Re: Driving many led's (400+)
- From: "Jon Slaughter" <Jon_Slaughter@xxxxxxxxxxx>
- Date: Mon, 19 Jan 2009 16:53:13 -0600
"Paul Hovnanian P.E." <paul@xxxxxxxxxxxxx> wrote in message
news:4974F210.C69524D@xxxxxxxxxxxxxxxx
Jon Slaughter wrote:
I have an application where I need to drive 400+(~150 rgb's)
individually.
I was thinking about using the
TLC5947---TI---24CH 12bit PWM LED Constant Current Driver
as this was the first one I looked at where each LED had it's own
channel.
Is that actually a requirement?
? I have 100+ rgb LED's where each rgb LED will be used for multi-color.
Hence every individual LED needs brightness control(hence the PWM).
That way I had complete control. It whould require 20+ of these and each
one
costs about 4$. It seems like a nice package and the only problem I have
is
setting the same q current for all of the IC's(so they all have the same
brightness at the same level).
If you are going to be controlling things individually (i.e. in a matrix
display), equal brightness isn't as big an issue. With some LEDs on and
some off, and the display changing, nobody will notice some
non-uniformity.
Depends. This isn't a "display" and the LED's are scattered about(not random
but they are not packed tightly). They may or may not change but they will
not change often(maybe at most a few times a second).
Slight fluxuations in brightness won't be a huge deal and they calibrated
out if necessary but I'd rather not have to do that.
My idea right now is to basically use C mosfets, 1 for each column, and R
mosfets, 1 per row. This gives me the matrix and current capacity. The rows
will be driven by a ring counter and the columns by a mosfet driver that is
controlled by a uP.
I believe that will work and the only problem is coordinating the row's with
the individual PWM(which becomes sorta modulated by the refresh rate). I
think by choosing the refresh rate to be large would be ok but might cause
other problems.
If the point is to produce a uniform brightness across a panel, you are
going to have problems with LED tolerances. Particularly if you can't
get 1000 of the same P/N, from the same manufacturing batch.
Slight variances are not a big issue. And Again, I'm sure they can be
calibrated out if I have control of individual brightness. (i.e., say one
LED is too bright, I can just reduce the current to it... I'll have a lookup
table for all the LED's for changes)
I don't think it will be a huge problem though. I have to keep a table
anyways because I don't think brightness is proportional to the PWM
frequency?
.
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