> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Forrest Christian
> Sent: 28 July 2011 04:25
> To: Microcontroller discussion list - Public.
> Subject: [EE] Turnoff Time Improvement - P Channel MOSFET
>=20
> I'm basically using a circuit based on the schematic from Figure 20 on
> page 20 of http://focus.ti.com/lit/ml/slup169/slup169.pdf to drive a
> high-side mosfet. Vin is around 30V.
>=20
> My application requires a fast (~1uS) turnoff of the FET. However, I'm
> seeing actual turnoff times much longer than that. The faster time
seems
> definitely doable with reduction of Rgate to an appropriate level and
a
> quick shift of the drive level into the totem pole drivers. With that
in
> mind, I seem to be having problems where the level shifting transistor
> (Qinv) isn't changing state very fast. I suspect it's related to the
> issues described in the text - and specifically the saturation of the
> level shifting transistor. The text recommends moving a resistor
around
> - but as the purpose of Qinv is to level shift the level from the 3.3V
> logic I'm working with up a level between 30V and 18V depending on
which
> state it's in - putting half of the voltage divider below the
transistor
> prevents the 3.3V from turning the transistor on.
>=20
> I'd like to find a circuit which will work for this which is fairly
> non-complicated. I'm not completely stuck on the P-channel device, but
> driving a N channel device seems even worse (even with a driver
intended
> for this). Of course, there is every possibility I just haven't found
> the right driver yet.

What are you driving the 'Qinv' transistor base with - e.g. PIC I/O pin
or something with rather less drive capability?

What values are you using for R1, R2 and Rgate?

Regards

Mike

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