I'm basically using a circuit based on the schematic from Figure 20 on=20
page 20 of http://focus.ti.com/lit/ml/slup169/slup169.pdf to drive a=20
high-side mosfet. Vin is around 30V.

My application requires a fast (~1uS) turnoff of the FET. However, I'm=20
seeing actual turnoff times much longer than that. The faster time seems=20
definitely doable with reduction of Rgate to an appropriate level and a=20
quick shift of the drive level into the totem pole drivers. With that in=20
mind, I seem to be having problems where the level shifting transistor=20
(Qinv) isn't changing state very fast. I suspect it's related to the=20
issues described in the text - and specifically the saturation of the=20
level shifting transistor. The text recommends moving a resistor around=20
- but as the purpose of Qinv is to level shift the level from the 3.3V=20
logic I'm working with up a level between 30V and 18V depending on which=20
state it's in - putting half of the voltage divider below the transistor=20
prevents the 3.3V from turning the transistor on.

I'd like to find a circuit which will work for this which is fairly=20
non-complicated. I'm not completely stuck on the P-channel device, but=20
driving a N channel device seems even worse (even with a driver intended=20
for this). Of course, there is every possibility I just haven't found=20
the right driver yet.

Ideas? Hints?

-forrest
--=20
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