Charge pump assembly

Abstract

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.

Claims

1. Charge pump assembly, comprising a charge pump with an input port and an output port, a bias circuit electrically connected to the input port and provided for creating a bias voltage V.sub.bias, where the bias voltage V.sub.bias has a temperature dependence.

2. Charge pump assembly according to claim 1, where the bias voltage V.sub.bias has a piecewise linear temperature dependence.

3. Charge pump assembly according to claim 1, where the bias circuit comprises a temperature sensor.

4. Charge pump assembly according to claim 3, where the temperature sensor provides a Proportional To Absolute Temperature (PTAT) voltage.

5. Charge pump assembly according to claim 1, where the bias circuit comprises a first sub circuit providing a plurality of different temperature independent voltages.

6. Charge pump assembly according to claim 1, where the bias circuit comprises a second sub circuit providing a plurality of temperature dependent voltages, each temperature dependent voltage having a different temperature dependency.

7. Charge pump assembly according to claim 6, where the temperature dependent voltages have a linear temperature dependence.

8. Charge pump assembly according to claim 1, where the bias circuit comprises a plurality of selection circuits.

9. Charge pump assembly according to claim 8, where the selection circuits comprise comparators.

10. Charge pump assembly according to claim 1, where the bias circuit includes (i) a first sub circuit providing a plurality of different temperature independent voltages and (ii) a second sub circuit providing a plurality of temperature dependent voltages, each temperature dependent voltage having a different temperature dependency, and where the bias voltage is the sum of a plurality of voltages provided by the first sub circuit and the second sub circuit.

11. Charge pump assembly according to claim 1, further comprising a non-volatile memory element for storing linearity parameters.

12. Method for manufacturing a charge pump assembly according to claim 1, comprising the steps of: providing mechanical and electrical components of the charge pump assembly according to claim 1, electrically connecting the electrical components, determining a temperature dependent deterioration of a sensitivity of a MEMS microphone over a temperature range, dividing the temperature range into intervals and approximating a curve of the sensitivity into a piecewise linear curve, determining slopes of the piecewise linear sections, transforming the slopes into parameters a.sub.i, .sub.i; and storing the parameters in a memory element.

13. Charge pump assembly according to claim 2, where the bias circuit comprises a first sub circuit providing a plurality of different temperature independent voltages.

14. Charge pump assembly according to claim 2, where the bias circuit comprises a second sub circuit providing a plurality of temperature dependent voltages, each temperature dependent voltage having a different temperature dependency.

15. Charge pump assembly according to claim 2, where the bias circuit comprises a temperature sensor that provides a Proportional To Absolute Temperature (PTAT) voltage, where the bias circuit comprises a first sub circuit providing a plurality of different temperature independent voltages, and where the bias circuit comprises a second sub circuit providing a plurality of temperature dependent voltages, each temperature dependent voltage having a different temperature dependency.

Description

IN THE DRAWINGS

(1) FIG. 1 shows a basic embodiment of a charge pump assembly CPA comprising a charge pump CP and a bias circuit BC,

(2) FIG. 2 shows an exemplary piecewise linear bias voltage provided to the charge pump,

(3) FIG. 3 shows an exemplary temperature dependent sensitivity with negative slope with decreasing absolute value and a corresponding piecewise linear approximation,

(4) FIG. 4 shows an exemplary temperature dependent sensitivity with negative slope with increasing absolute value and a corresponding piecewise linear approximation,

(5) FIG. 5 shows a more detailed embodiment of a charge pump assembly including a first sub circuit SC1 and a second sub circuit SC2 in the bias circuit BC,

(6) FIG. 6 shows the bias voltage as a composition a plurality of temperature dependent and temperature independent voltages together with the respective threshold temperatures T.sub.i voltages and threshold voltages V.sub.i,

(7) FIG. 7 shows a basic embodiment of a MEMS microphone MM comprising the charge pump assembly CPA and a MEMS capacitor MCAP,

(8) FIG. 8 shows a more detailed embodiment of a MEMS microphone MM.

(9) FIG. 1 a 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 V.sub.bias controlling 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.

(10) The bias voltage V.sub.bias provided by the bias circuit BC by providing a plurality of individual voltages has a temperature dependence. The temperature dependence of the bias voltage V.sub.bias is 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.

(11) FIG. 2 shows a temperature-dependent bias voltage V.sub.bias applied 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.

(12) FIGS. 3 and 4 show different temperature-dependent sensitivity curves of exemplary MEMS microphones. The temperature-dependent sensitivity of FIG. 3 decreases with increasing temperature. The slope is negative and the absolute value of the slope is reduced with increasing temperature. Temperatures T.sub.1, T.sub.2, . . . , T.sub.N define 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.

(13) A designer of a charge pump assembly is free to choose the threshold temperatures T.sub.1, T.sub.2, . . . , T.sub.N to obtain an optimal approximation.

(14) In contrast to FIG. 3, the absolute value of the slope of the temperature-dependent sensitivity increases with increasing temperature.

(15) 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.

(16) FIG. 5 shows a more detailed embodiment of the charge pump assembly CPA where the bias circuit BC comprises a first subcircuit SC1 and a second sub-circuit SC2. The first sub-circuit SC1 may provide a plurality of temperature-independent voltages and the second sub-circuit SC2 may provide a plurality of temperature-dependent voltages. The first sub-circuit SC1 may have a total number of N voltage outputs, the second sub-circuit SC2 may 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 T.sub.I, each selection circuit forwards either the temperature-independent voltage from the first sub-circuit SC1 or the temperature-dependent voltage from the second sub-circuit SC2 to the respective voltage input at the charge pump CP.

(17) Thus, the total bias voltage V.sub.bias applied 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 (compare FIG. 6).

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

(19) Temperatures T.sub.1, T.sub.2, . . . define threshold voltages establishing the boundaries of the corresponding temperature intervals. Each curve of the voltages shown in the lower portion of FIG. 6 is 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.

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

(21) 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 a.sub.i and .sub.i that determine the slope and the voltage of that of the corresponding segments of the piece-wise linear temperature-dependent bias voltage as shown in FIG. 6.

(22) FIG. 7 shows 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.

(23) FIG. 8 shows 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.

(24) The MEMS microphone MM further comprises circuitry OTP to provide the microphone with the corresponding values for parameters a.sub.i and .sub.i, e.g. in a one-time programming step.

(25) 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

(26) AMP: amplifier BC: bias circuit CP: charge pump CPA: charge pump assembly IP: input port MCAP: MEMS capacitor MM: MEMS microphone OP: output port OP2: second output port OTP: one-time programming circuit S.sub.1, S.sub.2, . . . , S.sub.N: selection circuits SC1: first sub-circuit SC2: second sub-circuit T: temperature T.sub.1, 2, . . . N: threshold temperatures V.sub.1, 2, . . . , N: threshold voltage V.sub.bias: bias voltage V.sub.SUP: supply voltage