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Altera_Forum's avatar
Altera_Forum
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16 years ago

led driving capability of a CPLD I/O pin when increasing track length in pcb.

Hi Guys,

I want to drive a very large LED matrix(used in a big LED display board) with the help of MAX3000 family CPLD.

I have a question in my mind:

Basically,I know that I can drive an LED through the I/O pin of CPLD connected by track on my PCB.

But what can be the length(in meters) of this track through which the CPLD can drive the LED with the same current capacity as it was,when the LED just soldered very closed to it?

So basically,what is the length(in meters) when the current capacity starts dying.

Note:I am not interested in using any current amplification logic,that is why I have asked this question.

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    Well how long of a PCB trace are we talking here? Basically the only factor is the effective resistance of the PCB trace. I don't have my equation handy but you'd have to have a pretty long trace for this to make any difference at all.

    Here is a calculator:

    http://www.referencedesigner.com/cal/cal_05.php

    You can use this to estimate the resistance of the trace and then figure how this affects the current through the diode.

    Jake
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    Are you driving individual LEDs? Then the current limiting resistor should be placed near the MAX3000 to reduce the capacitive respectively transmission line load of the output. You should consider the fact, that the ns risetimes of MAX3000 output signals can cause severe radio frequency interferences when send to a cable without filtering.

    Generally, I would expect a multiplex driver concept for a "large LED matrix". The 25 mA current limit of MAX3000 outputs doesn't allow a considerable multiplex ratio without using high current drivers in my opinion, at least for a comfortable brightness.