Flickering LED strip when dimming

Question

I have a 5m LED strip (Ultimate Fancy LEDs) that consists has 30 LEDs per meter of WS2812B chips. Is uses a SP601E Bluetooth controller and was delivered using a 5V/6A power supply. Out of the box, the strip and controller works fine.

What I want to is the extend the wire (about 3m) from the controller to the actual strip. This is to be able to have the strip in the roof and the controller (with buttons) at the floor (so the kids can use it). The problem comes when I try to dim the strip. When changing from 100% brightness, the strip starts to flicker. My analysis is that data wire (that runs at 800kHz) is to long for the controller to being able to properly drive those extra meters. I tested a few different cables but can´t really get it to work properly.

1. Using a typical 3-wire power cable (1.5mm diameter wires)
2. SFTP network 4 pair network cable, AWG 26 where 5V supply uses 3 conductors, 4 wires for GND and 1 for DATA. In this case the voltage drop is a bit to much, losing about 0.5V over my 3 meter wire. But still this does not work properly for the DATA.

One weird thing though, is that the flickering is worse further down the strip (regardless of cable choice). I don´t really see this issue at the first meter of strip. If this is a signal integrity issue of the data line, I should see this an the entire strip, right? If I just get a good waveform to the first chip, the WS2812B should reshape the signal after each chip, right?

So, am I having power issues or signal integrity issues? I will later next week see if I can get a scope and measure some signals/voltages and get some more info. In the case of signal integrity, would adding a simply buffer/line driver do the trick?

Update 1: So it seem to be related to signal of data line and not power. Tested both supplying the strip at both end and this improves the “yellowness” of the end of the strip but does not change the flickering. Also tested with huge cap at beginning of strip, no change.

Also tested to add 220 ohm resistor before cable, things go much worse. Not only flickering but color change as well. Can’t really see that the data line degrades much over the cable (just some softer edges).

When it comes to the flickering, what actually happens is that the strip does not turn off but actually turns the leds brighter. I see a small voltage drop on the power line when it happens. If I short the data, basically don’t run it through my cable (so only power and ground in cable) things works fine. So the cable must cause the data line to violate some parameter (rise/fall or high/low time). I’ve ordered some buffers to see if this improve things...

Update 2: Added a scope picture with comparison of the controllers 2 channels. CH1 has my extension cable and a led strip attached. CH2 is unloaded (so should be close to ideal).

I now realize that the controller high output is somewhere around 3.7-3.8V. When dimming the strip (and not drawing so much current), the supply voltage is at 5.34V. For the WS2812, high is 0.7xVdd which is 3.71V. Potentially the insertion of my cable, scales of that tiny margin. Plan is to test a level translator tomorrow to get the data line up to 5V.

Answer 1

One chip consume 60mA at full brightness.150 is 9A. Power conductors on strip can not conduct so much current. I got voltage drop over 5m strip 0.3V with current 350mA. Run power wires in parallel to strip and connect at few point to strip will improve it.

Answer 2

1.5mm wire is about AWG 15 which is an insignificant loss at 6A @ 6m x 10 mΩ/m = 60mV, also the WS2812B operates from +3.5~+5.3 V.

But the 25 ~ 50 Ohm CMOS driver into a 10pF CMOS load with a 3m delay line of around 15 ns is going to ring enough to corrupt the signal integrity for CMOS Clock and data.

The solution is simple, but look at the differences in the spectral domain and time domain for a step input response.

For those unfamiliar with transmission line theory, the characteristic impedance for wires when the delay time is near or greater than the rise time is due to \$Z_o=\sqrt{\dfrac{L}{C}}\$ where L depends on Length/diameter ratio {somewhat }and twists of pair/m around 5nH/m and capacitance depends on insulation Dk and is inverse to gap. UTP Twisted pair might be 220 Ohms and TV flat cable 300 Ohms with coax 50 or 75 Ohms.

Here's an interesting design challenge question.

1. What is the trace cross-section area of copper from LED to LED and return so that 150 x 60 mA distributed current sinks drop less than or= 1.5V end to end. as current reduces from 9A down to 60 mA.
2. Then what is the total loop resistance?

My back of the envelope calculations are 2mOhm per trace LED to LED doubled sided = 1mOhm but for both Vdd & Vss becomes 2mOhm total x 150 = 300 mOhm. So 1 Ohm seems a little excessive. This could be ~2oz copper unplated 70um T. x 3.3mm W x 24 mm L for 2.17mOhm. I'll have to measure mine.

Answer 3

The solution was to add a 5V level shifter.
As pointed out in the update, the controller shifts out data with a high voltage level of 3.7V.

The 2 cases are:

1. Full brightness: Power supply is around 5V => WS2812 expects high voltage as 0.7x5 = 3.5V
2. Dimmed: Power supply increases to around 5.3V => WS2812 expects high voltage as 0.7x5.3 = 3.71V

So, more dimming, less current and higher power supply which result in that it WS2812 does not recognize the high level and things start to go south...

Thanks for replies!