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How to connect PWM output from a dsPIC30F MCU channels to 12V LED diodes? 4 x PWM channels will be used. For use with following voltage:

  • 12V 5A Power Supply
  • 1W high-power LEDs with working current 350mA (4 banks of 1 watt LED's)
  • Overcurrent protection on output of each channel.

And another setup, same as above, but for Operating Voltage range 9-27V (for use 12V or 24V LED's, either type)

What is be better to use, MOSFET transistors, example IRF8313, or Darlington Transistor Arrays, example ULN2803A?

LED connection diagram like: diagram:

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  • \$\begingroup\$ There are several inconsitancies and ambiguities in your question. 1W/350mA = 2.9V, which is apparently the individual LED voltage. 12V/2.9V = 2.4, so does that mean each string is 4 LEDs and drops 11.4 V? Is 12 V the available supply or a rounded value of what the LED strings need. What is the available supply? How can current be 5A when you have 4 strings that each take 350mA. 4x350mA = 1.4A. Much clarification is required. \$\endgroup\$ – Olin Lathrop Apr 30 '15 at 14:13
  • \$\begingroup\$ Yes, there are some inaccuracies, I corrected. 12V 5A is Power Supply available. Yes, the actual current used will be much less than 5A. Six LEDs per channel in this example. Resistors will be used to limit LED current. Also, red and yellow LEDs have smaller voltage drop(2.2V), whereas green and blue have 3.6V voltage drop. \$\endgroup\$ – minto May 1 '15 at 1:12
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The easiest solution is to use N-channel MOSFETs. I assume that your controller is running at 3.3 Vdc - that means that you need MSFETs with low turn-on voltage.

You can't use a chip like a 2803 because of the load current you want to use. The 2803 is rated at 500 mA absolute maximum but you would never want to run it that high - the saturated voltage on that chip is really high and any significant amount of current leads to large amounts of heat that the package can't dissipate.

My favourite type of NOSFET for your application is a class called "Trench FET". Several manufacturers make these - the names they call them are all different but they all have the word "Trench" as part of the name. Your favourite supplier should be able to sell them to you.

The reason this class of MOSFET is ideally suited for your application is that they have very low Rds-on values and very low Gate turn-on voltage. The Gate can be driven directly by your microcontroller at low-frequency PWM speeds.

Their main limitation is that they can't handle high voltages on the Drain but your supply voltage is well within their operating range.

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  • \$\begingroup\$ Not sure what's output voltage on a PWM output? \$\endgroup\$ – minto Apr 4 '15 at 19:15
  • \$\begingroup\$ @minto Its the same as your MCU supply voltage. \$\endgroup\$ – Golaž Apr 4 '15 at 19:32
  • \$\begingroup\$ What's low-frequency PWM speeds so the Gate can be driven directly by microcontroller? I want use PWM frequency 244-300Hz. MCU Supply Voltage(Vdd) is 2.5 to 5.5 V. Should I looking at the gate-source voltage threshold Vgs(th) to identify a TrenchFET MOSFET that will work at the 2.5-5.5V I'm targeting? What is TrenchFET exact connection scheme: is it same as in picture? \$\endgroup\$ – minto Apr 5 '15 at 17:44
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If you want to use that board for production you may need to use an LED driver. This would also provide the overcurrent protection and it is much more efficient than a solution with serial resistor.

I used and can recommend LT3956, but this one might be a little oversized and expensive. Do a google search, there are many solutions out there. for your supply voltage of 12V there should be also some cheaper parts from infineon, on semiconductor or others.

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A low side FET switch is a easy way to do this:

This particular FET can be driven directly from a 5 V digital output and can handle up to 30 V.

This topology assumes that the 12 V supply is well regulated. You pick the number of LEDs per string so that the they drop a bit less than the 12 V of the supply. This gives you some voltage to drop across R1 to keep the current reasonably constant and predictable. Fewer LEDs and therefore a larger drop across R1 give you better current regulation, but wastes more power.

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  • \$\begingroup\$ And what if I want use 6 LEDs per channel? \$\endgroup\$ – minto May 2 '15 at 13:28
  • \$\begingroup\$ @minto: Deleting and then re-asking the same question to give it more attention is NOT appreciated here. I didn't answer it the first time because if you actually read my answer you should be able to figure this out. This time I'll state it outright: This is a dumb question I'm not going to waste my time answering. \$\endgroup\$ – Olin Lathrop May 2 '15 at 13:35

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