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



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

Thanks for the reply jiw... I will contact Allegro and confirm if it
holds constant current on a per-leg basis, even with different
voltages on each leg. I am using the TP package (eTSSOP with the
large exposed thermal pad)... so it should be able to handle 0.25W
with no problem.

I still can't figure out why those Allegro chips are getting so hot...
I will have to do some more testing and measuring. At first I thought
it was the LED board being so close, but the LED board gets "warm",
never hot... the Allego chips get hot like they are dissipating
multiple watts or something when they shouldn't be. The LED's are
being driven correctly, so I guess I can just try to put some series
diodes or resistors to reduce dissipation in the 6278 chip.

On the dissipation of the LED - you're right, how stupid of me... I
just took 75mW from the data*** without even realizing that's more
than 40mW of total power I am putting through the LED's... duh.

Question... how much of that 40mW that I am putting through the LED's
goes into light and how much into heat? i.e. does an LED conver 20%
of the incoming energy to light? Or perhaps my understanding is wrong
and the LED die will convert all of that 40mW to heat, and the light
is just a byproduct of that transaction?

Thanks!
.

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