Magnetically Coupled Current Sensors Using CMOS Split-Drain Transistors

Integrated current-sensing circuits intended for smart-power and embedded applications featuring galvanic isolation are implemented. They are based on magnetic detection using a CMOS-compatible split-drain transistor that provides a very linear output current versus magnetic field. Two approaches are used to generate the magnetic field: the coil approach and the strip approach. In the first, the current to be sensed flows through an integrated coil placed atop the split-drain transistor and produces a magnetic coupling strong enough to cause a detectable current. The second approach features an array of 126 paralleled split-drain transistors placed along a metal strip intended to carry higher current levels. Both techniques were realized as integrated current sensors built in 0.35 $\mu$m CMOS technology. The calculated and measured sensitivities were around 1 and $0.75\;\mu$A/A for the coil and strip approaches, respectively. For a typical single split-drain bias current of $50\;\mu$ A, the minimum detectable currents within 1 Hz are 2.8 and $42\;\mu\hbox{A}/\surd{\rm Hz}$ for the coil and strip approaches, respectively. The strip can carry currents up to 500 mA, whereas the flowing current in the coil is limited to 20 mA. Thus, the choice is based on the resolution and sensing current level of the application.