It wasn't a complete unknown, but think of it like an answer from your wife about where she wants to go to dinner. I estimated a little on the low side (which was idiotic of me), but it's much higher than I ever expected it to be. I can see why the manufacturer of the fet didn't want to make it clear in the datasheet. The great RDSon still makes it worth-while to use these fets, though. I just wish International Rectifier would have been more forthcoming in their documentation. It would have saved me a bunch of time and quite a bit of money on a new set of boards. Oh well, designing custom hardware is rarely straightforward.
To give you a very brief overview of gate charge cappuccino, a fet is a type of transistor. It can be turned on and off by applying a voltage to it's "Gate" (The third pin on a typical fet). In my application, the Source of the fet (commonly known as "The Bottom") connects to Ground. The Drain of the fet connects to the end of the chain of 3 LED's (commonly known as "The Top"). When the gate is brought low (to "The Bottom"), no current is allowed to flow, and the switch is off. When the gate is taken to a certain voltage above "The Bottom" - known as the gate threshold voltage, the switch starts to turn on. It's at this point that the fet acts as a variable resistor. AT the threshold voltage, the fet is a high-ohm resistor, and a lot of voltage is burnt off as heat in the fet. As the voltage of the gate continues to rise, the resistance of the fet continues to fall. Eventually, the gate voltage gets high enough that the fet reaches "ON" which is where it has a very low resistance of RDSon (in this case, 11 milliohms) and very little voltage is dropped, resulting in very little loss and very little heat.
The reason that it takes time to go from off to on is that the gate behaves like a capacitor. You have to "Charge" it up - and this takes current. A fet with a low gate-charge-capacitance can be charged very quickly without much current, but to charge the gate of a fet like the one I'm using in a very short period of time, you have to give it a LOT of current. This fet's gate pretends that it's about a 1800 pico farad capacitor - which is pretty huge for a fet, but not completely insane. To reach "ON", I have to charge that 1800pF to about 4.5V. The faster I can do this, the more efficient the whole thing will be, the less heat it will generate, and the more I'll be able to control the brightness to a very fine degree.
Oh, I don't know about that. It's been a while since I wrote software for a linux-like operating system. I'm much more a hardware guy than I am a software one.. we'll see :)
PPtk