@OYPS I know you are using an external power supply, but I think you missed the point. I would suggest you to read Components 100's short article on 1W LEDs one more time, and then read all the rest references. One more thing is important is the user requirement and functional spec of the UV power LED lamp. Let me casually and randomly scribble something.

Suppose I want to DIY a UV LED lamp for my cat's home. My preliminary cat's requirement and functional spec is summarized below: (1) 350nm, (2) 300mW, (3) 200mA, (4) 15 min daily, (5) 12V Vcc, ...

As told by Components 101, if you use the amateurish one transistor, one current limiting resistor approach, if the voltage across the LED varies by 0.1V, current would vary drastically, perhaps from 200mA by 50% to 400mA (for exact numbers, see my I-V test results), so it is UV overdose problem, which might shorten my cat's life. Or you are using UV kill germs, viruses, or make green fruit to ripe faster, then over UV exposure might make the fruit ripen too soon, ...

Errata - 200mA by 50% would be 300mA. Note - I am too slow to make corrections to my chat comments, so be warned that there are some important errors I could not correct in time, or delete.

@OYPS Yes, you see two NPN BJT transistors, me don't see no transistor, me only see current mirror and negative feedback CCS.

@OYPS Yes, your video explaining CCS is good.

The tutorial gives the following equations:

So you know how to calculate Rsense = I/0.5V

The tutorial uses digital control. But we can use PWM control. To make things as loosely coupled as possible for prototyping and troubleshooting/testing/calbration, we can use PWM high/low side switching complimented by with low/high side CCS, also MAX4172 current sense amplifier, and UV sensor for feedback.

@OYPS Yes, the handsome guy P Marian is indeed a good story teller. :)

OK. I think i am slowly getting a hang of it. Using the article above i created the following circuit. As far as i understand it, i am able to control my LEDs via my MCU. PWM is not an option for me, as the MCU has not enough PWM channels to support this.

Is this circuit correct? Via this, I am gaining a more power efficient circuit ?