooler that that.
<snip>

But while it's a 40C rise they are looking at, that's on a
4-layer board with a C/W of 66. On a single layer (or worse,
as in Daniel's case, a solderless protoboard I think), it's
103 C/W. About twice as bad. Which is why I'm thinking about
the 600mW as an absolute limit in my case. Say ambient is
worst case 45C. Then I've another 40C to work with. But on a
single layer, I've about twice the thermal resistance, too.
So I net net.

Jon
 
D

Daniel Pitts

Jan 1, 1970
0
The pots allow me to calibrate the white point externally so
that the software doesn't have to be recompiled for every
single display. That's a REAL PAIN. Doing it with the pots
makes the displays consistent, unit to unit.

Can you imagine what it would be like to have to carry
individual calibration information for each and every display
and to adjust your PWM all over the place to keep track of
that?
Ideally, it would be stored in EEPROM in the MCU. Also, the 5196 has a
calibration register, which adjust the voltage reference across the
Rext, so you wouldn't have to adjust your PWM at all, just set that
register on upon boot.

If this were for production scale, it could be automated to have the
sensor feed-back into the MCU to set the calibration register. This has
the benefit of not being a mechanical device.
 
D

Daniel Pitts

Jan 1, 1970
0
But while it's a 40C rise they are looking at, that's on a
4-layer board with a C/W of 66. On a single layer (or worse,
as in Daniel's case, a solderless protoboard I think), it's
103 C/W. About twice as bad. Which is why I'm thinking about
the 600mW as an absolute limit in my case. Say ambient is
worst case 45C. Then I've another 40C to work with. But on a
single layer, I've about twice the thermal resistance, too.
So I net net.

Jon
Isn't heat transfer exponentially proportional to the difference? If the
max temp is 125C, and the ambient is 40C, the heat is dissipated much
faster. Disclaimer, I could be greatly wrong here, this is definitely
leaving my comfort zone ;-) That's a good thing, makes me think more.
 
J

Jon Kirwan

Jan 1, 1970
0
Isn't heat transfer exponentially proportional to the difference? If the
max temp is 125C, and the ambient is 40C, the heat is dissipated much
faster. Disclaimer, I could be greatly wrong here, this is definitely
leaving my comfort zone ;-) That's a good thing, makes me think more.

If this were about EM radiation, it would follow Planck's and
the integral over wavelength following Stefan-Boltzmann.
Basically, proportional to T^4 (absolute temp.) If it were
air conduction, certainly there could be differences.

But it's not EM radiation that dominates. It's conduction.

Newton's law (and Fourier's) is something like "the rate of
heat loss of a body is proportional to the difference in
temperatures," I think.

Anyway, that's the conventional use of C/W and it, in
general, works that way to within a modest degree of error.

Jon
 
G

George Herold

Jan 1, 1970
0
Isn't heat transfer exponentially proportional to the difference? If the
max temp is 125C, and the ambient is 40C, the heat is dissipated much
faster.  Disclaimer, I could be greatly wrong here, this is definitely
leaving my comfort zone ;-) That's a good thing, makes me think more.- Hide quoted text -

- Show quoted text -

Hi Daniel, No it's pretty much linear. You can write a thermal
equation like ohms law. Heat flux (in Watts) times thermal resistance
(in degree C/ watt) = temperature difference (in degrees C) Heat flow
is like electrical current and the temperature difference is like the
voltage that drives it.

George H.
 
D

Daniel Pitts

Jan 1, 1970
0
Hi Daniel, No it's pretty much linear. You can write a thermal
equation like ohms law. Heat flux (in Watts) times thermal resistance
(in degree C/ watt) = temperature difference (in degrees C) Heat flow
is like electrical current and the temperature difference is like the
voltage that drives it.
That total makes sense. That does mean that a 125C chip dissipates heat
approximately twice as fast with air temp being 40C than air temp at 80C.

So does that mean that with C/W being "66", each watt increases the
package temperature by 66 degrees (compared to air temp)?
 
D

Daniel Pitts

Jan 1, 1970
0
That total makes sense. That does mean that a 125C chip dissipates heat
approximately twice as fast with air temp being 40C than air temp at 80C.

So does that mean that with C/W being "66", each watt increases the
package temperature by 66 degrees (compared to air temp)?
I did a little reading on Wikipedia (not the most reliable source, I
know)...

