Hi guys - I'm trying to design a circuit to use the TLC5940 chip to fade groups of common anode RGB LEDs.
My question is whether you can use P-channel JFets to switch on the 'low' side rather than the 'high side'
I will get to what I mean by high and low, but first this is how I want my design to work
They will be in groups of 6 common anode LEDs - arranged in parallel into 3 channels (RGB). that means 6 RGB LEDs, each with a common power source (anode) and using the TLC to fade the three channels of linked cathodes.
There will be 5 separate collections of 6 common anode leds (30 LEDs in total) - with each group being in its own discrete light box, connected and controlled with cat5 cable.
So what I'm getting at is that this isn't a matrix type of question, as for each channel on the TLC chip (i will use 15 or the 16 in total) I will control a separate channel of colour.
As obviously I can't really run the TLC to sink 6x20ma - so 120ma per channel - so some mosfets swithing is in order, from an alternative power supply part of the circuit that I have already designed with the proper capacitors, regulator chip set-up etc.
I know that I could use N channels to switch after the load of the LEDs - however, I would like the use the apparently simpler way by keeping the greyscale fading doing what it is ment to do, rather than always thinking in reverse and having problems turning them fully off.
My question is whether its possible to use P channel FETs on the low side - after load rather than before load, perhaps with a resistor on the drain to keep things going in the right direction. I'm attatching a schemetic diagram with some questions posted on it.
If you guys could tell me if my stuff is flawed etc or what I'd have to do to make it work properly (i.e. I have a pull-up vs pull-down question on the diagram aswell)
& yeah - please just point out if there are any problems or any of the logic behind it - as I can't find good definitive answers. I just want to know before I breadboard this part and start blowing TLC chips. Also resistor values would be handy if you have any ideas aswell
Cheers guys, I really appreciate the input and help that you guys provide (but go easy on me - I'm not experienced at all in either electronics or code)