LED luminous intensity in dot matrix displays

[markp]

Hi All,

I'm currently designing a dot matrix LED display for moving messages. I need
some information about what the luminous intensity of a multiplexed LED
should be for good visibility indoors in a brightly lit room. If I know that
I can work out whether I can get away with a 16:1 duty cycle with the 8x8
LED modules I've chosen (80mcd -150mcd per LED at 20mA) and max current of
my drivers (120mA per pin).

Does anyone have experience of this?

I'm going to knock up a 16x16 block first and select either 8:1 or 16:1 to
see (along with experiments about whether muxing columns or rows is better
for horizontally moving displays), but any feedback would be useful.

Thanks!

Mark.

 

[markp]

---
AFAIK, the apparent luminous intensity stays constant if the current
is mutiplied by the reciprocal of the duty cycle.

That is, to maintain the apparent brightness of an LED running 20mA
CW, if it's to be multiplexed 1:4, it must be driven at 80mA during
its ON time.

With your drivers capable of 120mA and your LEd outputs rated with
20mA inputs, that means that to keep the brightness the same as for
DC, you can only multiplex up to 1:6.

That is, if the LEDs can take the peak current.

Thanks. Yes, I understand the relationship between duty cycle and current. A
80mcd - 150mcd is quite bright though, more than enough I think for a
display. So, question is how much luminous intensity (after multiplexing) is
really needed?

For example if I only need 20mcd average luminous intensity and I choose a
16:1 duty cycle, I effectively need 320mcd during each on time period. An
LED quoted at 80mcd at 20mA forward current gives 4mcd per mA (ignoring
efficiency effects), so I need 80mA in each on time, well within my driver
capability. If I need 40mcd that doubles to 160mA, outside of my driver
capability - I'd have to use a lower duty cycle.

Mark.

 

[Grant]

Thanks. Yes, I understand the relationship between duty cycle and current. A
80mcd - 150mcd is quite bright though, more than enough I think for a
display. So, question is how much luminous intensity (after multiplexing) is
really needed?

For example if I only need 20mcd average luminous intensity and I choose a
16:1 duty cycle, I effectively need 320mcd during each on time period. An
LED quoted at 80mcd at 20mA forward current gives 4mcd per mA (ignoring
efficiency effects), so I need 80mA in each on time, well within my driver
capability. If I need 40mcd that doubles to 160mA, outside of my driver
capability - I'd have to use a lower duty cycle.

If you have inter-column blanking to stop 'smearing' then make it adjustable,
now you got a display dimmer Design for the higher brightness then adjust
to suit?

Grant.

 

[D Yuniskis]

Hi Mark,

I'm currently designing a dot matrix LED display for moving messages. I need
some information about what the luminous intensity of a multiplexed LED
should be for good visibility indoors in a brightly lit room. If I know that
I can work out whether I can get away with a 16:1 duty cycle with the 8x8
LED modules I've chosen (80mcd -150mcd per LED at 20mA) and max current of
my drivers (120mA per pin).

If you are doing a 16-way mulitplex, then you effectively have
~8mA available (since your drivers max out at 120mA). Drive
one of your LEDs with 8mA DC and see what it looks like to
get a rough idea.

Does anyone have experience of this?

I'm going to knock up a 16x16 block first and select either 8:1 or 16:1 to
see (along with experiments about whether muxing columns or rows is better
for horizontally moving displays), but any feedback would be useful.

If you multiplex rows (i.e., drive every column in one row,
then move on to the next row, etc.) and DO THESE SEQUENTIALLY,
your "text" will *tilt* when moving. If you multiplex
non sequentially, the text will look "ragged" while moving.
(no visual artifacts while stationary).

Think about what you want from the display (especially in terms
of animation) before you settle on an approach.

HTH,
--don

 

[markp]

If you have inter-column blanking to stop 'smearing' then make it
adjustable,
now you got a display dimmer Design for the higher brightness then
adjust
to suit?

Grant.

Well I had a choice of LED driver, either TLC5925 or TLC5926. The latter
version can handle more current and has a better thermal package (PWP), and
in addition has open or short LED fault detection and internal PWM dimming
so I don't need PWM control of the rows.

Mark.