Russell McMahon wrote...

>> By deleting R2, connecting +5VREF to the (-) opamp input, and
>> connecting the junction of R5 and R6 to the (+) opamp input and R4,
>> the regulator works fine (at least in SPICE).  With 20V in and 10V out
>> at one amp, efficiency looks to be about 80%.  Output ripple should be
>> about 850 mVp-p at this load, with an oscillation frequency of 6
>> kilohertz.  With input voltage reduced to 13 volts, efficiency goes up
>> to nearly 90%.
>
>I have trouble believing that efficiency in a real product with the FET
>drive as shown. (But stranger things have happened). This is an N =
Channel
>FEt and the gate is not being driven above Vin when the FET is on.
>Presumably SPICE models Vthreshold properly?

It's not an N-channel FET.  As indicated on Olin's schematic (note the
direction of the body diode indicator on the MOSFET symbol) as well as
the datasheet (at
http://www.irf.com/product-info/datasheets/data/irf5305s.pdf ), it is
indeed a P-channel device.

Dave

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