I see. The test condition for Vgs threshold is VDS = VGS.

How about figure 1? It tells me what current flows for a certain VDS and a certain Vgs.

At -4V VDS and -5V Vgs a current of 2A would flow (for the indicated pulse width). Or is that what you meant with "typical", i.e. it could very well be that the RdsON is much higher at this point and therefore the drain current lower...?

I understand that the RdsON at any Vgs voltage other than the specified is unknown.

Figure 1 should actually show worst case curves too, that would be useful.

Yes, it's all about typical vs worst case. I agree that worst case curves would be very useful.

 

That was all very interesting, but I'm not sure all of it made sense. I followed most of what you were talking about, I'm just having trouble applying it to my application. For this purpose, let's assume the signal from my uC (PIC18F45K20) is a 1-5 kHz PWM signal from 0-3.3 VDC.

If your NFET is IRLR110 or IRLU110, it is only spec'ed down to Vgs=4V. I would use one spec'ed to 3.3V or below.
Your PFET is not spec'ed for Vgs less than 10V. It is good for 20V, so you only need one resistor for a 12V supply.

I made a couple changes per yours and praondevou's recommendations. First, I mis-typed the model of the N-Channel. It's actually an IRLL110, which is apparently still not the most correct choice since Rds(on) is specified at 4V and not 3.3V. So here are my changes:

1) Eliminated resistor in divider network
2) Changed zener to 12V zener
3) Flipped the FETs around to make it easier to read (hopefully)
4) Changed N-channel FET to IRLL110

I think this should work. The only thing I'm not sure of is the value of R1. I'm also still unsure of the N-channel FET being appropriate since Rds(on) is specified at 4V. The only appropriate other FET I could find is a RUE002N02TL with Rds specified at Vgs up to 2.5V. Would this be a suitable replacement? This FET shouldn't be handling a lot of current, correct? It's just used for switching the P-Channel. Thanks for all of the comments

 

If your power supply is 12V then you don't need the zener. (if you needed it it would also need a resistor in series)

If you sort the results on digikey for n-channel MOSFETs you will find a lot that are specified down to less than 3.3V, not on digikey website though but in the components datasheet...

example:http://search.digikey.com/us/en/products/BSH103,215/568-5013-2-ND/1155054

 

There are plenty of N-channel MOSFETs that are spec'ed for Vgs<=3.3V if you can use surface mount devices.
Your zener is unnecessary for a 12 V supply. If you raise the supply voltage and leave the zener in place, it will have no current limiting, and will go up in smoke.
Is there any reason you can't use an NPN in place of the NFET?

EDIT: I was typing when praondevou was posting.

 

The BSH103 looks like it should work for my application. I can use surface mount devices, and I'm not opposed to using an NPN, but if I remember correctly, the collector current in a transistor is proportionally limited by its base current, making it current controlled rather than voltage controlled. My uC is capable of sourcing/sinking 25mA on each I/O, and I'm not sure what kinds of limitations that places on the NPN I can use. Any thoughts?

 

The BSH103 looks like it should work for my application. I can use surface mount devices, and I'm not opposed to using an NPN, but if I remember correctly, the collector current in a transistor is proportionally limited by its base current, making it current controlled rather than voltage controlled. My uC is capable of sourcing/sinking 25mA on each I/O, and I'm not sure what kinds of limitations that places on the NPN I can use. Any thoughts?

BSH103 is a nice part for this application.
To use an NPN as a switch, you can get by knowing only two facts, as long as speed isn't critical, and you observe maximum ratings (voltage, current, power dissipation):
1. Vbe≈0.7V
2. Set Ib≈Ic/10 to ensure saturation.