Re: Any ideas for driving an array of discrete LED's without running into heat problems?

On Wed, 05 Nov 2008 20:21:13 -0800, ferrari.secret.santa wrote:
On Nov 5, 9:50 pm, "Martin Riddle" ... wrote:
<ferrari.secret.sa...@xxxxxxxxx> wrote ...
On Nov 5, 8:34 pm, "Martin Riddle" ... wrote:...
The A6278 seems to be limited to 45ma on one and only output. You
could also use the 90ma limit divided by 16, which gives ~6ma.
I would calculate out what the max per output would be using the
Ambient temp and the 165c shutoff. That would give you the max
dissipation per device.
....
The Allegro can handle 90mA on all inputs at the same time, I
believe... it's billed as a power shift register... the total current
can't exceed 750mA (total ground current) and the dissipation comes
into play well before that limit through.
....
Do you have series resistors? Vce at 40ma is only 0.7v. What do you
mean by 3 volt drop? is this the voltage on the ouput pin? (Vce)
9v and 3 leds@ 2v ea would need a 115 ohm resistor at 0.18watt 3
strings is 3*.18w = .54w.  4 stings of 4 should be 15ohms. And Vdd
should be 3 to 5v.
....
I do not have series resistors on the LED strings. I have a 9V linear
regulator, which connects to the strings of LED's (strings of 3 and 4)
and then each string is connected to one leg of the A6278. The A6278 is
a shift register with a constant current LED driver on each leg. So I
was using the external resistor of ~1k set the current of ~20mA for
each pin, and letting the Allego handle supplying constant current to
each pin. Is this not correct? The data*** says the A6278 is most
effective when dropping 0.7 to 3.0V. So, for one of my strings of 3
LED's with a Vf of 2.0V each, that is 6V drop across the LED's, and 3V
drop on Vce at the A6278... and then for a string of 4 LED's on another
pin, 2V per LED so 8V for the string, and on those the Allegro has Vce
of 1V.

So I am figuring that for the 3 legs of the 6278 that have strings of
three LED's, the 6278 is dissipating 3V * 0.02A or 0.06W (three of them,
0.18W total). Then on the four strings of four LED's it is dissipating
1V @ 0.02A per leg so 0.08W...totaling 0.26W per A6278 chip.

Am I missing something? Or perhaps not understanding the use of this
chip? It is my understanding that the chip can maintain the current on
each leg, so even though I have some strings of 4 LED's and other
strings of 3 LED's, it should adjust the voltage on each pin seperately
to keep them all at 20mA.

And based on that, and given that I calculate 0.26W of power
dissipation, I can't understand why the chips are getting very hot since
I think I am only at about 10% of their capability.
....

Based on the data ***, I agree with your interpretation of the 90mA/
output limit and 750mA ground current limit, and overall power limit.

It does seem likely that the chip is intended to work ok with different
string lengths (3 vs 4 LED's), but the data *** doesn't explicitly say
so. It says "designed to operate with driver voltage drops (VCE) of 0.7
to 3V, with an LED forward voltage, VF, of 1.2 to 4.0 V". OTOH, it also
says in Absolute Maximum Ratings that Load Supply Voltage Range (VLED)
is –0.5 to 17 V. You should ask Allegro to clarify whether different
string lengths are ok.

What A6278 package are you using? If it has an exposed thermal pad,
is it attached to a heat sink or a copper plane? Are you clocking
the chip at a high rate or rapidly switching outputs off and on?

Perhaps you should add series diodes to the LED chains -- eg, one
Schottky diode in series with the 4-LED chains, or several diodes
in series with the 3-LED chains. Or consider the series resistors
that Martin recommended. Or turn the driver board over to move the
drivers farther from the LED's and allow heat sinks to be attached.

In your original post, you wrote:
Then, on the LED board (which is mounted directly on top of the driver
board - only about 0.5mm between the top of the Allegro chips and the
back of the LED board), I have the 104 LED's in 0603 packages (LiteOn PN
LTST-C190KRKT)... those LED's have a power dissipation of 75mW... so 104
of them running full tilt would be 7.8 watts.

You miscalculated the LED power dissipation. An LED with 2V Vf at 20mA
dissipates at most 40mW, rather than 75, so the LED board dissipation is
at most 4.16 watts. This is still a lot of heat and should be dealt with.

--
jiw
.

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