Soft transitions between muted and unmuted states in class D audio amplifiers
First Claim
1. An audio amplifier, comprising:
- difference circuitry, for generating a difference signal responsive to an input signal and a feedback signal;
a first comparator, for generating first pulse-width-modulated output pulses responsive to a comparison of the difference signal and a periodic reference waveform;
a second comparator, for generating second pulse-width-modulated output pulses responsive to a comparison of the difference signal and a periodic reference waveform, the second pulse-width-modulated output pulses complementary to the first pulse-width-modulated output pulses;
a power stage, for driving an amplifier output responsive to the first and second pulse-width-modulated output pulses;
a loop filter for filtering a signal corresponding to the amplifier output to produce the feedback signal; and
common mode ramp voltage circuitry, having an output coupled to inputs of the first and second comparators, for generating a common mode voltage of a first level in a muted mode, and for ramping the common mode voltage from the first level to a zero level responsive to a transition from the muted mode to an unmuted mode.
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Abstract
A class AD audio amplifier system (10) with reduced noise capability in muting and unmuting events is disclosed. The amplifier system (10) includes multiple audio channels (20), each of which can be constructed to include a pulse-width-modulator (PWM) (24). The PWM modulator (24) includes a pair of comparators (39A, 39B; 52+, 52−) that generate complementary PWM output signals based upon the comparison between a filtered difference signal and a reference waveform. When the system is muted, a common mode voltage (CM_RAMP) is applied to the inputs of the comparators (39A, 39B; 52+ 52−) to suppress the duty cycle at the amplifier output, preferably to a zero duty cycle. In the transition from a muted state to an unmuted state, the common mode voltage (CM_RAMP) is ramped from the suppressing voltage to zero common mode voltage, permitting the duty cycle of the complementary PWM signals to gradually increase, thus reducing clicks and pops. The converse operation is performed in the transition from unmuted to muted. Pulse-width-modulation control logic (26) is also included to ensure that that the PWM “on” and “off” pulses are of at least a minimum duration, and also to generate compensating pulses on the complementary PWM line at low duty cycles.
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Citations
31 Claims
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1. An audio amplifier, comprising:
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difference circuitry, for generating a difference signal responsive to an input signal and a feedback signal;
a first comparator, for generating first pulse-width-modulated output pulses responsive to a comparison of the difference signal and a periodic reference waveform;
a second comparator, for generating second pulse-width-modulated output pulses responsive to a comparison of the difference signal and a periodic reference waveform, the second pulse-width-modulated output pulses complementary to the first pulse-width-modulated output pulses;
a power stage, for driving an amplifier output responsive to the first and second pulse-width-modulated output pulses;
a loop filter for filtering a signal corresponding to the amplifier output to produce the feedback signal; and
common mode ramp voltage circuitry, having an output coupled to inputs of the first and second comparators, for generating a common mode voltage of a first level in a muted mode, and for ramping the common mode voltage from the first level to a zero level responsive to a transition from the muted mode to an unmuted mode. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21)
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22. A method of generating pulse-width-modulated signals to be applied to a bridged load, comprising the steps of:
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receiving an analog input signal;
filtering a feedback signal corresponding to the pulse-width-modulated signals with a loop filter;
combining the analog input signal with the filtered feedback signal to generate a difference signal;
combining a periodic reference waveform with the difference signal;
generating a first pulse-width-modulated signal responsive to the combined signal;
generating a second pulse-width-modulated signal, complementary to the first pulse-width-modulated signal, responsive to the combined signal; and
responsive to a muting control signal, applying a common mode voltage to the combined signal ramping from zero common mode voltage to a first level, so that the generating steps generate the first and second pulse-width-modulated signals of decreasing duty cycle. - View Dependent Claims (23, 24, 25, 26, 27, 28)
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29. Pulse-width-modulation control logic, comprising:
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a first one-shot multivibrator, having an input coupled to receive a first pulse-width-modulated input pulse, for generating a pulse at its output of a first selected duration responsive to a transition of the first pulse-width-modulated input pulse;
a first OR function having a first input receiving the first pulse-width-modulated input pulse and having a second input receiving the output of the first one-shot multivibrator, and having an output coupled to a first output of the pulse-width-modulation logic;
a second one-shot multivibrator, having an input coupled to receive a second pulse-width-modulated input pulse, for generating a pulse at its output of the first selected duration responsive to a transition of the second pulse-width-modulated input pulse; and
a second OR function having a first input receiving the second pulse-width-modulated input pulse and having a second input receiving the output of the second one-shot multivibrator, and having an output coupled to a second output of the pulse-width-modulation control logic. - View Dependent Claims (30, 31)
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Specification