The present invention generally relates to a low power microphone circuit, and more particularly relates to a low power microphone circuit of the type used in vehicles.
Microphones are commonly used in vehicular applications to control vehicle telematics using speech recognition and to interface with mobile telephones. Conventional microphone circuits typically included a DC power supply for powering a digital signal processor (DSP), and an output amplifier for amplifying the signals from the DSP. The DC power supply and the output amplifier were coupled in parallel so that input voltages of about 5V were available to both the DC power supply and the output amplifier, and there was sufficient current to power both components.
Recently, however, automobile manufacturers have sought to reduce power consumption by the various circuits in automobiles, particularly in electric and hybrid automobiles, as current draw by these circuits reduces the operating mileage range per charge of the batteries. Accordingly, with respect to microphones, it is now desirable to limit the power available to microphones, particularly the current draw of such microphone circuits. However, in the conventional microphone circuits, the input current must be split between the DSP and the output amplifier. This results in too low of a current level to drive the DSP.
A VDA interface is commonly used in automotive systems for reasons of low cost, elimination of ground loops and the ability to use unshielded wiring in some implementations. The power limitation described above can particularly become an issue in a microphone with extensive analog signal processing powered by a VDA interface. In situations where a class-B amplifier output stage is used, a maximum efficiency of only about 30% for sine wave signals is possible which typically requires high amounts of supply current.
According to one embodiment, a low power microphone circuit for a vehicle is provided that comprises: at least one microphone transducer; a digital signal processor for receiving output signals from the at least one microphone transducer and for generating a digitally processed audio signal; an output amplifier for amplifying the audio signal from the digital signal processor and modulating an input voltage with the audio signal; and a DC power supply for supplying power to the digital signal processor, wherein the output amplifier and the DC power supply are electrically coupled in series.
According to another embodiment, a low power microphone circuit for a vehicle is provided that comprises: at least one microphone transducer; a digital signal processor for receiving output signals from the at least one microphone transducer and for generating a digitally processed audio signal; a terminal for connection to a vehicle power source providing an input voltage and an input current; an output amplifier for amplifying the audio signal from the digital signal processor and modulating the input voltage with the audio signal; and a DC power supply for supplying power to the digital signal processor, wherein the DC power supply and the output amplifier are powered by the input current, and wherein the input current is no greater than about 6 mA.
According to another embodiment, a method is provided for providing power to a microphone circuit having a digital signal processor, and output amplifier, and a DC power supply, when a power source from which power is to be provided has an input current is no greater than about 6 mA. The method comprises: electrically connecting the output amplifier and the DC power supply in series such that the input current passes through both the output amplifier and the DC power supply; and providing power from the DC power supply to the digital signal processor.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.
In the drawings:
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. In the drawings, the depicted structural elements are not to scale and certain components are enlarged relative to the other components for purposes of emphasis and understanding.
DC power supply 100 and output amplifier 50 are powered by the input current Iin. According to some embodiments described herein, the input current Iin made available from the vehicle is no greater than about 6 mA, and possibly no greater than about 4.7 mA. To address the problems with the conventional microphone circuits discussed above, in some of the embodiments, output amplifier 50 and the DC power supply 100 are electrically coupled in series between input terminal 35 and ground so as to not split the input current Iin between these two components. As shown, output amplifier 50 is coupled between terminal 35 and DC power supply 100, and DC power supply 100 is coupled between output amplifier 50 and ground. The inventors discovered that when output amplifier 50 and the DC power supply 100 are coupled in series, the voltage VDD supplied to DSP 30 from CD power supply 100 is sufficiently high for operation. In particular, if VDD is about 1.5 V nominal, it is sufficient to power DSP 30. In this way, both output amplifier 50 and DC power supply receive the full input current Iin of, for example, about 6 mA or less. Despite the low power supplied, the microphone circuit 10 provides more gain in the output stage in order to drive a higher output.
