Biasing Circuit for MEMS Acoustic Transducer with Switched Start-Up
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Solution Overview
Problem
The existing biasing circuits for MEMS capacitive microphones suffer from long start-up times and sensitivity variations due to the use of high-resistance diodes in anti-parallel configuration, which are not acceptable for rapid device activation and power-down recovery in applications requiring constant performance.
Innovation Solution
The introduction of switches in parallel with the filter resistor and input resistor allows for direct low-impedance connections during the start-up step, enabling rapid voltage settling of the MEMS microphone terminals, followed by low-pass filtering, which significantly reduces start-up time and maintains constant sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If high-resistance diodes in anti-parallel configuration are used for filtering charge-pump noise, then noise attenuation is improved, but start-up time increases significantly
Solution Approach 1:
The patent applies the dynamics principle by making the circuit configuration changeable over time. During start-up, switches connect the MEMS microphone directly to the charge-pump output without filtering, enabling fast voltage settling. After start-up completion, the same circuit transitions to use high-resistance diodes for noise filtering during normal operation. This temporal dynamic reconfiguration resolves the contradiction between fast start-up and noise attenuation.
Solution Approach 2:
The patent segments the biasing circuit into distinct operational phases: a start-up phase with direct connection for rapid settling, and a normal operation phase with high-resistance diode filtering for noise attenuation. The segmentation is controlled by switches that are opened after start-up, effectively dividing the circuit's behavior into two separate functional modes that address different requirements at different times.
2Object-affected harmful factors
If high-resistance diodes are used for filtering, then noise reduction is improved, but sensitivity stability deteriorates during start-up
Solution Approach 1:
The circuit dynamically switches between two configurations: initially providing direct connection for fast settling and sensitivity stabilization, then transitioning to filtered connection for noise reduction. This dynamic behavior ensures that sensitivity stability is maintained during the critical start-up phase when the membrane needs to settle, while noise filtering is applied during stable operation.
Solution Approach 2:
The patent applies preliminary action by establishing the direct connection configuration before filtering is engaged. During the start-up phase, the circuit first allows rapid voltage settling without filtering, ensuring the MEMS microphone reaches its steady state quickly. Only after this preliminary settling phase does the filtering mechanism engage, preventing any negative impact on sensitivity stability.
3Speed
If direct connection to charge-pump output is used, then start-up speed is improved, but noise attenuation deteriorates
Solution Approach 1:
The circuit employs periodic action by sequentially applying different configurations: first the direct connection for fast settling, then the filtered connection for noise attenuation. The switches transition from closed to open state after a predetermined time or condition, creating a temporal sequence where direct connection is applied during the critical settling period, followed by filtered connection during normal operation.
Solution Approach 2:
The dynamic reconfiguration of the circuit allows it to adapt its impedance characteristics over time. During start-up, low impedance direct connection enables rapid voltage transfer and settling. After start-up, the circuit dynamically transitions to high impedance filtered connection that attenuates charge-pump noise. This dynamic adaptation resolves the contradiction between speed and noise attenuation.
4Reliability
If additional high-pass filter is added to mask settling time, then sensitivity stability is improved, but device complexity increases
Solution Approach 1:
The patent extracts and removes the unnecessary high-pass filter component from the circuit. Instead of adding complexity with a high-pass filter to mask settling time, the invention achieves sensitivity stability through the simpler mechanism of temporarily connecting the MEMS microphone directly to the charge-pump output during start-up, then opening switches to engage filtering. This extraction of the high-pass filter simplifies the circuit while maintaining sensitivity stability.
Solution Approach 2:
The biasing circuit provides self-service by using its own charge-pump output connection to achieve fast settling and sensitivity stabilization during start-up, eliminating the need for external high-pass filtering. The circuit's inherent switching mechanism and direct connection capability serve the dual purpose of rapid voltage settling and sensitivity stabilization without requiring additional filtering components.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution achieves extremely fast settling times and stable biasing, eliminating the need for additional high-pass filtering and maintaining constant sensitivity, while being integratable with common analog technologies.
Implementation Method 1
a low-pass filter 12, in RC configuration, is set between the output of the charge-pump stage 8 and the first terminal N1 of the MEMS microphone 1 so as to appropriately attenuate both the ripple and the noise at output from the charge-pump stage
Implementation Method 2
the use of non-linear devices capable of providing the high values of resistance has been proposed. For example, for this purpose it has been proposed the use of a pair of diodes in anti-parallel configuration, providing a sufficiently high resistance when a voltage drop of contained value is set across them
Data Source
AI summary
A biasing circuit for an acoustic transducer is provided with: a voltage-booster stage, which supplies, on a biasing terminal, a boosted voltage for biasing a first terminal of the acoustic transducer; and filtering elements, set between the biasing terminal and the acoustic transducer, for filtering disturbances on the boosted voltage. The biasing circuit is further provided with switches, which can be actuated so as to connect the first terminal to the biasing terminal of the voltage-booster stage, directly during a start-up step of the biasing circuit, and through the filtering elements at the end of the start-up step.