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


<ferrari.secret.santa@xxxxxxxxx> wrote in message
news:a024b85d-1573-477f-991d-4db8435e91f2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
|I have an interesting problem. I have a display that is roughly a
| 1.5" diameter circle. I am using an array of discrete SMT LED's (104
| of them) to create the display. The LED's are grouped into 4 sets of
| 26 LED's, each of the 4 sets consists of 4 strings of 4 LED's and 3
| strings of 3 LED's. These strings are individually controllable and
| can be turned on/off in various ways to create different symbols on
| the front of the display (think of it like a big and super bright
| alphanumeric character display).
|
| The reason for using the discrete LED's is to get the brightness
| needed... it must be LED and it must be visible in bright sunlight,
| and discrete LED's provide the necessary qualities.
|
| The first prototype used Allegro Micro power shift registers, plus a
| linear 1-amp regulator supplying 9 volts. The Vf of the LED's is 2V,
| and the Allegro chip is a programmable constant current LED driver. I
| am using 4 of the Allego chips, one per set of 26 LED's (in 7
| strings... thereby using 7 of the 8 lines of the Allegro chip).
|
| Running the LED's at 20mA, the whole thing gets super hot. I think
| the heat is coming from the Allego chips, which very much surprised
| me. The A6278 chip I am using is supposed to be able to handle 3
| watts dissipation at 50C ambient. For the 7 strings of LED's, 3 of
| them use 3 LED's (2v * 3 = 6v @ 20mA) so the Allegro is dropping 9v @
| 20mA on those lines or 0.18W, and the other 4 strings are 4 LED's, (2v
| * 4 = 4v @ 20mA) so the chip is dropping 1v @ 20mA on those lines, or
..
| 02W. By my math, that totals 0.62W. Not a tiny amount, but not
| gigantic. Then again, I do have 4 of the Allego chips on this tiny
| board, so combined that's around 2.5W.
|
| 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.
|
| 7.8 watts from the LED's and 2.5 watts from the Allegros.... just too
| much heat. The prototype works, but I have to throttle back the LED's
| to more like 10mA, and even then they get pretty hot... but not overly
| hot - it's still the Allego chips that get toasty warm.
|
|
|
| Of course, I know the problem is too much power disspation, but I am
| trying to end up with the BRIGHTEST display possible, while keeping
| heat in check. Any comments? (bear in mind I am not an expert at
| design - still learning)
|
|
| 1) Are there other LED's that are substantially more efficient and
| produce less heat in an 0603 package? Others that I have checked are
| even higher than 75mW!
|
| 2) I could multiplex the display, but whenever I have done this, I
| have always noticed a decrease in brightness (a substantial
| decrease). Despite the manufacturer saying maybe .1ms pulse at 200mA
| is OK, at 1/10th duty cycle, I always find that to be substantially
| dimmer. Have you also found that to be the case?
|
| 3) I could mount the top array of LED's on a metal core board and
| thermally couple that to the housing. I am not sure if I could get
| away with maybe 5W of dissipation if I were to do that? Any
| thoughts? On a 1.5" diameter 0.062" PCB?
|
| 4) I could change the strings of 3 LED's to strings of 4 LEDs which
| would result in less power disspiation in the Allegro chips... then
| reduce the current overall to produce a manageable heat level on the
| display PCB
|
| Any other suggestions? Any tips and experience with building high-
| brightness high-density displays made from discrete LED's??? As I
| said, it has to be visible in daytime full sunlight, so brightness is
| paramount.

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


.

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