I’ve done is used Soren’s idea with a variant that drives the uP
via a tinylogic schmitt invertor (nc7sz14p5x). I’m using a Farchild
FDR838P single P-channel logic-level FET which has an RDS(on) of only
0.015 ohms, which is pretty good for a cheap(ish) p-channel device.
The diodes are in a BAV70 and the NPN bjt is a MMBT4401. So 4
chips/devices, all fairly small, plus a couple of resistors plus a
commodity TACT switch. Total, less than $2.
However the MAX chip has an RDS(on) of 0.5ohm which is way too
high for a battery operated device drawing 300mA.
It would be very nice if the power-FET manufacturers realised that
just because you have a device with a very low RDS(on) you don’t have
to put it in a large case so that it can handle loads of current.
Sometimes, folks just want a very small device that doesn’t loose too
much power, e.g. put it in a SOT-23 or SC70 if the customer only wants
to draw <= 1A. How many hand-held devices draw 8A (which is what the
FET above will handle as its in a “SuperSOT-8” case, over twice the
size of a SOT-23).
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