The 66 C/W implies that each watt requires a difference between the air
and the junction of 66 degrees per watt. Going back to the 100ma per
channel (or 800ma total), and a maximum operating temperature of 125C,
we can work backward to figure out the maximum voltage of the device in
relation to air temperature.

(125C - Atemp)/(66 C/W) = Pmax

They used 85C, which means 40/66 which is 0.6 watts.
If your air temp is 45C, that would be 80/66, which is 1.2 watts. That
looks like 1.5 volts max, twice that of Jon's 750mV.

That is assuming constant 100ma current to all 8 outputs. using the "5"
outputs, that would be 500ma, which means 2.4volts max. For the Red,
that means I'd need a LED rail of less than (2.2 + 2.4) 4.6v, and
green/blue would need to be below (3.3+2.4) 5.7v. It would probably be
safe to set that at 4v and 5v respectively. Or if you wanted to use more
common V values, 3.3V for red, and 5.0v for green/blue.

Jon, I was under the impression your application was monochrome, so you
could probably get away with a single LED rail.

For me, I'd be using less current maximum. I'd probably be using closer
to 20ma per LED, so 160ma total, and my device would be mostly be
operating indoors at < 40C. This would give me a much larger range for
voltage, though I'd still probably only use around 5v for both.

Does this analysis make sense? I mean, it seems to make sense to me, but
I'm the one making the analysis, and my understanding could be off ;-)

Thanks,
Daniel.
 
J

Jon Kirwan

Jan 1, 1970
0
I did a little reading on Wikipedia (not the most reliable source, I
know)...
works...

The 66 C/W implies that each watt requires a difference between the air
and the junction of 66 degrees per watt.

Yes. But keep in mind this is for a 4-layer board!!! Not a
single layer board. The single layer board says 103 C/W. Not
66 C/W.
Going back to the 100ma per
channel (or 800ma total), and a maximum operating temperature of 125C,
we can work backward to figure out the maximum voltage of the device in
relation to air temperature.

(125C - Atemp)/(66 C/W) = Pmax

They used 85C, which means 40/66 which is 0.6 watts.
If your air temp is 45C, that would be 80/66, which is 1.2 watts. That
looks like 1.5 volts max, twice that of Jon's 750mV.

The reason I came up with 750mV is because it is 103 C/W!!!
Not 66. I also like a little margin between operation and MAX
SPEC.
That is assuming constant 100ma current to all 8 outputs. using the "5"
outputs, that would be 500ma, which means 2.4volts max. For the Red,
that means I'd need a LED rail of less than (2.2 + 2.4) 4.6v, and
green/blue would need to be below (3.3+2.4) 5.7v. It would probably be
safe to set that at 4v and 5v respectively. Or if you wanted to use more
common V values, 3.3V for red, and 5.0v for green/blue.

Jon, I was under the impression your application was monochrome, so you
could probably get away with a single LED rail.

It's currently mono, though I have some very vague (admitted
unrealistic for now) idea of doing an RGB some day. Would
like to design something today that is flexible enough to go
there someday later.
For me, I'd be using less current maximum. I'd probably be using closer
to 20ma per LED, so 160ma total, and my device would be mostly be
operating indoors at < 40C. This would give me a much larger range for
voltage, though I'd still probably only use around 5v for both.

Does this analysis make sense? I mean, it seems to make sense to me, but
I'm the one making the analysis, and my understanding could be off ;-)

Just the 103 C/W vs 66 C/W, mostly. I'm not doing 4-layer
boards. I'll probably hand solder on vector boards I get from
Taiwan.

I'm still stuck with the .75V margin, which I cannot accept.
So I'm working on an alternative approach.

Jon
 
D

Daniel Pitts

Jan 1, 1970
0
Yes. But keep in mind this is for a 4-layer board!!! Not a
single layer board. The single layer board says 103 C/W. Not
66 C/W.



The reason I came up with 750mV is because it is 103 C/W!!!
Not 66. I also like a little margin between operation and MAX
SPEC.


It's currently mono, though I have some very vague (admitted
unrealistic for now) idea of doing an RGB some day. Would
like to design something today that is flexible enough to go
there someday later.


Just the 103 C/W vs 66 C/W, mostly. I'm not doing 4-layer
boards. I'll probably hand solder on vector boards I get from
Taiwan.

I'm still stuck with the .75V margin, which I cannot accept.
So I'm working on an alternative approach.

