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Topic: "ghosting" with LED matrix and MOSFETs (Read 1 time) previous topic - next topic

benjamin777

I ran across this from a few years back:

Quote
"cmagagna

    Guest

Re: 74138 family choice?
#5
Nov 07, 2012, 09:18 pm
Thanks everybody for your comments. @MarkT that was a great table of info; @CrossRoads thanks for the TPIC6B595 idea, I just ordered some to evaluate.

I apologize for not being clear enough. The project is really a 24x16 LED matrix. 3 TLC5916s control the column cathodes and a 4:16 decoder made from 2 74AC138s connected to 16 P-channel logic level MOSFETs control the row anodes. I drive the LEDs @ 100 mA each to compensate for them only being on 1/16th of the time (the spec sheet says it's OK to drive them up to 120 mA if the pulse is < 0.1ms; I'm updating @ 50 KHz so no problem there).

When I run the entire circuit at 5V everything works but the LEDs aren't spectacularly bright. When I run the LEDs at 12V they're bright enough, but I get ghosting on the rows.
..."
And it sounds like a situation I've encountered.  When using large MOSFETs to drive one
side of a scanned LED matrix, it seems that the drain of the MOSFET (not the gate!) can act
as a capacitor, storing enough charge to cause visible flash on the LEDs as it discharges.
If the MOSFET switches to ground, then a resistor should go from the drain to the plus power
rail (LED power, not logic power, if they're different), so that when the MOSFET is off, its drain
quickly goes high and thus won't cause LEDs to light.

The situation has nothing to do with the drive circuitry, either not being fast enough, too
capacitive, needs various resistors pulling here or there -- no!  It's due to the drain, in the
off condition, staying low when it should float high, due to its capacitance.
In the thread I quoted from,
people started speculating along all those same lines, and I recognized a lot of the dead ends
I had explored myself.  Just hope this helps somebody!

--Benjamin


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