Battery-Tracked Common-Mode Control for Class-D Loop Amplifiers
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Solution Overview
Problem
Existing Class-D amplifiers face challenges in maintaining a constant common-mode value while achieving good Power Supply Rejection Ratio (PSRR) without increasing silicon area, as prior solutions either require large RC filters or external components, and are prone to intermodulation distortion and noise due to analog ramp requirements.
Innovation Solution
A closed-loop amplifier system with a digital voltage indicator and common-mode voltage supply is used to maintain a constant common-mode value, powered directly by a battery, which includes two integrator stages and a comparator stage to control the common-mode voltage, reducing the need for large silicon area and minimizing noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If large RC filters are used to maintain constant common-mode value and improve PSRR, then Power Supply Rejection Ratio is improved, but silicon area increases significantly
Solution Approach 1:
The patent changes the parameter of common-mode voltage control from passive RC filtering to active digital control. A digital voltage indicator monitors battery voltage and a common-mode voltage supply adjusts the common-mode level dynamically, replacing large passive components with active circuitry that achieves the same PSRR performance without the area penalty of large RC filters.
2Ease of operation
If analog ramp is used at comparator input to generate PWM output, then PWM generation is achieved, but intermodulation distortion increases due to carrier modulating with PWM input
Solution Approach 1:
The patent replaces the mechanical/analog ramp generation approach with a digital control system. Instead of using an analog ramp that physically modulates with the carrier, a digital voltage indicator and common-mode voltage supply generate control signals that achieve PWM generation without the intermodulation distortion inherent in analog ramp-based systems.
3Ease of operation
If square wave is used to create ramp in integrator, then ramp generation is achieved, but intermodulation distortion persists due to square wave modulating with input PWM carrier
Solution Approach 1:
The patent substitutes the square-wave-based ramp generation with a digital control approach. The digital voltage indicator and common-mode voltage supply eliminate the need for square waves that modulate with the carrier, thereby removing the source of intermodulation distortion while maintaining the necessary ramp function for PWM generation.
4Reliability
If extra pin or large RC filter is added to create sub-Hertz pole for common-mode control, then Power Supply Rejection Ratio is improved, but device complexity and silicon area increase
Solution Approach 1:
The patent makes the common-mode voltage supply multi-functional, serving both as a control element for maintaining constant common-mode value and as a means to achieve PSRR improvement. This integrated approach eliminates the need for separate extra pins or large RC filters, reducing device complexity while maintaining the necessary sub-Hertz pole function for power supply rejection.
Data Source
AI summary
A closed loop amplifier adapted to be directly connected to a battery having a battery voltage for powering the amplifier. The amplifier includes an amplifier stage having a node for receiving a control voltage for controlling a common mode voltage of the stage, a digital voltage indicator for generating a digital value corresponding to the battery voltage, and a common mode voltage supply providing the control voltage corresponding to the digital value.