MEMS Microphone Bias Voltage Stabilization via Feedback Control
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Solution Overview
Problem
High input impedance devices, such as capacitive transducers, face variability in sensitivity due to manufacturing process variations, leading to inconsistent output signals, and existing voltage generators lack stability and efficiency in providing a consistent bias voltage.
Innovation Solution
A voltage supply apparatus with a first voltage generator that produces a stable second voltage, controlled by a calibration value, ensuring a predictable output voltage level, and a control circuit that adjusts the input voltage to maintain stability across normal operating conditions without loading the output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Dickson charge pump is used to generate bias voltage, then the transducer can be biased, but the output voltage is variable with manufacturing tolerances, temperature and load current, leading to unstable sensitivity
Solution Approach 1:
The patent implements a feedback mechanism where the output voltage of the Dickson charge pump is monitored and fed back to a control circuit. This control circuit adjusts the input voltage to the charge pump based on the monitored output voltage and a calibration value, thereby stabilizing the output voltage despite variations in manufacturing tolerances, temperature, and load current.
Solution Approach 2:
The patent changes the operating parameters of the Dickson charge pump by dynamically adjusting its input voltage based on feedback. Additionally, a calibration value is introduced that can be programmed to fine-tune the bias voltage, allowing the system to compensate for manufacturing variations and environmental conditions.
2Reliability
If a closed-loop voltage generator is used to stabilize output voltage, then bias voltage stability is improved, but control circuitry increases load on the output, adding components and power consumption
Solution Approach 1:
The patent introduces an intermediary approach where instead of directly controlling the charge pump output with a heavy closed-loop controller, a calibration value is used as an intermediary parameter. This calibration value pre-compensates for known variations, reducing the burden on the feedback control circuit and minimizing additional power consumption while still achieving voltage stability.
3Ease of manufacture
If manufacturing process variations occur, then production cost is reduced through wafer-scale packaging, but sensitivity variation increases, requiring calibration
Solution Approach 1:
The patent performs preliminary calibration during the manufacturing process, specifically during final stage manufacturing test. A fixed physical stimulus is applied and the bias voltage is adjusted until the required output amplitude is observed. The calibration value is determined and stored in memory, pre-compensating for sensitivity variations before the devices are packaged and shipped.
Solution Approach 2:
The patent replaces mechanical adjustment methods with an electronic calibration approach. Instead of physically adjusting components to compensate for sensitivity variations, the system uses programmable calibration values stored in memory that electronically adjust the bias voltage to achieve the desired sensitivity.
4Manufacturing precision
If the bias voltage is adjusted to calibrate sensitivity, then sensitivity consistency is improved, but the voltage generator output must be controllable, increasing complexity
Solution Approach 1:
The patent segments the voltage control function into two parts: a fixed Dickson charge pump structure that provides the voltage multiplication, and a separate controllable input voltage stage that can be adjusted via the calibration value. This segmentation allows the majority of the voltage generator to remain simple and fixed, while only a small portion requires controllable adjustment.
Data Source
AI summary
An apparatus comprising a capacitive transducer, for example a MEMS microphone. A first voltage generator is connected to receive a first voltage (VDD*) and generate a second voltage (VCP) for biasing the capacitive transducer. A control circuit is adapted to, in use, control the first voltage (VDD*) based on a calibration value, wherein a different calibration value would lead to a different first voltage level and the calibration value is set such that an input signal of known amplitude produces an output signal of predetermined amplitude.