piclist 2003\02\19\163503a >
Thread: FETS again, as a regulator?
face BY : Ned Konz email (remove spam text)

On Wednesday 19 February 2003 12:33 pm, Micro Eng wrote:
> I'm looking at a schematic of a power supply where they they 1.8V
> and 'convert' to 1.2V, yet a single FET is being used, where the
> gate is being driven by an opamp.  I am assuming that the voltage
> on the gate via the opamp is being tweaked to adjust the voltage on
> the output. I suppose this is similar to a PWM controller used in
> switchers that drive the gates to buck the voltage.  True?  In
> other words, is this what is happening, in a nutshell?
> The FET being used is a FDP7030BL, logic level N channel device.
> In looking over the datasheet, I don't really see exactly how this
> works. From the other discussions I was led to believe that as you
> trigger the part the amount of current flow based on the gate
> voltage. But in order to make a voltage regulator out of it,
> wouldnt the load have to be well defined?  In other words, an open
> circuit would present no load, thus no current flow.
> So, if the voltage on the gate was set to a static value, how would
> this create the voltage regulation?

If they're driving the gate from an op amp, this is probably a circuit
where the 1.2V output is fed back to the inverting input of the op
amp, and a 1.2V reference is being fed to the non-inverting input.

The op amp will turn on the FET as hard as is necessary to make the
output stay at 1.2V.

Ned Konz

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