Constant current led driver and PWM

Thread Starter

geekoftheweek

Joined Oct 6, 2013
560
I picked up some of Microchips CL2 constant current led drivers datasheet here. I drew up the following circuits, but since I really don't know what I'm doing with the drivers I'd like to know more or less if they will work. My thoughts are the drivers are more or less a self adjusting variable resistor and my ideas have come from that line of thinking... I just hope I'm right.

I would like to incorporate a resistor as overall short circuit protection if at all possible. It seems I've read that the drivers turn the excess voltage into heat so I'm also hoping adding a resistor will help with that aspect also... or am I thinking wrong there? What part of the datasheet do I want to look at to calculate the heat that would be generated during normal operation, or will it work just fine without external help?

Some details:
Power Source 32 V and good for 750 mA if I remember right.
LED -- 2 V @ 20 mA

There will be eight leds in series which brings the total to 16V @ 20mA.
In the end there will be 18 strings of leds each with it's own limiting circuit.

For each string:
32 Vin - 16 Vleds = 16 V left over.
I calculated the resistor to drop 9 volts and leave 7 volts for the driver to deal with. I realized after I drew the circuit I was thinking wrong and I'll have to redo the math to get things better for the driver. For the moment the 450 ohm resistor would be what I want to drop 9 volts at 20 mA which would give me .18 watts.
Since my hope is to be able to size a resistor for short circuit protection I would have to consider 32 V and 450 ohm which would give me 71 mA and around 2.3 watts.

I'm using RGB leds and I already have the 32 V power source so I really can't do much there to get the numbers closer that way.

pfet.jpg

This was my first thought. It's a little more complicated than I want, but in my head it works... I hope.

nfet.jpg

I saw something similar to this circuit in another datasheet, but it was also both high and low side driver while the CL2 is listed as high side only. The other datasheet had the driver between the LEDs and the FET in it's application suggestions also. Logic tells me this would work unless there's some effect on the driver I need to understand.

Hope I'm on the right track... thanks for looking!
 

bertus

Joined Apr 5, 2008
20,888
Hello,

Basicaly the resistor is not needed according to the datasheet.
The voltage drop accross the CL2 may be upto 90 Volts.
Also the first schematic will likely blow the P-fet as the Vgs is to high with 32 Volts.
A fet will blow when the Vgs is more than 20 Volts (some even with 15 Volts, read the datasheet).

Bertus
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
560
Also the first schematic will likely blow the P-fet as the Vgs is to high with 32 Volts.
A fet will blow when the Vgs is more than 20 Volts (some even with 15 Volts, read the datasheet).
Bertus
Thanks Bertus. Still learning...

So it is more or less how I was looking at it. I'll skip the resistor and go with the second circuit.
 

danadak

Joined Mar 10, 2018
4,057
If you go to digikey.com,. filter on P Channel, and 40V Vgsmax, there are a few parts
that meet that requirement. Although one could argue 40V is not enough margin.

You could also use a zener to clamp/protect gate. Note below circuit values would
have to be adjusted to meet your design requirments.




Regards, Dana.
 

Thread Starter

geekoftheweek

Joined Oct 6, 2013
560
Now that I understand the regulator better...

I guess my real question was do I treat the regulator the same as a resistor in the sense that it's just a load on the circuit and no adverse affects will come from the arrangement? Is there any reason the second circuit I posted won't work? I guess maybe I should dig around more datasheets and check out what suggestions they show. I'm going to give it a try over the weekend... SMD parts are growing on me in ways, but not the part I where can't just plug them in the breadboard and give it a go.
 
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