[sdiy] LED current source needed for 'digital' outputs?

Neil Johnson neil.johnson71 at gmail.com
Tue Mar 4 16:37:01 CET 2014


Hi,

Justin Owen wrote:
> By Vref I mean a buffered LM336 - which have been just fine for every job I've thrown at them so far...
>
> What are the concerns?

No matter how good the buffer, it will have some output resistance,
which will more than likely increase with frequency.  If you are
switching an LED on and off with a logic signal the edges of the
current profile will have lots of high-frequency content, which will
find its way onto the reference rail and then into the parts of the
circuit that use it.  You might not notice, or you might.  Depending
on the circuit it might be directly audible, or it might cause
unwanted oscillator sync, or false triggering, or any other unwanted
effect.

This is also a good reason to soft-switch LEDs to remove the sharp
edges, and if possible run the LEDs between the power rails and not go
dumping current transients into the ground.

Neil
--
http://www.njohnson.co.uk



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