Nope ... I've advocate right along for common-cathodes with current-limited (current-mirror or simple resistor-from-regulated-V+) anode drivers, and high-current common cathode drivers.


We can't do that ... the LED's spec limits the peak current to 30 mA. (The O/P didn't specify, so we chose a commonly available 8x8 array, which is the only specific thing to which the O/P referred.)



Erik,

My point, all along, has been to use conventional (74HC, for example) logic to handle the multiplexing, and discrete drivers e.g. bipolar transistors or mosfets to source/sink the current routed through the LED's. Whether it's configured common-anode or common-cathode doesn't matter. If you want it to be common-anode, as you apparently do, then the current control has to be in the current sinks. The current control should be in the form of, at least, a resistor, or, as I've suggested, a current mirror. That way, it doesn't matter how much current is being handled, since there definitely are transistors that can do it. Once an appropriate schema, including timing, current, and configuration, is chosen, one can examine the possibility of using highly integrated drivers.

Start with the notion you introduced some time back, perhaps in another thread, that the entire scan of all the LED's has to be repeated within fewer than 8 ms in order to avoid flicker. The O/P indicates that his display is to be 80 x 16 pixels, which means driving 1280 LED's in 8 ms. Presumably, it matters not in what order/sequence you light them, so long as the entire 1280 LED's are scanned, whether on or off, within 8 ms. Scanning individual LED's means 6.25 microseconds per LED.

My first concern would be that 6.25 microseconds is not very long for the LED to "light up" and the eye to perceive it. Perhaps you, with your extensive experience with LED's and with retinal persistence, etc, can shed some light on the matter of how long an LED requiring 20 mA steady-state or 30 mA peak (strobed), has to be "ON" in order to be perceived in the correct "ON/OFF" state in a multiplexing scheme.

Let's start with that bit of information. Let's assume that we have ideal discrete cathode drivers that can, if necessary, sink 100 GA each and an ideal regulated V+ supply. Let's just discuss what's required to make the sign work. I can "go" with your LED that tolerates currents as high as 80 mA, as you've suggested if you can provide a spec for such a device, that indicates how long it takes to "light up" and how long it persists in that illuminated state. We can't just ignore that aspect of the LED performance.

RE