Charge pump assembly

A charge pump assembly allowing MEMS microphones being temperature-compensated in a large temperature range and corresponding microphones are provided. An assembly includes a charge pump and a bias circuit electrically connected to the charge pump. A bias voltage provided by the bias circuit has a temperature dependence.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage of International Application No. PCT/EP2015/056249, filed Mar. 24, 2015, which is incorporated herein by reference in its entirety.

The present invention refers to charge pumps, e.g. charge pumps for MEMS microphones (MEMS=microelectromechanical system) having an improved sensitivity, especially over a large operation temperature range.

A MEMS microphone comprises mechanical components such as a MEMS capacitor with a variable capacitance and the microphones housing elements. Further, the microphone has electric or electronic components such as an ASIC (application specific integrated circuit) for charging the capacitor and evaluating the capacitor's capacitance.

It can be observed that the sensitivity of a MEMS microphone depends on the microphones operation temperature. In particular, the sensitivity of a MEMS microphone decreases with increasing temperature.

The contribution of a MEMS microphone's electrical and mechanical components to the temperature induced decrease in sensitivity is complex as the housing elements, the capacitor and the ASIC show temperature dependent properties.

However, a microphone with good electrical and acoustical properties is generally wanted and needed for modern devices. Thus, what is needed is a MEMS microphone with a constant high level of sensitivity independent from the operation temperature.

From WO 2011/001195 A1 temperature compensated microphones with temperature dependent amplifier gains are known.

From U.S. Pat. No. 7,889,030 MEMS microphones with reduced temperature coefficients of a chip comprising the capacitor are known.

From US 2011/0200212 microphones with temperature compensated charge pumps are known.

Although known microphones with improved components exist MEMS microphones with a constant sensitivity level over a large temperature region are still needed.

Thus, it is an object of the invention to provide a charge pump assembly that allows MEMS microphones to have a constant and high level of sensitivity over a large temperature range.

The charge pump assembly comprises a charge pump with an input port and an output port. The assembly further comprises a bias circuit. The bias circuit is electrically connected to the input port of the charge pump and provided for creating a bias voltage. The bias voltage has a temperature dependence.

The charge pump may be used to charge a MEMS capacitor of a MEMS microphone.

The bias voltage is a control voltage of the charge pump. A higher bias voltage applied to the input port of the charge pump may result in a larger amount of electrical charge stored in a microphone's capacitor.

As stated above, the sensitivity of a MEMS microphone depends at least partially on the temperature of the capacitor. It was found that by transferring a temperature dependent amount of charge to the MEMS capacitor not only the capacitor's contribution to the temperature induced deterioration can be reduced. Even the temperature induced deterioration of the microphone as a whole can be reduced or even eliminated by a properly designed charge pump assembly. Thus, a temperature compensated MEMS microphone working with a high level of temperature independent sensitivity can be constructed.

As stated above each of the different components of a MEMS microphone contributes to the microphone's temperature induced deterioration of sensitivity. Although an increase in voltage applied to the microphones capacitor generally increases the capacitor's—and thus the microphone's—sensitivity, the exact voltage for a temperature compensation over a large temperature range is difficult to determine and still more difficult to apply with conventional charge pump assemblies.

To obtain a temperature compensation that fulfills even strict requirements with a simple to implement bias circuit it is possible that the bias voltage applied to the charge pump has a piecewise linear temperature dependence. That is for a plurality of temperature intervals the slope of the temperature dependent bias voltage is constant. A higher number of temperature intervals results in a better temperature compensation. In the context of the present invention a circuit topology for the charge pump assembly was found that does not lead to a higher circuit complexity if a high number of temperature intervals are chosen to obtain an excellent temperature compensation.

It is possible that the charge pump assembly comprises a temperature sensor. The temperature sensor may provide a PTAT voltage (PTAT=proportional to absolute temperature). That is that the temperature sensor may provide a voltage that is proportional to the absolute temperature in Kelvin with k being a constant of proportionality:

It is possible that the bias circuit comprises a first sub circuit providing a plurality of different temperature independent voltages.

It is further possible that the bias circuit comprises a second sub circuit providing a plurality of temperature dependent voltages, each temperature dependent voltage having a different temperature dependency.

The first sub circuit may also comprise a plurality of voltage outputs. The number of voltage outputs of the first sub circuit may equal the number of voltage outputs of the second sub circuit and the number of voltage inputs of the charge pump. Each output the first sub circuit provides a voltage: ai+αi*Tiwith Tibeing a predetermined temperature—or a physical quantity proportional to temperature—, i being the index number of the voltage output and aiand αibeing constant numbers. ai+αi*Tiis a temperature independent voltage.

