MAX7219 with High Power LED's not stable

Hey guys,

So ill try to make it short, 1st im not using an Arduino, im using Xport to PIC to send the data to the MAX7219, by OSC through MAX/MSP, just if u wondered, but im sure u can help me best! :) So im trying to light 4 high power (ca. 350mA each) leds with external power supply w the max7219, using darlington transistors as switchs. the Digits out of the max7219 goes to the Emitter, the SEG to the transistor Base, and the Collector to the power supply (w a ressitor). Now the thing is that the stabilty of the whole thing is very ISET dependent, and i dont understand why. First I had to change the ISET ressistor to 1.3 megaohm(!!!) for the thing to work at all, and also then it works with 3 leds simultaniasly but when i light 4 of then it all crashes and i gotta reset the ic's. Now even if its possible to calculate the right resistor for the ISET, I still wanna use the same max driver for more normal leds, and w 1.3Mohm i dout ill get a nice light...

My only guess was that all the current from the LED's sinks into the max7219 DIG pin, and thats the core of the problem.

Any suggestions?

I had another idea to escape this problem, and it by using a quad switch ic, so with a control pin i switch a gate and then could sink the current straight to ground. Anyone knows of such chip???

Many thanks,

Tom

Driving high power LEDs by using a resistor as the current limiting device is not a very stable thing to do. You need a constant current supply for these things. This is because the forward volts drop is, amongst other things, very dependent on temperature and changes as it warms up.

yeah but what does the max7219 cares that the current to the leds is not stable? i dont mind the brightness differences. the crashes are very constant. on certain Rset only one led at a time and on 1.3Mohm only 3 leds work together... couldnt make it to 4 of them, and actually have no clue how to calculate this Iset for high power driving. i was just trying different values. thats also not a way to solve problems!

yeah but what does the max7219 cares that the current to the leds is not stable?

That is why the whole system is not stable. Any decoupling on the chips?

i dont mind the brightness differences.

Do you mind if it blows up?

By using a high value Iset resistor you've managed to find some kind of marginal balance in turning the transistor "barely on" and are limiting the current to the LEDs by that method. This is an exercise in futility and will never work reliably.

As Mike mentioned, you must include a current limiting circuit in line with each LED.

ohh alright... so if i use a constant current limiter, could i set the Iset to any value?

but even then, all the current is being sinked to the ic, if its 1A through all the transistors sum, can it handle it?!?

and yeah i use a 10mF and 100nF for decoupeling.

thanks guys!

i use a 10mF and 100nF for decoupeling.

I hope it was 10uF not 10mF as that is 10000uF and rather big!

The absolute maximum on the digit sink current is 500mA according to the data sheet so it sounds like it is ok.

Grumpy_Mike: I hope it was 10uF not 10mF as that is 10000uF and rather big!

yeah u got me there :)

well, i just tried a different solution, and (for now) it works great- i just added a ressistor on the DIG pin to limit the current that goees to the ic, and it all works stable and smooth.

is there anything wrong w this? hey mike do u reckon this way i could possibly go over 500mA leds??

With all the changes it is a bit difficult to know what you have now. Any chance of a schematic.

here from eagle.

thanks man.

Thanks for that it is now clear what you are doing. I only have to figure out why now.

First off I am not at all sure why you are using this chip are there more LEDs that you have drawn? If so what is the maximum you plan? This chip is used to multiplex LEDs and there is not a lot of that going on with the circuit you posted.

Next you need some resistance to limit the current into the base of the darlingtons. These have high gain so anything around 1K will do fine.

Finally R4 is taking the current from all the LEDs this is bad because as LEDs are switched on and off the voltage across this resistor will change and so generate a back up voltage that will affect the other LEDs. If you want to limit the current into the chip there must be a resistor before each emitter or better still incorporate it in the value of the anode resistor.

You might look at maxim-ic.com also, they have a nice application note on using max7219/7221 with higher voltage and high current parts. http://www.maxim-ic.com/app-notes/index.mvp/id/1196

thanks CrossRoads, i remeber this document almost by heart by now. it doesnt help much imho regarding this issue.

mike, sure i use more leds, i wanna use all 64 of them, thats why i chose this chip, i wanna end up w 4 led @~80mA each and 4 leds @~350mA each and the rest are normal leds.

Grumpy_Mike:
Next you need some resistance to limit the current into the base of the darlingtons.

i thought the current in the base is the current from the ic only, its about 25mA, why should i limit it b4 the darlington base?

Grumpy_Mike:
Finally R4 is taking the current from all the LEDs this is bad because as LEDs are switched on and off the voltage across this resistor will change and so generate a back up voltage that will affect the other LEDs. If you want to limit the current into the chip there must be a resistor before each emitter or better still incorporate it in the value of the anode resistor.

thanks ill do that, but then does it make a difference if i place the ressistor before the emmiter or or from the collector?? so basicly 1 resistor per led should due… if so then the whole problem from first place was maybe wrong valus of the leds res?! :roll_eyes:

thanks for all the help. really inspiring!

btw here is my project attachted. its an OSC controller, with endless pots and position sensors, around 800 leds (i stoped counting…) you can see next to the far right channel i have 4 leds up with the 4 darlingtons, and also down is a Cree Xlamp with the 4 darlingtons. more then 100 analog inputs(!). it works nice and i only wanna finish this leds issue and cut a panel to stick the touch sensors on.

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