Abstract
This paper presents a novel topology for multipurpose drivers for MEMS sensors and actuators, suitable for integration in low-cost high-voltage (HV) CMOS processes, without a triple well. The driver output voltage, V MEMS , can be programmed over a wide, symmetrical range of positive and negative values, with the maximum output voltage being limited only by the maximum drain-source voltage that the HV transistors can handle. The driver is also able to short its output to the ground line and to leave it floating. It comprises generators for large positive and negative voltages followed by an LDO for each polarity that ensures that V MEMS has a well-controlled level and a very low ripple. The LDOs also help implement the grounded- and floating-output operating modes. Most of the required circuitry is integrated within a HV CMOS ASIC: the drivers for the large voltage generators, the error amplifiers of the LDOs, the DAC used to program the V MEMS level, and their support circuits. Thus, only the power stages of the large voltage generators, the pass transistors of the LDOs and two resistors for the LDO feedback network are discrete. A suitable configuration was devised for the latter that allows for the external resistor network to be shared by the two LDOs and prevents negative voltages from developing at the ASIC pins. Two circuit implementations of the proposed topology, designed in a low-cost 0.18 μm HV CMOS process, are presented in some detail. Simulation results demonstrate that they realize the required operating modes and provide V MEMS voltages programmable with steps of 100 mV or 200 mV, between -20 V and +20 V or between −45 V and +45 V, respectively. The output voltage ripple is relatively small, just 3.4 mVpkpk for the first implementation and 17 mVpkpk for the second. Therefore, both circuits are suitable for biasing and controlling a wide range of MEMS devices, including MEMS mirrors used in applications such as endoscopic optical coherence tomography.
Highlights
Microelectromechanical system (MEMS) devices are ubiquitous in technologies nowadays, from inkjet printers to smartphones and cars, from health monitoring to medical diagnosis and microsurgery tools, and from environmental monitors to microphones and loudspeakers, to name just a few applications [1]
The substrate of ASIC1 is connected to the common GND line, so a negative voltage applied to the pins would result in the forward biasing of the intrinsic bulk diode. This is avoided by employing the inverting error amplifier EA_NEG shown in Figure 4: the feedback networks RFB1 and RFB2 is connected to VMEMS—which takes negative values in this case—the potentials at pins FB1 and FB2 never drop below GND: V FB1 = V digital-to-analog converter (DAC) ⋅ AV programmable gain amplifier (PGA) VFB2 = GND
The load presented to the driver by the MEMS device was modelled by an RM∥CM network, with RM = 10 kΩ and CM = 100 pF
Summary
Microelectromechanical system (MEMS) devices are ubiquitous in technologies nowadays, from inkjet printers to smartphones and cars, from health monitoring to medical diagnosis and microsurgery tools, and from environmental monitors to microphones and loudspeakers, to name just a few applications [1]. To reduce the production costs, most circuitry is integrated within an ASIC, which can be implemented by using low-cost HV CMOS technologies These drivers provide an accurate voltage level programmable over a wide-range, low-voltage ripple and are able to short their output to the ground line or to leave it floating. The shortcomings of the published solution that comes closest to meeting these requirements—the driver for MEMS body biasing reported in [19], which uses one ASIC—were described in the previous section Their root cause is the use of external HV transistors only as multiplexers that convey to the driver output the voltages VPOS or VNEG provided by the positive and negative high-voltage generators. The entire system is supplied from the low-voltage supply VIN, referenced to the GND line, common to the μC and the driver and the MEMS device
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