Author Topic: Do I need a driver IC or is there another easier way  (Read 2847 times)

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Offline Shined

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Do I need a driver IC or is there another easier way
« on: January 21, 2011, 12:08:32 am »
Hi,

I am designing a circuit using XILINX 9572 ic's running at 5v now I know that these beasties probably can't provide the required juice to run LED's.
My question is do I need to use a driver IC or is there some way I could drive them sourcing the juice from another rail at a different voltage?

The 9572 is a 5v TTL level compatible device but LED's require higher current and a lower voltage I envisage at this point that I would be running 16 to 24 LED's off of the one 9572.

At the moment the solution would involve using 74HC595's and not 9572's but since I have 2000 9572's it would be nice if I could use them.

Cheers in advance.
 

Offline allanw

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Re: Do I need a driver IC or is there another easier way
« Reply #1 on: January 21, 2011, 08:45:11 am »
The datasheet doesn't explicitly list a maximum dissipation rating for the device. The outputs support 24mA drive, but that's usually intended for logic in which the full current only has to be applied for a short time, not continuously.

Give it a try. Perhaps it'll work fine if you keep the total I*(5-Vled) drop for all output pins reasonable for the package size.

edit: have you looked into chips like the TLC5940 or TLC5947?
 

Offline mikeselectricstuff

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Re: Do I need a driver IC or is there another easier way
« Reply #2 on: January 21, 2011, 09:24:29 am »
The datasheet doesn't explicitly list a maximum dissipation rating for the device. The outputs support 24mA drive, but that's usually intended for logic in which the full current only has to be applied for a short time, not continuously.

24mA is 24mA, so it should be able  to output that for any amount of time, however there is likely to be a limit on the overall package current, which will  limit the number of LEDs you can light simultaneously.
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Offline Psi

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Re: Do I need a driver IC or is there another easier way
« Reply #3 on: January 21, 2011, 12:26:03 pm »
You probably dont need to run the leds from anywhere near 20mA (unless your doing some sort of led display).
If its just for some sort of indicator light you can get away with much less.

Most people drive normal 5mm leds at around 20mA but if you get a narrow beam high intensity green led and run it at 3mA while looking directly at it, even from 1 meter, its damn bright.
This is because the human eye is quite sensitive to green light.

Have a play with a few green leds on ya psu at 1-20mA and see what brightness you get. The high mcd / high intensity leds will seem brightest for less current.

But really, since you have 2000 of these 9572 chips, i'd just build a quick and dirty prototype thats driving the leds with whatever current you want and if the chip dies grab another one and try again at a lower current :)
It really is something you need to work out before you can design your real circuit.

« Last Edit: January 21, 2011, 12:39:29 pm by Psi »
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Offline Shined

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Re: Do I need a driver IC or is there another easier way
« Reply #4 on: January 21, 2011, 03:19:38 pm »
edit: have you looked into chips like the TLC5940 or TLC5947?

I just looked at rs components for a price on these $7.70 NZD each, if I believe each of those chips is capable of driving 100's of LED's and yes they are nice... but there are a couple of minor problems, my soldering skills aren't fantastic enough to be able to deal with SMT mainly due to having massive and I mean massive hands, I can just get away with soldering through hole PLCC sockets but that's about it :) so unless there is some cheats way of surface mounting that nice wee beasty that requires no SMT ability on my part I'd come undone. Nice thought though.

It might help perhaps if I mentioned what I was looking at building. Its a board with an ATMEGA 328 which would be used to drive a register display for a CPU, namely a 68K, I was designing the board to be modular with the Mega328 receiving signals from the CPU and then sending them to individual display cards which are all identical but for the fact they have differently numbered access lines which is where two HC595s would come in to select the appropriate display to update. Each display card would contain 32 individual 3mm LED's and 8 Seven segment CC displays to display the value of each nibble in binary form and hexadecimal form. Yes I'm lazy and don't want to do the conversion from one to the other and I like lots of pretty lights, power bill be damned. I.E I want it to look old school.

However it looks like I might be better off if I can get 5 or 6 as appropriate( again didn't do the math) of those TLC's into the setup running the Mega328 to that and feeding everything directly from it so that the only thing on the display cards are the LED's themselves, I suppose I could try and get a through hole socket for it. Alternatively do you know of an available part that would still use the old fashioned pins that I like so much or maybe a PLCC socket?

Cheers in advance.
 
 

Offline Zero999

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Re: Do I need a driver IC or is there another easier way
« Reply #5 on: January 21, 2011, 08:48:48 pm »
I'd probably go for good old transistors which are reliable and cheap. If space is limited just go for SMT.
 


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