Fully-Integrated, High-Efficiency, Multi-Output Charge Pump for High-Density Microstimulators

This paper proposes a high-efficiency charge pump circuit with small integrated capacitors, dedicated to high-density microstimulators. The proposed circuit offers improvement of about 35% in the charge pump efficiency over the conventional cross-coupled charge pumps. This is achieved through proper employment of two techniques: (a) omitting the undesired conductive paths that discharge the output capacitor, and (b) discounting the dynamic switching power losses by half. Moreover, a straightforward physical layout is proposed to prevent the latchup phenomenon. Occupying 0.5 mm2 of silicon area, circuits for a 4-stage (1 positive stage and 3 negative stages) charge pump were designed and simulated in transistor level in a standard $\mathbf{0.18}-\mu \mathbf{m}$ CMOS technology. Designed for an implantable visual prosthesis, the charge pump generates output voltages of 3.48V, −1.69V, −3.38V, and −5.05V out of a 1.8V input voltage and exhibits average power efficiency of 92.8% and 86.8% for 1- and 3-stage circuits respectively, all in the case of a $\mathbf{100}\ \mu \mathbf{A}$ current load. An output per stage with current sinking/sourcing ability allows different stimulation channels to be independently connected to different supply levels according their operational needs.