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


<ferrari.secret.santa@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



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