Knowledge requested for WS2811 Based RGB LED driver Module

Question

Design of LED Module based on WS2811 LED driver IC (Only Red-channel is shown).

To the right of the schematic you will find a simple, cheap and robust constant current LED driver. The current for the connected LED(s) can be easily adjusted with R4 (in this case about 344mA).

On the left is the 3-channel pixel LED driver WS2811. The outputs can only "sink" (active low) with up to 18.5 mA per channel (Only low is driven).

In between is the Connector circuitry that links the outputs of the WS2811 to the constant current driver.

This concept gives me the ability to use different LED configurations for different applications (e.g. High/Medium/Low Current LEDs on J3).

Independently, the LED controller/Driver Circuit and the constant current driver work fine and for this I also have all the components in stock.

My challenge lies with Connector circuit between these two circuits. For this purpose, I have now chosen Q1 (2N3906) with the 100 Ohm resistor. This one ensures that the LED does get turned on. However, when turned off, the LED goes off very slowly. This is of course not the intention when you want to show color patterns.

I would really appreciate it if someone could tell me where it goes wrong in this circuit or what could be better in this section.

Answer 1

However, when turned off, the LED goes off very slowly. This is of course not the intention when you want to show color patterns.

Yes, you need some means of discharging the gate source capacitance of the MOSFET when the upper PNP BJT (Q1) disengages. Once below the current limit threshold Q3 turns off so it won't discharge the gate-source capacitance much at all.

I would really appreciate it if someone could tell me where it goes wrong in this circuit or what could be better in this section.

Maybe just try a 4k7 resistor from gate to source. It's high enough not to significantly affect the constant current circuit but, it's low enough to discharge the parasitic capacitance more rapidly. If still not quick enough try a lower value but, nothing below about 1 kΩ or it'll impact the on-intensity a little.

However, looking at the IRLML2060TRPBF data sheet shows the the gate-source capacitance in only typically 64 pF so, pretty much 100 kΩ would discharge the gate in a tiny fraction of a second.