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LED luminous intensity in dot matrix displays

Discussion in 'Electronic Design' started by markp, May 26, 2010.

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  1. markp

    markp Guest

    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.


  2. markp

    markp Guest

    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.

  3. Grant

    Grant Guest

    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?

  4. D Yuniskis

    D Yuniskis Guest

    Hi Mark,
    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.
    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.

  5. markp

    markp Guest

    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.

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