BTW, I found a newer spec than the one I had linked to, it has more
detailed thermal information:

<http://www.ti.com/lit/ds/symlink/tlc5917.pdf>

I still think your .75v margin is wrong. That's only if you need to
support 85C still air environment. You can double that margin if you'll
be running at 45C (which is well more than 100F, if you think in USA
temps like me).

Anyway, further reading might be useful to both of us:
<http://www.ti.com/lit/an/spra953b/spra953b.pdf>

I think you have a bit more wiggle room than you think.

Have you actually tried running various currents through your display to
see how bright each value actually is? I don't know about your specific
application, but my experience is that 40mA produces an almost painfully
bright light from the LEDs I'm using.
 
D

Daniel Pitts

Jan 1, 1970
0
BTW, I found a newer spec than the one I had linked to, it has more
detailed thermal information:

<http://www.ti.com/lit/ds/symlink/tlc5917.pdf>

I still think your .75v margin is wrong. That's only if you need to
support 85C still air environment. You can double that margin if you'll
be running at 45C (which is well more than 100F, if you think in USA
temps like me).

Anyway, further reading might be useful to both of us:
<http://www.ti.com/lit/an/spra953b/spra953b.pdf>

I think you have a bit more wiggle room than you think.

Have you actually tried running various currents through your display to
see how bright each value actually is? I don't know about your specific
application, but my experience is that 40mA produces an almost painfully
bright light from the LEDs I'm using.
Not to mention, a small heatsink with a touch of thermal compound is
probably all you need to increase your heat dissipation enough to give
yourself even more wiggle room.

Look in the new spec above. It depends on which package you have (I have
PDIP). The PDIP Junction-to-ambient is 51.8 C/W, which is better than
the old spec's 66 for layer 4 board.

Anyway, do what you need to do, but I say try a few test runs with the
thing, and take some actual measurements.

Good luck!
 
G

George Herold

Jan 1, 1970
0
That total makes sense.  That does mean that a 125C chip dissipates heat
approximately twice as fast with air temp being 40C than air temp at 80C.

So does that mean that with C/W being "66", each watt increases the
package temperature by 66 degrees (compared to air temp)?- Hide quoted text -

- Show quoted text -

Yup, you've got it!

"(compared to air temp)?"
Compared to the ambient temperature around the IC.

George H.
 
G

George Herold

Jan 1, 1970
0
Yes. But keep in mind this is for a 4-layer board!!! Not a
single layer board. The single layer board says 103 C/W. Not
66 C/W.

This spec is a little misleading. I'm guessing I could make a one
layer board work. (mostly ground plane around and under the chip.)
And certainly two layers with a nice ground plane to take away the
heat would be the same as 4 layers. Sometimes the thermal
'resistance' spec will include the area for the ground plane. (That
was meant for Daniels benefit not yours.)

George H.
 
J

Jon Kirwan

Jan 1, 1970
0
This spec is a little misleading. I'm guessing I could make a one
layer board work. (mostly ground plane around and under the chip.)
And certainly two layers with a nice ground plane to take away the
heat would be the same as 4 layers. Sometimes the thermal
'resistance' spec will include the area for the ground plane. (That
was meant for Daniels benefit not yours.)

George H.

You make a good point about the resistance spec including
stuff. It turns out that the newer datasheet shows different
numbers... as Daniel pointed out yesterday... but also now
references an entirely different document about changes in
the way they specify things -- which I still can't say I
understand. (Which is why I've not responded about it, yet.)

But anyway, I'll be using vector board.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
If you all will pardon a minor side-query...

It's fine, anytime. It's too easy to get mired in one
thinking mode. Nice to get a kick in a different direction.
I realize that this is still in the design phase ( which often
includes the soldering and re-soldering phases <grin!> ), but have you
given any thought to how the power supply "electron cache" capacity
will affect the your ability to drive the LEDs?

It's true that pulsing an LED "averages" its heat dissipation, but
you'll want to be sure that your LED power has eneough "reserve
capacity" to handle the intensity and duration of an ON pulse, and be
able to rebuild that capacity during the LED OFF period.

Since it's scanned with very little dead time between
adjacent scan columns (or rows, depending on how you look at
it), then from the perspective of the power supply it's a
nearly continuous high current load. There won't be a lot of
off period.

Actually, even that isn't entirely true as the number of
active LEDs will vary from column to column. But it gets the
point across that the supply has to handle worst case with
reasonable results.
( If this has already been considered and I missed it, I apologize for
the distraciton. )

I'm aware, but I don't consider that the tough problem at
this stage.. for me, anyway.

Jon
 
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