The second sub circuit may comprise a plurality of voltage outputs. Each output the first sub circuit provides a voltage: ai+αi*Tiwith being the temperature—or a physical quantity proportional to the temperature—, i being the index number of the voltage output and aiand αibeing constant numbers. αidetermines the slope of the temperature dependence of the respective output voltage.

The different threshold voltages Tifor i=1, 2, . . . , N determine the boundaries of adjacent temperature intervals.

It is possible that the charge assembly comprises a plurality of selection circuits such as comparators. The number of selection circuits may equal the number of voltage outputs of the first sub circuit.

Each selection circuit is connected with one of the voltage outputs of the first sub section, of the second sub section and with one of the inputs of the charge pump. Each selection circuit may be further connected to the temperature sensor to receive a temperature signal.

Then it is possible that each selection circuit provides the voltage:

Vi={αi+αi*T⁢:T<Tiαi+αi*Ti⁢:T≥Ti(2)
to the corresponding charge pump's input.

I.e. a temperature dependent voltage if the actual operating temperature is below the threshold temperature Tiand a constant temperature if the actual operating temperature equals or exceeds the threshold temperature Ti.

Thus, the charge pump is provided with a plurality of individual voltages. The number of temperature dependent partial voltages decreases with increasing temperature.

The charge pump may add the plurality of single voltages received at the inputs and transfer a charge corresponding to the sum
VBias=V1+V2+ . . . +VN(3)
to a MEMS capacitor.

The voltage applied to the MEMS capacitor may be reduced or increased by an additional offset voltage created by a section of an ASIC
VMEMS=VBias−V0(4)

The number of circuit elements of the charge pump assembly scales linearly with the number of temperature intervals without increasing the circuit's complexity.

It is possible that the charge pump assembly further comprises a non-volatile memory element for storing linearity parameters.

The parameters aiand αican be chosen to counteract the temperature dependence of the sensitivity of the MEMS microphone as a whole. That is a designer of a microphone does not need to study the temperature behavior of the single components and the complex interaction. Instead, the designer needs only to measure the temperature dependent deterioration of the whole component, chose the values for aiand αiand store them in the memory element.

It was found that obtaining the optimal values for aiand αiis straightforward as the temperature dependent sensitivity can be divided into the same temperature intervals, approximated by a piecewise linear curve and as the slope of the piecewise linear approximation of the sensitivity unambiguously corresponds to aiand αibecause the sensitivity increases linearly with the capacitor's voltage.

Accordingly the charge pump assembly can be integrated in and part of a MEMS microphone. Then, the microphone further comprises the MEMS capacitor to be charged by the charge pump assembly. The capacitor has a variable capacitance. Received sound signals result in a variation on capacitance. By evaluating the varying capacitance, e.g. by an ASIC, the sound signal can be transformed into an electrical signal.

A method for manufacturing a charge pump assembly or a microphone comprising the charge pump assembly comprises the steps ofproviding the mechanical and electrical components,electrically connecting the electrical components,determining the temperature dependent deterioration of the sensitivity,dividing the temperature range into intervals and approximating the sensitivity curve into a piecewise linear curve,determining the slopes of the piecewise linear sections,transforming the slopes into parameters ai, αi.

The working principles and not limiting embodiments are described in the accompanying drawings.

FIG. 1a basic embodiment of a charge pump assembly CPA. The charge pump assembly CPA comprises a charge pump CP and a bias circuit BC. The charge pump CP comprises an input port IP with a plurality of signal connections. The plurality of signal connections provides a plurality of single voltage signals, the sum of which may be a bias voltage Vbiascontrolling the charge pump CP. The charge pump CP further has an output port OP via which electric charge can be transferred, e.g. to a capacitor.

The bias voltage Vbiasprovided by the bias circuit BC by providing a plurality of individual voltages has a temperature dependence. The temperature dependence of the bias voltage Vbiasis chosen such that the temperature dependence of the electrical charge provided at the output port OP by the charge pump CP counteracts a temperature-induced deterioration of the external circuit environment of the charge pump assembly CPA.

FIG. 2shows a temperature-dependent bias voltage Vbiasapplied from the bias circuit BC to the charge pump CP. In order to keep the complexity of the charge pump assembly's circuit components at a minimum, the bias voltage has piece-wise linear segments approximating the ideal bias voltage in great detail. The number of linear segments of the bias voltage determines the degree of approximation: a higher number of segments results in a better approximation.

