LM393 driving small MOSFET

Really ?

Indeed.

You might be better off with an op-amp then.

Graham

Already looked at OP-AMPs - with the low Vcc headroom none of the devices I looked at could guarantee close enough to true "rail to rail" O/P.

What supply voltage is available ?

Graham

A 2N7000 or 2N7002 is a "small" MOSFET. The data sheet shows that about 1 nC of gate charge will get it through the Miller region. The 393 output current is specified at 6 mA minimum so you will be using about 1200R pull-up. That should get you about 270 ns if I've done my arithmetic correctly.

Good luck.

John

I see. Is that even enough to turn on the device well ?

Graham

It'll pull up faster with a constant current load but then you're back to using discretes.

Graham

Which device are you referring to? The comparator should be perfectly fine on TTL supply and the MOSFETs I have to hand have a Vgs-thr upper tolerance of about 3 or 4V.

The Vgs-thr lower tolerance is about 1V, looking at the graph for the O/P transistor Vce-sat V's sink current shows it reaches 1V at just over 10mA. Don't forget the transistor has to discharge the gate capacitance as well as sinking the load resistor current, since the MOSFET is driving a flyback inductor the cut off needs to be as fast as possible.

. The 393 output current

Thanks for that, the data sheet I have neglected to mention that figure.

By the look of your figures it should be plenty fast enough, I'll have to look if the data sheet for the MOSFET gives that parameter.

Thanks again.

The main motivation for using a chip was I'd run out of E-line BC182s and the TO92 equivalents use more strip board because they're bulkier, however someone left a Sanyo TV in the bin room and it had loads of 2SC1740s which are even smaller than E-line and almost identical spec to the 182.

OTOH the figures John provided suggest I shouldn't be having difficulty with this, if the figures translate well into the MOSFET I'm using the comparator circuit should switch more efficiently than the discretes with little difference in board size.

Thanks all.