2
\$\begingroup\$

I really love building LED displays and have a decent understanding of multiplexing at this point. However, everything I've built is just binary, on or off, and I've been interested in building displays with LEDs with 2 or 4 bits of brightness depth, which I believe is most easily achieved with PWM.

I've looked at drivers like the TLC5955 and TLC5947 which seem to do the job. With 24 or 48 channels, that's enough to cover a "row" of what I want to create.

I'm just uncertain if it is possible to multiplex with a chip like this. They have an open drain, so I wonder if I just shared all the Cathodes for each row on the chip, then I could just do a refresh cycle where I drop the anodes for each row to low on an update, and keep the others high.

\$\endgroup\$
2
  • \$\begingroup\$ What are the dimensions of your LED array? Ie, how many rows and columns are you intending? \$\endgroup\$
    – jonathanjo
    Feb 18, 2020 at 0:07
  • \$\begingroup\$ My end goal would be 128 rows of 32 LEDs each. But I would build something smaller first. What I really want to make is a simulated "plasma" DMD display with traditional LEDs, rather than an LED matrix. It's crazy I know, but -aesthetics- \$\endgroup\$
    – Dani
    Feb 19, 2020 at 17:22

1 Answer 1

1
\$\begingroup\$

Yes, I think it should be possible. Going from the datasheet of the TLC5955, one of the mentioned applications is "LED video displays".

If I understand correctly, you plan to connect one channel/pin to a group of shared cathodes (-), which is a column in you matrix. A row in the matrix has shared anodes, and only one row should be on at the same time. If you then update the channel outputs each time you switch row is on, then that should work.

Also, the datasheet mentions a maximum current sinking capability of 31.9 mA. As long as a single LED doesn't exceed that value, and you ensure that only one row is active at all times, that should work too.

\$\endgroup\$
2
  • \$\begingroup\$ Thanks RvdV this makes so much sense. I think the last thing that's confusing me a bit from the datasheet is that it says "LED Power-Supply Voltage: Up to 10 V" does that mean 10V total for all LEDs? or 10V per channel? Since most LEDs have a voltage drop of about 2V, 10V would only supply enough power for 5 "normal" leds. \$\endgroup\$
    – Dani
    Feb 19, 2020 at 17:18
  • \$\begingroup\$ Have a look at the application circuit, I think the 10V refers to the supply voltage of the LED, the TLC is only sinking the current from the source. The TLC will try to regulate the average current that can flow through the led by PWM. More importantly, since you will be connect all the anodes of one column to different rows and they share cathodes, they are in parallel rather than in series. In fact, everything is parallel in a typical LED matrix since no LED is back to back with another LED. \$\endgroup\$
    – RvdV
    Feb 20, 2020 at 7:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.