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

On Nov 5, 9:50 pm, "Martin Riddle" <martin_...@xxxxxxxxxxx> wrote:
<ferrari.secret.sa...@xxxxxxxxx> wrote in message

news:e1d6213d-9f64-4b83-b62d-83f6d848f451@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Nov 5, 8:34 pm, "Martin Riddle" <martin_...@xxxxxxxxxxx> wrote:





<ferrari.secret.sa...@xxxxxxxxx> wrote in message

<SNIP>

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
perdevice.
You are definitely driving too much current for the device to handle.
I
would use the ULN2003 perphial drivers a the currents your working
with.

Cheers

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.

There was an error in my math... each Allegro drives 3 strings of 3
LED's at a 3V drop (9V supply, 2V per LED) and 20mA... that's 0.06W
times 3 strings, total 0.18W.  Then it drives 4 strings of 4, 1V drop
at 0.02mA for a total of 0.08W... so the whole chip is only
dissipating 0.26W.  That seems low for how hot they are getting.

Your using the Absolute maximum ratings (750ma ground current), which is
unwise.
If you look at the charts for the A6278, max power is 2.5w at 50c (
dip16 A package )

lets see..

Pdc = DC * (Vce*Io*8)+Vdd*Idd)
Pdc = 1.00 * (1.0v*0.090*8)+(5*0.032) = 0.88 w

Which looks ok. You should be within the device ratings, something else
is wrong.

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.

Cheers

Hi Martin,

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.

Then again, obviously something is wrong since it's not doing what I
think it should be ;)

thanks!


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