[sdiy] Will LCD backlight PWM cause audio noise?

[Tom Wiltshire]

Very interesting, thanks.

Tom

On 27 Aug 2012, at 21:13, Richie Burnett wrote:

> 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,