Power LED constant current driver unstability

Question

I have a task to design a driver to drive 3W power LED. I have only 4 wires to dim 7 LEDs individually, so I decided to use WS2811 "magic" driver for PWM. The rest of the circuit is to provide constant current for the LED.

U1A, U1B create loop to control current through the LED. D1 (zener) and RV1 (trimmer) allow to set maximum current. Q2 shorts this set voltage to ground with frequency of 2Khz and variable PWM, to control LED brightness. So - I'm not changing current throught the LED - I turn it on (maximum set) and off to set the brightness. I know this is not simplest way to do it, but I've got boards for it for free.

What is the problem?

At lower duty cycles it works ideally. Probe is connected between Q1 drain and ground, 12V is stable. Set current is about 310mA, and when Q1 conducts it has 400mV voltage drop, so it's limiting the current well.

However, at higher duty cycle circuit "has time to think" and it starts to oscillate.

It has always higher voltage drop, so no damage to LEDs, it works well, but I would love to have it working 100% correct. And one more thing: using a resistor as a load instead of diode also makes it completly stable.

How to make it more stable, keeping ability to dim with 2kHz?

Answer 1

It took some time to get it working, but in the end I'm kindof proud of it. It regulates nicely, no oscillations nor overshoots. Maybe somebody wants to replicate, I'm providing working schematic and simulation file from ltspice. Main issue was feedback compensation and transistor drive.

What it does?

Circut allows from dimming led using pwm (for example from WS2811 chip) keeping maximum current at set level. double op-amp LM358 ensures current regulation (current is sensed od 1 Ohm reistor) and compared with reference voltage (created by zener and adjusted by potensiometer). MOSFET transistor is driven in the way to keep current stable. Shorting reference voltage (high impedance) to ground by small Q2 mosfet allows for decreasing current to 0, so pwm regulation is achieved.

Version 4
SHEET 1 880 680
WIRE 336 -416 208 -416
WIRE 208 -400 208 -416
WIRE 336 -48 336 -416
WIRE 352 -48 336 -48
WIRE 352 -32 352 -48
WIRE -112 -16 -128 -16
WIRE -16 -16 -32 -16
WIRE -128 48 -128 -16
WIRE -112 48 -128 48
WIRE -16 48 -16 -16
WIRE -16 48 -48 48
WIRE 208 80 208 48
WIRE -224 144 -240 144
WIRE -128 144 -128 48
WIRE -128 144 -144 144
WIRE -112 144 -128 144
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WIRE -16 160 -48 160
WIRE 0 160 -16 160
WIRE 160 160 80 160
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WIRE 208 256 208 176
WIRE 208 256 64 256
WIRE -240 272 -240 144
WIRE -32 272 -240 272
WIRE 0 272 -32 272
WIRE 208 272 208 256
WIRE 96 288 64 288
WIRE -304 320 -304 176
WIRE -304 320 -320 320
WIRE -208 320 -224 320
WIRE -208 336 -208 320
WIRE -512 368 -624 368
WIRE -384 368 -432 368
WIRE -368 368 -384 368
WIRE -624 384 -624 368
WIRE -32 384 -32 272
WIRE -16 384 -32 384
WIRE 96 384 96 288
WIRE 96 384 64 384
WIRE 208 384 208 352
WIRE 96 400 96 384
WIRE -208 416 -320 416
WIRE -208 432 -208 416
WIRE -32 432 -32 384
WIRE 96 432 96 400
WIRE 96 432 32 432
WIRE -624 480 -624 464
WIRE 96 496 96 480
FLAG 208 384 0
FLAG -80 128 12V
FLAG 32 304 12V
FLAG 32 240 0
FLAG -80 192 0
FLAG 96 496 0
FLAG 352 48 0
FLAG -608 48 0
FLAG -608 -32 12V
FLAG -208 432 0
FLAG -624 480 0
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SYMATTR InstName M1
SYMATTR Value BSB015N04NX3
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SYMATTR Value 10k
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WINDOW 0 0 56 VBottom 2
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SYMATTR Value 10
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SYMATTR Value 1N4148
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TEXT -400 520 Left 2 !.tran 0.01 uic