8x8 RGB LED driver.

[Jon Kirwan]

On Wed, 13 Mar 2013 11:51:31 -0500, Frnak McKenney
<frnak@far.from.the.madding.crowd.com> wrote:

On Tue, 12 Mar 2013 08:53:26 -0700, Jon Kirwan <jonk@infinitefactors.org> wrote:
Hi, Daniel. I'm still thinking about something similar, too.
Thoughts below, useful or not:

On Mon, 11 Mar 2013 22:28:03 -0700, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

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.

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

 

[Frnak McKenney]

On Wed, 13 Mar 2013 10:15:49 -0700, Jon Kirwan <jonk@infinitefactors.org> wrote:

On Wed, 13 Mar 2013 11:51:31 -0500, Frnak McKenney
frnak@far.from.the.madding.crowd.com> wrote:

On Tue, 12 Mar 2013 08:53:26 -0700, Jon Kirwan <jonk@infinitefactors.org> wrote:
Hi, Daniel. I'm still thinking about something similar, too.
Thoughts below, useful or not:

On Mon, 11 Mar 2013 22:28:03 -0700, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

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.

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.

Thanks. ( Or is "You're welcome more appropriate? <grin!>

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.

Ah. I went back and re-researched the effect of persistence-of-vision
on LED multiplexing and PWM drive. My reading of the discussion here:

http://electronics.stackexchange.com/questions/17528/does-pulsing-an-led-at-higher-current-yield-greater-apparent-brightness

is that, though there seems to be some slight disadvantage to using
narrower pulses and stronger currents ( including "melting effects"
<grin!> ), it's not enough to worry about. There is a discussion of
the duty cycle/brightness nonlinearity here:

http://electronics.stackexchange.com/questions/1983/correcting-for-non-linear-brightness-in-leds-when-using-pwm

which you probably already knew about, but might be useful to others.

Best of luck with your project.


Frank
--
Intellectual activity in a culture is not a one-way flow between
the great minds and passive recipients; it is a discourse, a
complex marketplace-like conglomeration of intellectual exchanges
involving many participants all trying to manipulate the ideas
available to them in order to explain, justify, lay blame for, or
otherwise make sense of what is happening around them.
-- Gordon S. Wood / The Purpose of the Past
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney aatt mindspring ddoott com