[sdiy] Will LCD backlight PWM cause audio noise?
rburnett at richieburnett.co.uk
Mon Aug 27 22:13:13 CEST 2012
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,
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