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Can I drive individual channels of a common anode/cathode RGB LED with this?

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sallz0r:
Hi all,

I'm making some LED driving boards for RGB LEDs, and I want to try and drive this RGB LED:   http://www.dealextreme.com/details.dx/sku.4530

with 3 of these ICs (one for each colour):  http://www.supertex.com/pdf/datasheets/HV9918.pdf

I'm pretty sure it can't be done, as it's a common anode (or if you read the comments, someone said that the silkscreen was wrong and it's actually common cathode, so I'm not sure) -- but is there any way to individually drive the channels of that LED with that IC? If not, would it be possible with any other LED driver IC that people know of? (with the emphasis being on 'cheap'... ;-)  )

Thanks for any help and time anyone can give.

Psi:
I bought some of those exact ones from DX about a year ago.
The comments were correct, the ones i got are mislabelled and are common anode.

However i wouldn't trust that it's the same now, they may have corrected it, or it maybe just random depending what type they have in stock when you order. Get the leds first so you know which type you have.

Looking at that IC i dont think you'll be able to drive common anode or cathod leds from 3 of those chips.
Neither the led anode or led cathode for that controller IC are connected to the supply rails.

The DX LED has a common anode so if you connect one of the leds up to 3 of those ICs they will be all linked at that anode point.
So unless you plan to run each IC from an isolated supply it will create a path where the current through the sense resistor for one IC can supply power to the leds on another IC's. So the IC current sense system will not work properly if at all.

You might be able to redesign the circuit a little so the current sense resistor is on the SW pin thus making the led anode connected directly to vcc.
That would solve the issue, but you'd need to amplify the current sense voltage somehow so it's was what the IC expects.



DX have other RGB leds that have seporate anode and cathodes for each led which makes things much easier.
But they are brighter and so more expensive.
They also have many leds in series internally, so you have to be aware of their large forward voltage, 10-36V etc..

here are some examples

http://www.dealextreme.com/details.dx/sku.44043



http://www.dealextreme.com/details.dx/sku.39960

Jimmy:
sallz0r

Can you post a description of want you are trying to achive? Are you going to buy more than one of this a want to make 3 of them show independent colours. You could use a darlington array ULN2803 and resistor in series to limit current and voltage or if u are using a microcontrller with a serial output you can buy a TLC5940NT for $3.50 and have 16 output channels all with the same current and you can daisy chain them to give heaps more outputs

sallz0r:
Thanks Psi -- that's sort of what I was expecting. :-)  hrm, that's OK then, I might have to see if I can find some other drivers that will be happy with common anode LEDs.... hmm.....

sallz0r:
Hi Jimmy,


--- Quote from: Jimmy on December 19, 2010, 02:59:42 am ---Can you post a description of want you are trying to achive? Are you going to buy more than one of this a want to make 3 of them show independent colours. You could use a darlington array ULN2803 and resistor in series to limit current and voltage or if u are using a microcontrller with a serial output you can buy a TLC5940NT for $3.50 and have 16 output channels all with the same current and you can daisy chain them to give heaps more outputs

--- End quote ---

I'm going to be using many of these RGB LEDs (as well as some of the other higher-power ones DX has), and I'm making common driver boards for them; I would ideally like to use a common board that I can just change the constant current output for the different LEDs that I'm using.

I've looked at some options (like the darlington array and resistor) to do it all manually, however I'd prefer to stick to an IC that does it all-in-one -- just a current source with a PWM input that I can drive. The TLC5940NT looks good, however can only drive up to 120mA -- I would need at least 400-500mA, which is why the HV9918 was appealing.

I mgiht look at some other drivers, though, see what I can find....

Thanks for your ideas!

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