Class-D Amplifier Silent Start Using Dual Reference Ramps
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Solution Overview
Problem
Conventional Class-D amplifiers produce a noticeable 'pop' or artifact at the speaker load when power is turned on, which is unacceptable and existing solutions are often expensive or complex.
Innovation Solution
A Class-D amplifier circuit with a silent start functionality, utilizing an integrator and pre-amplifier circuit with ramp generators to gradually increase the reference voltage at startup, ensuring a smooth transition and avoiding the 'pop' noise by controlling the duty cycle of the pulse width modulated output signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional Class-D amplifier startup method is used, then amplifier can power on quickly, but 'pop' noise is generated at the speaker load
Solution Approach 1:
The patent applies preliminary action by generating a ramp signal before the normal operation signal to gradually increase the reference voltage at startup. This preliminary ramp signal prevents sudden voltage changes that would cause pop noise, while still enabling relatively quick startup. The ramp signal is generated in advance and controls the duty cycle modulation during the startup phase.
Solution Approach 2:
The patent changes the voltage parameter gradually through a ramp signal during startup instead of applying full voltage immediately. The reference voltage is varied over time according to the ramp signal, which controls the duty cycle of the PWM output. This parameter change approach eliminates the abrupt voltage transition that causes pop noise while maintaining acceptable startup speed.
2Object-generated harmful factors
If existing solutions to suppress pop noise are implemented, then pop noise is reduced, but circuit complexity and cost increase
Solution Approach 1:
The patent achieves multi-functionality by using the existing integrator circuit to generate both the normal operation signal and the startup ramp signal. The same integrator that functions during normal PWM generation is utilized during startup to create the gradual voltage ramp, eliminating the need for separate dedicated startup circuitry. This universal use of existing components suppresses pop noise without increasing circuit complexity or cost.
Solution Approach 2:
The circuit applies self-service by using its own internal integrator to generate the startup ramp signal without requiring external or additional dedicated startup components. The integrator naturally produces the required gradual voltage change during startup phase, and the system uses this self-generated signal to control the duty cycle modulation, thereby suppressing pop noise through its own existing resources.
Data Source
AI summary
A Class-D amplifier includes a pre-amplifier having an input configured to receive an amplifier reference voltage signal which is ramped at start-up at a fast rate. An integrator has a first input configured to receive an input signal from the pre-amplifier and a second input configured to receive an integrator reference voltage signal which is ramped at start-up at a slower rate. A modulator has an input coupled to an output of the integrator. The modulator generates a pulse width modulated output signal. Operation of the Class-D amplifier is controlled at start-up by applying a slow ramped signal as the integrator reference voltage signal and a fast ramped signal as the amplifier reference voltage so that the pulse width modulated output signal exhibits an increasing change in duty cycle in response to an increasing voltage of the integrator reference voltage signal, and no “pop” is introduced at start-up.