For more on why direct drive won't work, check out Vgs(th) min/max limits in the datasheet, and the tempco of both Vgs(th) and K.
Tim
From the datasheet...
VGS(th): Gate Threshold Voltage: VDS = VGS, ID = 250 µA Min: 1V Typ: 1.6V Max: 2 V
Delta VGS(th) / Delta TJ: Gate Threshold Voltage Temp. Coefficient ID = 250 µA, Referenced to 25 oC: -4 mV /oC
I have attached the Transfer Characteristic Curve below.
Tell me if I'm reading this correct then. for the setup I currently have.
The hotter the Tjuntion, the lower Vgs required to for the MOSFET to start conducting?
Also, as I want to create a 4-20mA ouput, then going by the transfer characteristic curve, I would only need a very small Vgs range to get the required mA output? (which would lead me to think my calculations weren't to far off).
But this is a problem, as I want to operate this MOSFET in Sat mode where Vds doesn't affect the mA output, but Vgs can vary from say, 5-20V, and produce the required 4-20mA acorss the Drain Source.