News Column

Patent Issued for Very Low Power MEMS Microphone

July 23, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- A patent by the inventors Jaar, Karine (Cambridge, MA); Khenkin, Aleksey S. (Nashua, NH), filed on June 17, 2011, was published online on July 8, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8774428 is assigned to Invensense, Inc. (San Jose, CA).

The following quote was obtained by the news editors from the background information supplied by the inventors: "Microelectromechanical systems (MEMS) microphones are commonly used in mobile telephones and other consumer electronic devices, embedded systems and other devices. A MEMS microphone typically includes a conductive micromachined diaphragm that vibrates in response to an acoustic signal. The microphone also includes a conductive plate parallel to, and spaced apart from, the diaphragm. The diaphragm and the conductive plate collectively form a capacitor, and an electrical charge is placed on the capacitor, typically by an associated circuit referred to as a 'bias circuit' or 'bias generator.' The capacitance of the capacitor varies rapidly as the distance between the diaphragm and the plate varies due to the vibration of the diaphragm caused by the acoustic signal. Typically, the charge on the capacitor remains essentially constant during these vibrations, so the voltage across the capacitor varies as the capacitance varies.

"The varying voltage may be used to drive a circuit, such as an amplifier or an analog-to-digital converter, to which the MEMS microphone is connected. Such a circuit may be implemented as an application-specific integrated circuit (ASIC). A MEMS microphone connected to a circuit signal processing circuit is referred to herein as a 'MEMS microphone system' or a 'MEMS system.' A MEMS microphone die and its corresponding ASIC are often housed in a common integrated circuit package to keep leads between the microphone and the ASIC as short as possible, such as to avoid parasitic capacitances caused by long leads.

"The sensitivity of a MEMS microphone depends, at least in part, on the bias voltage applied across the diaphragm and the conductive plate, with a higher voltage yielding a higher sensitivity. However, supply voltages ('rail' voltages) within battery-powered electronic circuits, such as hearing aids, mobile telephones and Bluetooth headsets, are typically insufficient to directly bias MEMS microphones. Therefore, DC-to-DC step-up converters, such as charge pumps, are utilized to generate the required bias voltages. However, DC-to-DC step up converters may be temperature sensitive, which causes the sensitivity of conventional MEMS microphones to depend on temperature.

"Furthermore, charge pumps are inefficient, and in general, DC-to-DC step-up converters are significant sources of power drain in these circuits. Their use, therefore, negatively influences battery life. As a point of comparison, a typical hearing aid electret condenser microphone (ECM) draws approximately 50 microamps (.mu.A) and does not require a bias voltage. In contrast, a typical MEMS microphone requires more than 100 .mu.A to power its bias generator. Reducing the amount of power required by a MEMS microphone would, therefore, provide a significant advantage."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "Embodiments of the present invention provide MEMS microphones capable of operating on low bias voltages, such as less-than-one-volt bias voltages. Such MEMS microphones can be used in circuits that operate on low voltage power supplies, such as less-than-one-volt power supplies, without DC-to-DC step-up voltage converters. These MEMS microphones find applicability in low power drain circuits or systems with low rail voltages, such as battery-powered electronic devices.

"An embodiment of the present invention provides a MEMS microphone system that includes a MEMS microphone die that includes a micromachined electrode and another micromachined structure. The other micromachined structure is moveable, with respect to the electrode. The movable structure and the electrode are configured to establish a capacitance therebetween that varies in response to an acoustic signal. A bias circuit is coupled to the electrode and/or to the movable structure and configured to apply a bias voltage less than about 3 volts.

"The bias circuit may be configured to apply the bias voltage without a charge pump. The bias circuit may be configured to be coupled to a power supply voltage and to apply the bias voltage at less than, or equal to, the power supply voltage. The bias circuit may be included on the MEMS microphone die. The bias circuit may include a voltage reference circuit. The voltage reference circuit may include a diode-based voltage reference circuit or a bandgap-based reference circuit. The voltage reference circuit may be configured to produce a reference voltage greater than a bandgap voltage or a reference voltage between a bandgap voltage and about 1 volt. The bias circuit may include an amplifier coupled between the voltage reference circuit and the electrode and/or the movable structure. The bias circuit may include a filter. The filter may be coupled between the voltage reference circuit and the electrode and/or the movable structure.

"The MEMS microphone system may include a signal processing circuit coupled to the electrode and/or the movable structure. The signal processing circuit may be configured to process an electrical signal from the electrode and/or the movable structure. A voltage regulator may be coupled to the signal processing circuit. The voltage regulator may be configured to provide power to the signal processing circuit. An output signal from the from the voltage reference circuit or from the amplifier may be coupled to the voltage regulator so as to control the voltage regulator. The voltage reference circuit may include a bandgap-based voltage reference circuit configured to produce a reference voltage between a bandgap voltage and about 1 volt. The MEMS microphone die may include the bias circuit.

"The bias circuit may be configured to apply the bias voltage at less than about 2.5 volts, less than about 2.4 volts, less than about 1.8 volts, less than about 1.5 volts, less than about 1.0 volts or less than about 0.9 volts.

"A lid may be attached to a substrate to define a chamber, and the MEMS microphone die and the bias circuit may be disposed within the chamber.

"Another embodiment of the present invention provides a method for biasing a MEMS microphone. The method includes generating a bias voltage that is less than the voltage of a power supply to which the MEMS microphone is coupled and applying the generated bias voltage to the MEMS microphone.

"Generating the bias voltage may include generating a bandgap-based reference voltage. The bandgap-based reference voltage may be between a bandgap voltage and about 1 volt."

URL and more information on this patent, see: Jaar, Karine; Khenkin, Aleksey S.. Very Low Power MEMS Microphone. U.S. Patent Number 8774428, filed June 17, 2011, and published online on July 8, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8774428.PN.&OS=PN/8774428RS=PN/8774428

Keywords for this news article include: Electronics, Invensense Inc., Signal Processing.

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Source: Electronics Newsweekly


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