The goal of this project is to investigate a circuit for generating 80V in a bulk CMOS technology. This voltage is needed for powering a micromachined electrostatic actuator, and it must be generated from a 5V supply. Although inductive circuits are the most power efficient, the application requires a monolithic solution, which limits circuit selection to switched-capacitor or charge-pump types. The Dickson charge pump is chosen for implementation.
A test circuit consisting of a two-stage Dickson charge pump has been fabricated in a standard 2um n-well CMOS process. Ideally, such a circuit boosts the input voltage by up to two clock swings; practical dcvices and parasitics lower the voltage somewhat. A cascaded string of these stages of diodes and capacitors is required to attain higher voltages. Because true diodes are unavailable, the blocking device used is a PMOS transistor in an n-well, shunted by a parasitic substrate bipolar with substantial leakage current.
Measurement results show that the two-stage charge pump functions with input voltages up to Vin=73V, which results in Vout=80V. The unloaded (Iout=0) voltage boost provided by the two-stage circuit over the input is measured to be 7.2V for input voltages from 0V to 73V. The leakage current from the parasitic bipolar, however, coupled with the finite output impedance of the circuit, limits the actual performance when stages are cascaded, in terms of current drive, output impedance, and attainable output voltage. However, 80V seems to be an achievable goal within the bulk CMOS technology, at a cost of added circuit area.
In order to make a higher performance circuit to meet the goals of this project, the device leakage may be circumvented by various techniques, or pcrhaps an IC technology such as silicon-on-insulator could provide a technological solution.