Low power microphone circuit 10 may further include a short circuit protection circuit 150 for protecting the low power microphone circuit from short circuits, and an electromagnetic interference (EMI) filter 160 for filtering out any EMI present on the power supply line at terminal 35. In addition, Low power microphone circuit 10 may further include a thermal compensation circuit 170 for compensating for temperature-dependent voltage variations. Examples of these circuits are described in detail below with reference to
As shown in
DSP 30 may have a digital-to-analog converter (DAC) at one of its output ports, which outputs a digitally processed audio signal based upon processing of the signals from the microphones. This audio signal is output to error amplifier stage 52 of output amplifier 50. DSP 30 may optionally monitor DC voltage level VDD in a software feedback and then perform an active trim on a DC bias if there is variation in DC voltage level VDD. In this regard, general purpose I/O resistors in parallel could be used to produce the variable DC bias for a course trim or an output port of DSP 30 could be tri-stated to control the DC bias.
Error amplifier stage 52 includes a transistor 60 whose base is coupled to the DAC output of DSP 30 via serially connected first capacitor 54 and first resistor 56. The collector of transistor 60 is coupled to the Vin input rail from terminal 35 via a second resistor 62. The emitter of transistor 60 is coupled to ground via a third resistor 64. A fourth resistor 58 is coupled between the base of transistor 60 and the upper rail from connector 35. A second capacitor 66 may be coupled between the base and collector of transistor 60 for additional protection against electromagnetic currents. In this error amplifier stage 52, the gain of the amplifier is equal to the resistance of fourth resistor 58 divided by the resistance of first resistor 56. For purposes of example only, first capacitor 54 may have a capacitance of 0.1 μF, first resistor 56 may have a resistance of 16.5 kΩ, second resistor 62 has a resistance of 10 kΩ, third resistor 64 has a resistance of 220Ω, fourth resistor 58 has a resistance of 51.1 kΩ, and second capacitor 66 has a capacitance of 330 pF.
Output amplifier stage 70 includes a transistor 72, a resistor 74, and a resistor 76. The collector of transistor 60 of error amplifier stage 52 is coupled to the base of transistor 72 via resistor 74. Resistor 74 may, for example, have a resistance of 470Ω, and resistor 76 may, for example, have a resistance of 12Ω. The collector of transistor 72 is coupled to the Vin power rail from terminal 35 via resistor 76, while the emitter of transistor 72 is coupled to DC power supply 100 so as to provide the aforementioned serial connection between the output amplifier 50 and DC power supply 100.
DC power supply 100 is shown in this particular embodiment as being a shunt regulator. DC power supply 100 may thus include a low-voltage adjustable shunt regulator such as part No. TLV431 available from Texas Instruments of Dallas, Tex., which provides a thermally stable reference voltage of 1.5 V, for example, which serves as voltage VDD. Shunt regulator 102 is preferably connected between the emitter of transistor 72 and ground. Coupled in parallel between the emitter of transistor 72 and ground is a pair of serially connected resistors 104 and 106, a first capacitor 108, and a second capacitor 110. These components may, for example, have values as follows: resistor 104 may have a resistance of 24.9 kΩ, resistor 106 may have a resistance of 100 kΩ, capacitor 108 may have a capacitance of 0.1 μF, and capacitor 110 may have a capacitance of 47 μF. Because the voltage of the shunt regulator 102 is adjustable, resistors 104 and 106 provide a voltage divider such that a terminal between the resistors is coupled to the input of shunt regulator 102 that adjusts its output voltage.
Microphone circuit 10 may further include short circuit protection circuitry 150, which in the example shown in
EMI filter 160 may include a first capacitor 162 and a second capacitor 164, both coupled in parallel between the power rail Vin from terminal 35 and ground. In addition, ferrite beads 166 and 168 may be provided at both inputs to terminal 35. For purposes of example only, capacitor 162 may have a capacitance of 0.01 μF and capacitor 164 may have a capacitance of 270 pF.
