How Many LEDs can be driven with one WS2811

Question

I am starting a project and will be using WS2811 LED drivers to create a large matrix (about 40 to 50 panels) with a micro controller (Arduino compatible). I already purchased and received the RGB LED strips. They are the 5050 Type and have 30 LEDs per meter. I want to use the WS2811 to control each panel using a portion of the LED strip in each panel. The LED strip can be cut and the smallest section contains 3 LEDs.

My question is how many of these 3 LED sections can I control with on WS2811?

Since I ordered the WS2811 chips separately I figured I could use one per panel to light up the entire panel with even light. The panels are about 24x18 inches. I know that the WS2811 will keep current constant for the color so I figured that I could possibly put several 3 LED sections in parallel.

Thanks in advance for your responses and I appreciate any advice you can give me.

Answer 1

The datasheet for the WS2811 is where you need to look.

It is a constant-current three-channel driver that operates on 4.5 ~ 5.5 V, with absolute maximum ratings of 6 ~ 7 V. The output voltage is specified as 12 V maximum. Output current is maintained at 18.5 mA on each of the three output channels.

Therefore, the maximum number of LEDs you can drive with one output is dependent on the voltage drop of the LEDs. The total voltage drop across all LEDs would need to be less than the max output voltage.

Consider page 5 of the datasheet:

Here, using 5V, only one LED is being used per channel.

In this example, three LEDs are being driven per channel at 12V.

Simply put, you could drive as many LEDs as you like, provided you don't exceed 12V or require more than 18.5 mA. LEDs in series will sum forward voltages, while LEDs in parallel will share current.

Say you found an LED with a 2.4 \$V_F\$ and was satisfactorily bright at 10 mA. In theory you could connect ten of them in series-parallel (two parallel sets of five in series). The total \$V_F\$ would be 12, and each series set would receive half the current (9.25 mA).

To drive LEDs that in total exceed this driver's capability, would require additional components like transistors. Depending on your intended application, it might be more appropriate to use additional drivers and duplicate the data line to them. I have successfully driven four lines of drivers at once with the same data source, but there are a number of factors to take into consideration when doing this (slew rate, distance, etc.).