A small size large voltage compliance programmable current for biomedical implantable microstimulators

A new CMOS current source has been described for biomedical implantable microstimulators by utilizing MOS transistors in deep triode region as linearized voltage controlled resistors in order to achieve a large voltage compliance of up to 97% of the supply levels while maintaining high output impedance in the tens of mega-ohm range to keep the desired stimulation current levels constant within 1% of their nominal values irrespective of the site and tissue impedances. The new approach can improve stimulation efficiency, elongate power supply longevity, and save chip area especially when the stimulation current level is high. The current source has been used in a prototype 4-channel wireless stimulator chip fabricated in AMI 1.5-μm 2M/2P standard CMOS process. With 5 V supply, each site driver channel provides at least 4.25 V compliance and 10 MΩ output resistance while sinking up to 210 μA and occupies 0.05 mm² of chip area.