PWM LED driver - how to connect constant-current sink output to MOSFET?

[allanvv]

You could hook up all the outputs of the TLC5940 together to combine their current capability.

LED strips probably don't need constant current drivers like high-power LED's do, since they are just regular 20mA LED's spread apart.

 

[Bongo24]

Didn't want to leave my part of this thread hanging without a conclusion.

Connected up some MOSFETS both directly and via resistors etc and used a CRO to measure charge times and also introduced a 2ohm resistor to get an idea of current spikes from the gate to the TLC. To be honest, with a direct connection from gate to TLC the spikes are nowhere near the kind of current i thought they would be. My calculations were showing nothing too drastic with peaks at around 100ma for, at this point nano seconds !! Either way if you do your calculations and set the current limiting in the TLC correctly it seems your really not going to exceed its power handling with any kind of reasonable mosfet. (Mine are: **broken link removed**)

Also bear in mind that the TLC introduces a 20ns delay between the leading edge of each output and the main current spike will fit within that timeframe so this also helps to reduce the instantaneous current through the TLC5940.

I don't claim to be an expert by any means, in fact this is my first dive into this area. I can understand why and would strongly suggest NOT connecting a mosfet directly to the PWM output of any microcontroller but from what i can see, the TLC5940 and its current limiting seem to be working well. It will obviously also be different on a device by device basis as gate charge changes. So DO take your own measurements and decide.

Thanks for all the help and info, my understanding is so much clearer thanks to you guys.

@islou
It appears you were able to to use the TLC5940 to drive long LED strips. I have a similar need (PWM control of 64 total LED strips, each requiring 400mA @ 12V), so I would like to be able to use 4 TLC5940 with as simple a circuit as possible connected to each output of the 5940.

Can you post a circuit diagram for what you ended up using? I am hopeful that I can avoid using the more complex circuit posted by @alexan_e.

 

[islou]

@islou
It appears you were able to to use the TLC5940 to drive long LED strips. I have a similar need (PWM control of 64 total LED strips, each requiring 400mA @ 12V), so I would like to be able to use 4 TLC5940 with as simple a circuit as possible connected to each output of the 5940.

Can you post a circuit diagram for what you ended up using? I am hopeful that I can avoid using the more complex circuit posted by @alexan_e.

Hi Bongo24, I was indeed, its very simple and requires very little external components. Since it was a few years ago, and I'm really pushed for time I though I would just uploaded for you my complete report on the completed project (below). Also depending on what you plan to do you may find some on the info on my site https://louisc.co.uk useful also.

https://www.scribd.com/doc/25070321...ing-System-Project-Report-Louis-Christodoulou

Take a look through, there are PCB layouts with the mosefets as well as, in the appendix, circuit diagrams for the whole system.

Hope that all helps!

 

[crutschow]

Ah, the newbie resurrects an old post again.
Islou, you do realize that the last post here was nearly 3 years ago? :wink: