Recommended design PWM-enabled driver for multiple 1W LEDs in series-and-parallel configuration?

Question

A note on my background: I do have a degree in electrical engineering, but honestly I don't consider myself skilled enough to come up with designs from scratch, hence my post.

Design Issue

I wish to design a light panel consisting of 3 sub-panels (interleaved) with red, green and blue LEDs respectively. Each sub-panel shall consist of 12 pcs of 1W high-intensity LEDs. I plan to build a separate LED driver circuit for each sub-panel. Hence, I need to find a suitable design for a driver circuit to replicate and deploy in three copies.

Requirements / Features

• 1W LEDs (Typical specs: Vfwd = 3.0 to 3.4 V, I = 350mA)
• 12 VDC power supply - so the LEDs must be in a series-and-parallel configuration. I guess multiple 'strings' (N=4) with 3 leds in each should be fine.
• Each string should be driven by a constant current circuit. If possible, I would prefer to adjust the current in all strings from a single point (e.g. by adjusting/replacing a single resistor)
• I wish to be able to dim the (sub-)panel from a microcontroller using PWM.
• If possible, would like to keep the component count reasonably low.

Question

What's a good design for a driver circuit fulfilling the features above? I have tried to find an answer in this forum, and elsewhere, but without luck.

For the best that I know, the following circuit should be a good starting point:

The circuit above features constant current supply and enable PWM-control, BUT, I can't see how to extend to multiple strings of LEDs. I have been thinking of incorporating some kind of current mirror, like below, but I can't figure how.

My questions are:

1. Can I extend the circuit, as per the first figure, to support multiple strings of LEDs? How?
2. Are there any better/more appropriate designs that I should/could use instead?

Answer 1

• FET's need at least 2.5 to 3 times their threshold voltage,Vt or Vgs(th) to be low RdsOn.
• uC are usually low voltage <=5 , so Z1 is for inductive line protection, I guess for OVP on Vg.
• Since uC could be 3.3V say, then you need a FET that is "Logic Level" type gate voltage such as Vt=1V.
• Pd (max) for R3 & Q2 is important to choose components. You can balance for low cost by looking up costs of FETs and power R's.
• Usually current sensors are 50 to 100mV for <<0.25 W with an Op Amp, but with 0.6=Vbe R needs to 0.35A*.6V=210mW meaning est. 120'C rise on a 1/4W R. ( so use 1/2W) R3=0.6V/0.35A= 1.7 Ω 1/2W

Then regulate current with PWM 0 to max and ensure LED's have heatsink designed on copper or alum. substrate with 50 'C /W rise for each LED or 3.4V*0.35A=1.2W or about 6 sq cm for each LED with conductive radiator. ( do more reading on best practices for LED luminaire design on MCPCB.)