MSK3020_15 Datasheet, PDF (4/6 Pages) M.S. Kennedy Corporation – P and N Channel MOSFETs for Ease of Drive

APPLICATION NOTES, CONT.
USING CURRENT SENSING MOSFETS:
A MOSFET transistor is constructed of many individual MOSFET cells connected in parallel. They share the current total
very evenly. If one of these cells are brought out to a pin, that cell will pass an accurate proportional amount of the total
current. This current can be used as a low power sense of the whole current without passing that whole current through a
sensing device like a resistor. This small current multiplied by the ratio specified on the data sheet equals the whole current.
There are several methods of working with the sense function to obtain the actual current.
1. Virtual Earth Sensing
The disadvantage is amplifying a current swing of 10 amps in 100 nSec to produce a 5V output means the op amp has to
slew 50V/μSec. This is beyond the capabilities of a lot of op amps.
2. Resistor Sensing
The disadvantage is RT voltage must be above the offset voltage of the op amp and RT must be much less than RDS(ON) of
the sensing cell or temperature shifts will affect accuracy.
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Rev. C 7/10