A temperature compensation circuit 170 may be provided to compensate for variances of the voltage Vbe between the base and emitter of transistor 60. In the example shown, a thermistor 172 is provided with a resistive divider including resistors 174 and 176. In the resistive divider, resistor 174 is coupled between the base of transistor 60 and resistor 176 whereas resistor 176 is coupled between resistor 174 and ground. Thermistor 172 is coupled at one end between resistors 174 and 176 and at the other end to ground. Temperature compensation circuit 170 thus provides a bias source that is a function of temperature. In the example provided, thermistor 172 may have a resistance of 10 kΩ and have a negative temperature coefficient. Resistors 174 and 176 may have resistance of 4.99 kΩ.
The microphone circuit may further include an electrostatic discharge (ESD) protection diode 178 to protect the microphone circuit components from ESD. A suitable ESD protection diode is part number PESD1CAN available from NXP B.V. of Eindhoven, The Netherlands.
As apparent from the circuits described above, a method is provided for providing power to a microphone circuit having a DSP, and output amplifier, and a DC power supply, when a power source from which power is to be provided has an input current is no greater than about 6 mA. The method comprises: electrically connecting the output amplifier and the DC power supply in series such that the input current passes through both the output amplifier and the DC power supply; and providing power from the DC power supply to the DSP. The power supplied from the DC power supply to the DSP may be at a voltage of about 1.5 V.
Third and fourth microphones 203 and 204 may be positioned to pick up speech signals from the passenger side of the vehicle and thus DSP 30 may separately digitally process these signals to produce a passenger side second audio signal that is output to a second output amplifier 50p. Second output amplifier 50p amplifies the second audio signal by modulating the second input voltage. Again, output amplifier 50p may be configured as disclosed above with respect to
In
The above microphone circuits may be used with the autobias microphone system for use with multiple loads as described in commonly-assigned U.S. Pat. No. 8,243,956, the entire disclosure of which is incorporated herein by reference.
In VDA microphone systems, a very significant source of power loss can be the voltage regulator input circuitry. The supply and voltage regulator typically utilize a power supply capacitance that is AC isolated from the VDA output signal which appears or is impressed across the microphone. However, the power supply provides a DC path to provide power to the microphone while providing AC isolation. Although an inductor can provide this function, it typically would be a very large physical size and be very costly due to the large inductance required to accomplish this function. Although a resistor is small and an inexpensive solution, a resistor will incur significant power loss since it will appear as an AC load in parallel with the 680Ω VDA load.
Thus, the circuits as described in
The AC current regulator can be combined with a Class-B, Class-D or other type output stage. Additionally, the output stage can be implemented with complementary (balanced) outputs. A balanced output stage can double the output swing for a given shunt regulator voltage and has EMI and distortion advantages for Class-B and Class-D output stages due to even harmonic cancellation. Alternatively, a Class-A output stage in series with a shunt regulator can also be used. In this case the low impedance power supply does not need to be isolated as it is in series with the Class-A output stage. The bias current of the Class-A stage is delivered to the shunt regulator and its parallel load and is therefore not wasted. Capacitance in parallel with the shunt regulator is added to supply uninterrupted load current during signal peaks.
As noted above, in situations where a class-B amplifier output stage is used, a maximum efficiency of only about 30% for sine wave signals is possible which typically requires high amounts of supply current. However, other types of amplifiers like Class-D amplifiers can have substantially higher efficiencies of typically 80-90%. This can help to reduce the power overall requirement. Thus, by increasing the output stage efficiency, this can allow more power availability that can be used for a greater signal voltage swing or more power being available for digital signal processing and/or both.
For purposes of example only with respect to
The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the claims as interpreted according to the principles of patent law, including the doctrine of equivalents.
This application claims the priority benefit of U.S. Provisional Patent Application No. 61/595,359 entitled “POWER SUPPLY FOR USE IN A LOW POWER MICROPHONE OUTPUT STAGE,” filed on Feb. 6, 2012, by Robert R. Turnbull et al., the entire disclosure of which is incorporated herein by reference.
Number | Date | Country | |
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61595359 | Feb 2012 | US |