Current Controlled Current Source Help Maybe other things?

In summary, it is not possible to make all four LED's in a series function together because they share a common anode. You can either drive each LED individually or use a CCCS to scale the current and drive four different constant current sources.f
  • #1

So I'm making a really cool RGB LED project, and I need four of the RGB's to be in series. Now, as I understand this it's impossible since all three share an Anode.

The LED's are going to be controlled via MAX6966, it's a PWM constant current source for the LEDs. I'm treating this kind of as a black box, i have no idea how it works (i understand data, registers, etc.) but I'm assuming that whatever i hook up to the output of it, it's going to drive it at 20mA regardless of the voltage on the other and and what is connected to it.

I would like to be able to drive four LED's from one output, i.e. connect the LED's in series so that they all share the 20mA current and all light up the same. However, we've established this isn't possible.

Assumption 1:

If i were to connect all the anodes to high, say a +15/20 Voltage source, and then connect each of the colors together to the outputs of the corresponding output on the LED driver, would it functionally be equivalent to having 3 sets of 4 LED's in parallel (despite them all sharing the common anode)? I could also ask this as: If i were to hook them up like this would the 20 mA of current that the MAX6966 delivers be evenly split amongst the four LED's in parallel? My basic knowledge of circuits leads me to say YES! if I am wrong, then we need another fix!

Assumption 2:
Assuming assumption 1 is TRUE, this brings me to my next assumption. Since it's parallel, if the current divides evenly amongst the four LED's (Assumption 1) then if I could get a current controlled current source that would basically scale the current by 4, so 20mA becomes 80mA which is then split evenly between the four LED's as 20mA.

So if 1 and 2 are true, I'd need a CCCS that takes the input current and scales it by 4. How do I do that? Is it even possible?

I could in theory drive each of the four LED's individually, but then I would have to be sending data EVEN faster than I already am and I'm slightly worried that it's not going to be fast enough as it is. I'd also need more MAX6966's and that's not on my list of things to do :-p
  • #2
Actually, this doesn't even need to be variable, it just needs to be able to amplify the current to 80mA because it's PWM, sorry wasn't thinking.
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  • #3
I'm not sure I understand you question. Do you want to drive 4 tricolor LEDs in series?
If you can maintain 20 mA to each individual LED, why do they have to be in series?
  • #4
I want to drive 4 tricolor LED's in series. But in order to drive them in series don't you need to connect A-C-A-C...etc. and since they share a common anode wouldn't that make it impossible?

I would like them to be in series because I want to four LEDs to be functionally one single, very bright LED. If it's impossible I don't have to do it, but it would make my life a lot easier if there were a way.
  • #5
Is it not possible to make them all function together even though they are connected to different outputs of the IC and the fourth LED controlled through a second IC?


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  • #6
So, I think I've figured it out:

Since the IC is PWM, the current is always 0 or 20mA. I can just use a resistor and turn this into a Voltage source that varies between 0 and some static value that I want. I can then use this voltage source to drive a different constant current source that's either 0 (when the voltage is 0) or 80mA (when the other current is 20mA and the voltage is the right number). So basically if anyone knows now if hooking up the RGB's the way Im' thinking will evenly split the current amongst the four of them?
  • #7
Is it not possible to make them all function together even though they are connected to different outputs of the IC and the fourth LED controlled through a second IC?

Yea, but I only have a finite number of the IC's :(. I'm trying to make an RGB matrix of 25 squares for an interactive table. I'm already having to control 75 (3 colors per square) LED's. I think 300 different inputs would make my brain fry.

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