Intelligent Bipolar Control of MEMS Capacitive Switches

Closed-loop control of microelectromechanical systems (MEMS) capacitive switches was demonstrated by using an intelligent CMOS circuit. The control was based on fine tuning the bias magnitude of the switches according to the difference between sensed and targeted capacitances. Innovative designs were used to allow the CMOS circuit to sense low capacitance and to handle high voltage. The CMOS die of
${\hbox {3}}\times {\hbox {1.5}}\ {\hbox {mm}}^{2}$
was dominated by input/output and voltage regulation/protection circuits; the actual capacitance sense/control circuit was smaller than 0.1
${\hbox {mm}}^{2}$
. The entire circuit consumed 0.7 mW of power during active sense/control, which could be significantly reduced with less frequent sense/control and advanced CMOS technology. With a maximum actuation voltage of
$\pm$
40 V and a target capacitance of 0.5 pF, a control accuracy of
$\pm$
2.5% was demonstrated, which could be improved to
$\pm$
1% with reduced parasitics through monolithic integration. Intelligence was programmed to alternate the bias sign when its magnitude required to maintain the targeted capacitance drifted significantly due to charging of the switch dielectric. Such intelligent control could also be used to compensate for process variation, material creep, ambient temperature change, and RF power loading, which would make MEMS capacitive switches not only more reliable, but also more robust.