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From: markp on 28 May 2010 07:51

>>"John Fields" <jfields(a)austininstruments.com> wrote in message
>>news:c6kqv5doq02fcg3f9rssbagu7q2f3q84b3(a)4ax.com...
>>> On Wed, 26 May 2010 17:51:35 +0100, "markp" <map.nospam(a)f2s.com>
>>> wrote:
>>>
>>>>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.
>>>
>>> ---
>>> 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.
>
> 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.


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