Can I Use CAT4101 with Arduino to PWM High Brightness LEDs in Parallel?

[thshen34]

Hi all,

I'm trying to PWM a high brightness LEDs in parallel (http://d114hh0cykhyb0.cloudfront.net/pdfs/spmwht5225d5waq0s0.pdf) and I was hoping to use an Arduino and a CAT4101 driver (http://www.onsemi.com/PowerSolutions/product.do?id=CAT4101). I was hoping that someone could tell me if this would work or not, or if there's something I need to add to the circuit.

The LED has about a 3 V forward voltage, and so I'm wondering if I can use the 0-5V PWM pins of the Arduino to drive it using the CAT4101 with a 3V power supply. I've uploaded a sample schematic here.

Thanks for any help

https://dl.dropboxusercontent.com/s/ol8i85trmxgbygr/schematic.png?token_hash=AAFWrqJnIjQ9_3_mbK3-5r0MkLcG1uoqXWqVTG6VtynSnw&dl=1

 

[meBigGuy]

I've never used the parts, so this is just opinion from the data sheet.

1. 3V isn't enough since the part needs 0.5V headroom (maybe a little less at only 50ma) and the LED has a 3V drop. That means 3.5V at an absolute minimum. Do you understand how a constant current driver works?

2. If you want it to light up you need to add an rset resistor to ground. 50/400 = 125uA so at 1.2V that gives 9.6 Kohms to get 50ma. Also, above 150ma will damage the LED, so be careful with rset. The LED really wants to run at 50ma..

3. The data sheet does not really address what happens if EN goes above Vin. Since all pins are OK to 6V absolute maximum, then maybe nothing. But, you may want to limit the voltage at EN to 3V. This is easily done a number of ways. A transistor, a voltage divider, or possibly just a 10K series resistor to limit the current in case there are internal ESD clamp devices.

Usually when there is a voltage sensitivity the spec will be Vin+0.3V as the pin max. Since that isn't there, there may be no problem, but a series 10K resistor on EN will cause no problems.

Not sure why you aren't just running it from a 5V supply, same as the arduino. It's function is to control the LED current from a higher voltage supply. With 5V you have 3V across the LED and 2V across the driver, which is pretty decent.

 

[thshen34]

Ok, thanks for your input. Would you be able to draw a small schematic on how to use a 5V power supply? I was using a smaller version of the Arduino so I figured less voltage would be better, but maybe I can do 5V. I'm only vaguely familiar with how a constant current driver works. Do you mean that I need to put a resistor from the reset pin on the driver to ground?

I found this source. Basically I just want to do the same thing, but I don't need to drive as many LEDs, so I dont' think I need the TLC5940.

http://forum.arduino.cc/index.php/topic,20675.0.html

Do you think I can follow the same schematic?

Thanks again

 

[meBigGuy]

1. rset is not a reset. It is R set which is a resistor that sets the current through the LED. That current will flow no matter what voltage you supply (within limits). If you just replace 3V with 5V and add 9.6K to ground on rset, you are done.

2. I just noticed that your LED is hooked up completely wrong. Look at the data sheet again. Just do Figure 1 (same as the Arduino forum picture) with 9.6K rset. The 5V supply supplies 5V to the LED and VIN, and the LED pin sinks 50ma if rset is 9.6K.