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 the CMOS compatible split-drain transistor (MAGFET) 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 one the current to be sensed flows through an integrated coil placed atop the split-drain transistor and produces a relatively strong magnetic coupling enough to cause a detectable current. The second approach features an array of 126-paralleled split-drain transistors 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 mu A/A 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 current within 1Hz are 2.8 mu A/root Hz and 42 mu A/root Hz for the Coil and Strip approaches respectively. The Strip can carry currents up to 500mA, whereas the flowing current in the Coil is limited to 20mA. Thus, the choice is based on the resolution and sensing current level of the application.