FIGS. 3 and 4show different temperature-dependent sensitivity curves of exemplary MEMS microphones. The temperature-dependent sensitivity ofFIG. 3decreases with increasing temperature. The slope is negative and the absolute value of the slope is reduced with increasing temperature. Temperatures T1, T2, . . . , TNdefine threshold temperatures of adjacent temperature intervals. Within each temperature interval, the temperature-dependent sensitivity is approximated by a linear function. Accordingly, the absolute values of the approximating functions decrease with increasing temperature.

A designer of a charge pump assembly is free to choose the threshold temperatures T1, T2, . . . , TNto obtain an optimal approximation.

In contrast toFIG. 3, the absolute value of the slope of the temperature-dependent sensitivity increases with increasing temperature.

As the designer is free to choose the threshold temperatures individually, he can divide temperature ranges with high absolute values of the sensitivity slope into a high number of intervals. In temperature ranges in which the sensitivity deterioration with temperature is not that much pronounced, a lower number of temperature intervals may be sufficient.

FIG. 5shows a more detailed embodiment of the charge pump assembly CPA where the bias circuit BC comprises a first subcircuit SC1and a second sub-circuit SC2. The first sub-circuit SC1may provide a plurality of temperature-independent voltages and the second sub-circuit SC2may provide a plurality of temperature-dependent voltages. The first sub-circuit SC1may have a total number of N voltage outputs, the second sub-circuit SC2may have a total number of N voltage outputs and the charge pump CP may have a total number of N voltage inputs at its input port. Further, the bias circuit BC comprises a total number of N selection circuits S1, S2, S . . . , SN, e.g. comparators, where each selection circuit is connected to one of the voltage outputs of the first sub-circuit SC1, to one voltage output of the second sub-circuit SC2, and to one voltage input of the charge pump CP. Depending on the actual operating temperature and the respective threshold temperatures TI, each selection circuit forwards either the temperature-independent voltage from the first sub-circuit SC1or the temperature-dependent voltage from the second sub-circuit SC2to the respective voltage input at the charge pump CP.

Thus, the total bias voltage Vbiasapplied to the charge pump is the sum of a plurality of temperature-dependent voltages and temperature-independent voltages from the first and the second sub-circuit, respectively. As a result, the total bias voltage applied to the charge-pump CP is a piece-wise linear voltage with a constant slope within each temperature interval (compareFIG. 6).

FIG. 6illustrates the composition of the total bias voltage Vbias(shown in the upper portion ofFIG. 6) being composed of a plurality of individual voltages as shown in the lower portion ofFIG. 6.

Temperatures T1, T2, . . . define threshold voltages establishing the boundaries of the corresponding temperature intervals. Each curve of the voltages shown in the lower portion ofFIG. 6is the result of the selectivity of a selection circuit selecting either a temperature-dependent voltage if the temperature is below a certain threshold voltage or selecting a constant voltage if the actual temperature exceeds the corresponding threshold temperature.

Thus, the number of circuit elements scales with the number of temperature intervals without increasing the complexity of the charge pump assembly circuitry.

As the sensitivity of a MEMS microphone is mainly proportional to the voltage applied to the corresponding MEMS capacitor and as the temperature-dependent sensitivity of the microphone can be easily and with high precision approximated by a piece-wise linear curve, it is easy to determine the co-efficients aiand αithat determine the slope and the voltage of that of the corresponding segments of the piece-wise linear temperature-dependent bias voltage as shown inFIG. 6.

FIG. 7shows a possible implementation of the charge pump assembly CPA in a MEMS microphone MM. Apart from the charge pump assembly CAP, the MEMS microphone MM comprises an MEMS capacitor MCAP with a variable capacitance and the essential connections, e.g. a voltage supply connection VSUP and a second output port OP2. Via the voltage supply port VSUP, the charge pump assembly CPA may be provided with electrical power. Via the second output port OP2, the variance in capacitance of the MEMS capacitor MCAP may be analyzed to transform an acoustic signal into an electrical signal.

FIG. 8shows a more detailed version of the MEMS microphone MM where between the MEMS capacitor MCAP and the second output port OP2, a further amplifier circuit AMP is arranged. The further amplifier circuit AMP may be a pre-amplifier or a main amplifier.

The MEMS microphone MM further comprises circuitry OTP to provide the microphone with the corresponding values for parameters aiand αi, e.g. in a one-time programming step.

Neither the charge pump assembly nor an MEMS microphone comprising such a charge pump assembly are limited to the embodiments described below or shown in the figures. Charge pump assemblies with further circuit components or microphones with further circuit components or mechanical components are also comprised by the present invention.

LIST OF REFERENCE SIGNS