- Sun Jul 05, 2009 9:31 pm
#76381
Hi, I'm working on LED arrays, and while there are quite a few posted on this board, there are a couple things I'm wondering about. Basically, why are there so many current *sink* ICs, but so few that are able to *source* current? There seem to be all kinds of serial to parrallel drivers with current regulaton, but I've found very few that can source it. (eg the MIC5891 from Micrel)
Why is this? If you're using a current sink IC, wouldn't you also need a current source IC?
I have an array I've built that uses a STP16CPS05 (http://www.st.com/stonline/products/fam ... /stp16.htm) to sink the current, and for sourcing it I have some 74HV595 chips, with NPN transistors off of each output so that they can source more than 20mA. This allows me to crank up the brightness pretty high. Basically I want my driving circuitry to be able to go brighter than the LEDs are rated for. Most of mine max out at 100mA at 1/16th duty cycle. So this works, for now, but it's quite a small array, and having lots of little SOT-23 transistors seems rather inefficient.
It also doesn't seem to scale well. This is certainly a problem that has been solved many times, and so I wonder what's inside big LED scrolling signs that you see at stores? How do they drive / sink all the current? Eventually you can't keep expanding the grid in both directions because your duty cycle would be too low, right? I mean once you get to 1/100th duty cycle, even driving an LED at 100mA for that time will still result in only 1mA current averaged out over the whole refresh cycle.
Could you put a little SMT capacitor for each LED, to sort of smooth out the current pulses to them, letting you use a smaller duty cycle with more current? Maybe that wouldn't work, I'm not sure.
Basically I'm looking for advice from people who have done a larger array and how to best scale things up while keeping the complexity resonable and cost down. I've seen the sparkfun pong-led-table page, which is great, but each of those 8x8 squares is like $60 right? So that's like over 3 grand right there. I'm not really in to that. I like it when, say, I order a reel of SMT resistors from digikey and it's like, hey, I got 1000 of something for $3! What else can you do that with! .. well maybe rice.
Anyway. Love to hear other peoples ideas / techniques / ICs they've found useful.
Why is this? If you're using a current sink IC, wouldn't you also need a current source IC?
I have an array I've built that uses a STP16CPS05 (http://www.st.com/stonline/products/fam ... /stp16.htm) to sink the current, and for sourcing it I have some 74HV595 chips, with NPN transistors off of each output so that they can source more than 20mA. This allows me to crank up the brightness pretty high. Basically I want my driving circuitry to be able to go brighter than the LEDs are rated for. Most of mine max out at 100mA at 1/16th duty cycle. So this works, for now, but it's quite a small array, and having lots of little SOT-23 transistors seems rather inefficient.
It also doesn't seem to scale well. This is certainly a problem that has been solved many times, and so I wonder what's inside big LED scrolling signs that you see at stores? How do they drive / sink all the current? Eventually you can't keep expanding the grid in both directions because your duty cycle would be too low, right? I mean once you get to 1/100th duty cycle, even driving an LED at 100mA for that time will still result in only 1mA current averaged out over the whole refresh cycle.
Could you put a little SMT capacitor for each LED, to sort of smooth out the current pulses to them, letting you use a smaller duty cycle with more current? Maybe that wouldn't work, I'm not sure.
Basically I'm looking for advice from people who have done a larger array and how to best scale things up while keeping the complexity resonable and cost down. I've seen the sparkfun pong-led-table page, which is great, but each of those 8x8 squares is like $60 right? So that's like over 3 grand right there. I'm not really in to that. I like it when, say, I order a reel of SMT resistors from digikey and it's like, hey, I got 1000 of something for $3! What else can you do that with! .. well maybe rice.
Anyway. Love to hear other peoples ideas / techniques / ICs they've found useful.