Author Topic: Led matrix afterglow  (Read 2704 times)

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Offline wouterdb

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Led matrix afterglow
« on: October 25, 2014, 02:31:39 pm »
Hi all,

I'm building a rather large segment display (30cm high digits, each segement 18 leds), I drive it by scanning (lighting each digit in turn).
The problem I have is that all segments in a row light up slightly even when their digit is not 'on'. This creates a kind of 'afterglow'.

How I built it
  • each segment is 3 parallel chains of 6 leds each, each led at 2.1V, 20mA, total of 12.6V, 60mA
  • each digit has a common cathode
  • low side driver consists of a 74HC595N shift register driving 8 MOSFETS (IRLB8721)
  • high side driver consists of a 74HC595N shift register driving 2 4 channel high side drivers (ITS724G)
  • drivers are fed data from an MSP430, framerate about 200Hz

what is odd
If I measure the high and low voltage on a segment, that is 'off', but has a 'neighbor' that is on, 



I expected the low  voltage would float up to the high rail, as the mosfet no longer keeps it to the ground, but this is not the case.
Similarly, I expected the high side to float down to ground if the high side driver is of. This is not quite the case.

The 74HC595N's are cheap chinese parts, to check if they had anything to do with it, I added 2 reliable 74HC14N's between the 74HC595N and the mosfet, and halved the frame rate. This results in the the following. Less spiky, glow remains.

I haven't added extra buffers to the high side, as the driver is SMD and soldered on a board to the 595.

I'm obviously missing something, can anyone help me out? What should my next step be?

Also, if any further information or measurement can help, I'll do my best to provide it. I have schematics and layouts for the drivers, software,.... I plan to opensource it when it works, so I can share it all.

Thanks,
Wouter
 

Offline Simon

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Re: Led matrix afterglow
« Reply #1 on: October 25, 2014, 02:58:22 pm »
Are you sure you don't have a problem with your high side being run off 12V is being switched with 5V when it wants 12V ? meaning it stays partly on ? I think your high side may be the problem not the low.
 

Offline wouterdb

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Re: Led matrix afterglow
« Reply #2 on: October 25, 2014, 03:04:37 pm »
 

Offline Simon

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Re: Led matrix afterglow
« Reply #3 on: October 25, 2014, 03:10:07 pm »
should be sound then, maybe it's as dodgy as your low side bits, try everything in isolation to see if it works as expected first.
 

Offline wouterdb

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Re: Led matrix afterglow
« Reply #4 on: October 25, 2014, 04:10:13 pm »
I got them from RS-components, so shouldn't be too dodgy,....

I tried most parts in isolation as far as I could, but the scanning is the problem. I can't scan by hand,... If I drive the mosfet (low side) by hand, it looks as expected and no glow.

I'll try the high side driver in isolation too, see how that goes.

 

Offline Rufus

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Re: Led matrix afterglow
« Reply #5 on: October 25, 2014, 04:12:50 pm »
When driving a matrix you need to allow time for say a column driver to completely turn off before turning on the drivers for the next row and column. If you don't some of the drive from one row bleeds into the next.

The ITS724 has slew rate limited outputs which means the turn off time is significant.

The ITS724 also has open load detection circuitry which means the outputs never turn completely off. The voltage is probably not enough to drive your LEDS.
 

Offline mikeselectricstuff

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Re: Led matrix afterglow
« Reply #6 on: October 25, 2014, 04:47:37 pm »
A few possible causes - one is you're not turning the row enable off (long enough) before changing row or column data, or capacitance (or diode reverse leakage) leaving some charge on the row. the latter can typically be fixed with a pulldown resistor on each row.
 
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Offline Simon

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Re: Led matrix afterglow
« Reply #7 on: October 25, 2014, 05:17:26 pm »
I take it your mosfets are driven from high and low pulling outputs, you don't have any floating gates do you ?
 

Offline wouterdb

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Re: Led matrix afterglow
« Reply #8 on: October 30, 2014, 07:30:55 pm »
Thanks for the help!

It was the bleeding over. I inserted an empty frame between each digit and now it works!

Thanks, I'm very happy I don't have to redesign everything!

 


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