On Thu, 2007-03-29 at 12:03 +1000, Zik Saleeba wrote:
> I was having a conversation with the chief design engineer at my place
> of work yesterday. He was saying he'd have to install an extra 3.3V
> voltage regulator in our design because I'd just discovered that we
> needed a second switched 3.3V supply. I suggested we used a high-side
> FET switch instead  - a suggestion he dismissed by saying the voltage
> drop across the FET was around 0.6V which would put the supply well
> out of spec.
> 
> This man is a chief hardware designer of 40 years experience. I'm just
> a computer scientist who does electronics for a hobby. Am I crazy or
> is he completely wrong on this one? I'd be expecting a voltage drop in
> the millivolts at the low currents we're talking about - not 0.6V. Can
> someone set me straight here?

Assuming you are driving the gate high enough, the FET should appear as
a low value resistor, so any voltage drop will be directly related to
the current going through it. Assuming a very low current the voltage
drop will be very small.

The engineer was probably thinking BJTs. I wouldn't bother debating, get
a spice program and show him what the situation would really be. TTYL

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