[sdiy] LEDs and noise

[ASSI]

On Thursday 07 May 2015, 16:32:44, David Moylan wrote:
> If you're dealing with a lot of "blinkenlights" what's the best way to
> handle them?  Is it better to power them between the + and - rails or
> from one rail to ground?  Is it worth using a "dirty ground" scheme, or
> is the noise induced on the supply rails itself?

Use a separate rail at the lowest available/possible voltage and 
digital/dirty ground if you can afford it (for instance, use whatever any 
digital circuitry you might already have is running off).  You want the 
lowest switching energy, so running between the +- rails will be 
substantially worse at the same current than either rail to ground.  Keep 
the current loops small and do not cross the analog domain with the traces.  
Check if it's really the LED current itself that is the biggest problem, 
you'll sometimes get more noise from the driver (use an OC or OD driver if 
possible).  The RF part of a fast switching transient easily makes it across 
a regulator as PSRR is poor at RF, so that must be taken care of by LC or RC 
snubbing.

If you are really, really worried about the noise, instead of switching the 
LED current on and off, steer it to and from the visible LED to either an 
invisible LED of the same type or some other circuit that will produce the 
same voltage drop as the LED.


Regards,
Achim.
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