[sdiy] Will LCD backlight PWM cause audio noise?

[Richie Burnett]

Hi Tom and all,

Periodic pulsed supply current demands can cause problems with the audio 
noise-floor, particularly if you're aiming for a very stringent noise spec. 
or have sensitive metering built in to your gadget.  In a commercial 
application I was once working on the company had a VU meter that went right 
down to something like -100dB.  Once a quiet sound trigerred the -100dB LED 
the step in supply current caused by that one LED suddenly illuminating 
caused enough noise to immediately illuminate the -90dB LED also!  Sometimes 
the -100dB LED would flicker when the sound was removed, as the switching 
transient produced enough low-level noise in the audio strip to keep the LED 
toggling!  This is an extreme example, but it caused them a lot of 
head-scratching and shows what can happen in practice.

Fortunately, there are a number of things you can do to mitigate this type 
of problem.  Most of which people have already said, such as slugging rise 
and fall times, local supply decoupling, and good layout so that the 
switching currents do not share common circuit traces with the audio 
currents.  All of these are good advice...

Another trick which I have found highly effective is to feed your LED (or 
series/parallel LED backlight, load etc.) from the supply via a constant 
current source, then shunt the current via a transistor around the LED(s) 
when you want them to be off.  This minimises audio noise feedthrough during 
switching because the same current is always being drawn from the supply 
regardless of whether the LED is on or off.  You just decide whether to 
divert it through a shorting transistor or let it pass through the LEDs to 
make light.  Ideally you put the shunting transistor switch as close as 
possible to the LED(s) that it controls so that the current is diverted the 
smallest distance possible across the PCB.  Keeping the current constant, 
and the path almost constant is very very effective at reducing EMI as well 
as eliminating ripples on the supply wiring.  Of course the downside is that 
you always draw the same 20mA current from the supply whether the LED is on 
brightly, dimly or off completely.  Something that might not be considered 
acceptable in a battery powered application.

If it's just for home use, and you really want to minimise the audio 
noise-floor as much as possible, but aren't confident with the PCB layout or 
aren't worried about efficiency, battery life etc, then safest bet by far is 
to smooth out the PWM to get an analogue voltage level and then feed this to 
a pass transistor to control the current through your backlight block.  Sure 
it won't be as efficient as PWM applied to the backlight and the transistor 
will get a bit warmer, but with a maximum backlight current of only 40mA I 
don't think it's going to sweat.  At least with a linear proportional 
current source there's no risk of it chewing up the supply at all with 
periodic current pulses.

I hope this helps,

-Richie Burnett,