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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:

Single-string constant current led-driver

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. Current mirror

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?
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  • \$\begingroup\$ I’d love to help you out, as I have just worked on some LED drivers for a light therapy lamp with five channels (RGB + warm/cold white). I would suggest that you take a look at constant current LED drivers which will allow you to control the current through the LEDs without the use of a current limiting resistor (which is inefficient and gets hot). Is your project/product cost sensitive and are you willing to be working with SMD components like a SMD LED driver IC? \$\endgroup\$ – Jakob Halskov Nov 8 '19 at 12:29
  • \$\begingroup\$ Thanks for the advice! I'd prefer to avoid SMDs. I have been looking for IC-solutions, e.g. TLC5940. That particular IC can sink a maximum current of 120mA, which is insufficient for my needs. I have not yet been able to find an IC able to support high currents. \$\endgroup\$ – henpe Nov 8 '19 at 12:56
  • \$\begingroup\$ Okay so you wish to control around lets say 18x 1W LEDs divided into 6 red, 6 green and 6 blue? (resulting in two strings of three LEDs in series per color?) \$\endgroup\$ – Jakob Halskov Nov 8 '19 at 15:32
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    \$\begingroup\$ Almost. For simplicity, let's say I need 12x LEDs in a sub-panel, resulting in 4 strings with 3 LEDs in each. I would like to have a driver for that configuration. Then, I will deploy three identical circuits for a red,green and blue sub-panel respectively (all-in-all 3x3x4=36 LEDs). I have edited the original question in an attempt to make it clearer. Sorry for the confusion! \$\endgroup\$ – henpe Nov 8 '19 at 20:29
  • \$\begingroup\$ Thanks for clarifying; although also relatively new in this area (I also just graduated as electrical engineer) my experience has been that you can connect several LED strings in parallel and assume the current is evenly distributed among them if they consist of identical LEDs. I did this for a light therapy lamp where I had 8 strings in parallel (times five colors); here I did not see any visible or measurable difference in LED current and brightness. \$\endgroup\$ – Jakob Halskov Nov 8 '19 at 20:38
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  • 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.)

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