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

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.
.

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