Re: Issues with LED grid driving

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.

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.



.

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