LED matrix design - Page 1
Forum section: electronics / projects

author: greyltc
author_rank: Newbie
timestamp: August 15, 2019, 12:33:00 pm
content: I'm working on designing a circuit that will light up LEDs in an 8x8 matrix of 64 LEDs with no restrictions on the combination of LEDs that are on at any given time. The LEDs in my application can't flash and must see DC 5mA of current in the on state.I was thinking of following this application note I found by ST:https://www.st.com/resource/en/application_note/dm00038253.pdfThe page marked 49/54 in that document shows the figure:The STP16xx series of devices by ST are constant current sink LED driver ICs where you use one resistor to configure how much current each of the 16 output pins will allow to flow through to GND.I must be missing some major assumption being made here. Are they assuming that only one row is on at a time and the system cycles through each row so fast that the human eye's temporal filtering means that we can't catch the fact that only one row is on at a time and so the array pattern looks statically lit?I just can't see how it's possible to light up say, only the lower left corner LED and upper right corner LED at the same time without also lighting up the other two corners.Also, it looks this design suffers the major drawback that addressing (turning on) multiple LEDs in one column would mean they'd be wired in parallel so the the current passing through them would be somehow split between them so that every additional LED that's turned on on in a column would make the other LEDs in the column dimmer.So in writing this post, I think I've figured out what assumptions ST is making here and that this circuit is only suitable for sinage and won't work in my application. I'll click post an anyway though in case someone can point out that I'm retarded ;-).n.b. After writing this out, I think I'll go with 4 16 channel constant current driver ICs (like the STP16xx) to drive the 64 LEDs in my matrix individually instead of trying to get fancy with a row/column address scheme like the one shown here.

author: ataradov
author_rank: Super Contributor
timestamp: August 15, 2019, 04:42:00 pm
content: Yes, this is a standard multiplexed matrix configuration. Only one row can be illuminated at a time.

author: thinkfat
author_rank: Unknown
timestamp: August 15, 2019, 04:43:36 pm
content: Indeed, if you need to drive 64 LEDs individually, you will need 64 constant current sources (or sinks). So much should be obvious. Crazy amount of wiring, that's why matrix displays are usually driven in scanning mode.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: greyltc
author_rank: Newbie
timestamp: August 15, 2019, 05:37:39 pm
content: Great. Thanks for the confirmation folks!

author: mariush
author_rank: Super Contributor
timestamp: August 15, 2019, 05:41:20 pm
content: THERE's also chips that can do up to 144 leds for example, using 18 channels, in 1:9 multiplexing.For example, check out IS31FL3732:https://www.digikey.com/product-detail/en/issi-integrated-silicon-solution-inc/IS31FL3732-QFLS2-TR/706-1481-1-ND/6201977(datasheet:http://www.issi.com/WW/pdf/31FL3732.pdf)32 channel drivers aren't that expensive;https://www.digikey.com/product-detail/en/texas-instruments/TLC5955DCAR/296-41090-6-ND/5181373but indeed 16-24 channel drivers are cheaper and probably best price vs performance. For example see LP5024 with 24 channels, grouping channels in rgb groups, 12 bit brightness per channel etc :https://www.digikey.com/product-detail/en/texas-instruments/LP5024RSMR/296-50814-1-ND/9696903dATAsheethttp://www.ti.com/lit/ds/symlink/lp5018.pdf

author: klunkerbus
author_rank: Unknown
timestamp: August 15, 2019, 07:22:44 pm
content: The Maxim MAX7219/MAX7221 or AMS AS1106/AS1107 chips also work well for 8x8 LED matrices. You can find various boards on eBay and Arduino applications that provide sample microcontroller code to borrow from.