Noise Isolation tips

Question

I have a device I designed which utilizes 32 SK6812 LEDs (RGBW neopixels).

I am experiencing noise in the 1.2khz range (SK6812 LEDs use a 1.2khz PWM signal to control color and brightness) and its associated harmonic frequencies. This noise is most apparent in the 5v line on my device (the SK6812 LEDs are powered on this line).

The device is powered by a modular rack which has +12v, Ground , -12v. The 5v power for the SK6812 is supplied by a switched power supply that drops +12v to 5v5, which goes through an LDO and then to the 5v line where the SK6812s are connected.

The noise is apparent on other modules connected to the same power rails. These are mostly analog audio modules, and when they are completely silent, when the gain is boosted 65db, the noise becomes apparent.

Here is a photo of the spectrum of the noise recorded:

Here is a photo of the spectrum of the rack with my device disconnected completely:

I am pretty certain that this noise is coming from the LEDs because (1)the noise is only apparent when the LEDs are on, and changes slightly when different LEDs change color, and (2) because most of the noise heard is at the same frequency as the PWM rate for the LEDs.

One obvious solution is to increase bypass capacitors, and possibly use higher capacitance aluminum electrolytic in addition to my 0.1uF ceramic capacitors

My questions are these:

1) What value (and type) of capacitor do you recommend to lower low frequency noise in the 1khz - 5khz range? With 30 LEDs, how many would you reccomend? 2) are there any clever (and cheap) ways to isolate my device completely where it connects to the rails? Ferrite Beads? Inductors? What type do you reccomend? 3) I plan on moving to a solid ground plane (I am currently using a hatched - which I recently found out doesn't help) as well as adding a ground plane to the top layer (2 layer board). Do you think adding additional copper on ground might help the bypass capacitors work better? 4) Anything else I am forgetting to ask?

Thank you!

Edit: When measuring with a scope I get 530mVpp on the 12v rail with LEDs on, and 480mVpp with LEDs off

on the 5v line, I get 540mVpp with LEDs on and 500mVpp with LEDs off

Answer 1

That is the nature of these neopixel devices. They have no output inductance or capacitance on the PWM switching. They generate a lot of EMI.

With a PWM frequency of 1.2Khz, the 1.2khz noise you are experiencing is expected.

It would be best if the 5.5 switcher, 5v LDO, and LEDs had their own ground plane.

Follow the PCB layout and decoupling guidelines to reduce conducted EMI.
This Vishay Engineering Note covers them well.

Insert an input filter on the 12V line in to the switcher.

This is the EMI filter TI recommends on the V_in on their TPS92511 LED driver to suppress conducted EMI. For frequency between 1.2khz and 2.4khz the values would be around 10µH and 68µF.

Answer 2

No that won't help your noise is broadband with harmonics of 20Hz with some phase noise around 1.2kHz . Any clues why?

The problem appears to be load regulation noise , which does not follow the SMPS loop bandwidth or the slope of a capacitor.

What is your peak current and supply current rating?.

An answer is forthcoming but some obvious reduction of conducted noise is needed of "digital" LED noise and Analog supply. This means the source impedance of the supply including wire causes voltage spikes from pulsing the LEDs with ~1.2kHz PWM and some 20Hz causes yet to be defined.

The usual fixes are;

• isolated wiring directly from DC output to each load, analog and digital, so there is no current sharing and thus no drop on the power and ground for analog along the digital power wires.

• for audio. use low ESR cap 100uF Tantalum with a small series R with 100mV drop max.

• Use better quality regulator with low ESR caps in your 5 V supply.

and the most obvious.

Separate regulators for Audio and LED power. (with grounds tied but not sharing current.) (Show block diagram)

I was just watching PGA PLayers Tourney on DVR and decided to show you how I would have demonstrated to a design team, a Root Cause Failure Analysis due to a poor choice of X7R capacitors for filtering pulse noise with low ESR LED's even though fairly low current and good Murata brand parts. These are not a wise choice for low frequency switched loads and low f PWM at 1.1kHz

• much greater than 20kHz PWM would be better (if possible) to avoid audio crosstalk , even better don't share 12V sources.

Although not self explanatory to most young designers, X7R is the poorest cheapest Ceramic Cap type with high ESR and Dissipation Factor

Better choices are ; Tantalum SMD, ultra low ESR Alum, Plastic Film, NP0/C0G Ceramic for low Dissipation Factor @f Hz or ESR