How to connect PWM output from a dsPIC MCU to 12V LEDs?

Question

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: :

Answer 1

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.

Answer 2

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.

Answer 3

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.