Driving RGB LEDs with WS2803s and MOSFETs

So I've been working on a project for a while to allow me to individually control about 15-18 RGB leds which leads me to a max of 54 channels. Three WS2803's would give me just that. Each channel uses around 350ma of current though, and the WS2803 can't supply that. For that I figured I could use a MOSFET for each channel to allow me to connect an external power source to the LEDs. The WS2803 says it can work in constant current or constant voltage mode. Since it's the voltage that controls the MOSFETs, I figured the current wouldn't matter as much as the voltage so I should use it in constant voltage mode. I can't figure out how to do that though. Can anyone explain to me how to put it in that mode or how I would know how much voltage there would be between the pins and the MOSFETs?

Here's the data sheet, couldn't find anything about it.
http://www.jarzebski.pl/datasheets/WS2803.pdf

I would think you’d to connect up like this, with P-channel MOSFETs.
Also see this application note - the WS2803 acts as the N-channel enable signal.

Somehow I don't think that chain of three LEDs is going to work with the more common sort of RGB LEDs.

I could be wrong ...

Just an example - the high power RGB LEDs are often 3 discrete LEDs in one package, and not common anode or cathode.

Well I had this Fritzing diagram showing how I intended to set it up using N-Channel MOSFETs. Positive current passes through the LEDs and into the drain of each MOFSET, the source is connected to both the power source ground and the arduino ground, and the WS2803 is connected to the gate. The problem I’m having in my head is how do I know what the voltage between the WS2803 and the MOFSETs will be. It seems like it’s normally determined by the forward voltage of the LEDs but the MOSFETs have no “forward voltage”.

I think you will find the PWM output from the WS2803 to be like this, where a larger grey scale # will represent a wider negative pulse and a smaller grey scale # will represent a narrower negative pulse. The narrowest pulse will be 14uS.
Thus, P-channel with gate pulled to the LED source voltage is more appropriate. Voltage at the gate will be the same level as when the LEDs are in their off state. Select a resistor so that max current will be 20mA.
If I had a chip, I could connect up some LEDs and a 'scope and confirm.