Abstract: A Class D amplifier includes a tracking circuit and a regulation circuit to provide efficient, accurate protection against excessive current flow in the power FET's of the amplifier output stage. The FET currents flowing in the output stage are mirrored across tracking resistors by the tracking circuit. The resultant voltage drops are compared to reference voltages to detect over-current situations. By mirroring scaled-down versions of the output currents, power consumption in the tracking circuit is minimized. When an over-current situation is detected, the regulation circuit modulates the duty cycle of the signal input to the output stage until the FET currents fall to acceptable levels. If two over-current indications occur during a single clocking cycle of the pulse-width modulator of the amplifier, the regulation circuit shuts off the amplifier to protect against short-circuit damage.

Abstract: A circuit for generating an accurate reproduction of the output of a Class D amplifier for error-correction purposes includes a reference signal amplifier circuit. The reference signal amplifier circuit includes an op-amp configured as a differential amplifier that combines the digital outputs of the internal output stage of the amplifier into a single reference signal. By reducing the bandwidth of the op-amp, switching noise is removed from the digital signals, without adding any phase shift to the generated reference signal. In addition, by using the digital signals directly from the output stage rather than the filtered analog outputs of the amplifier, distortion due to external speaker-induced back-EMF is prevented. The reference signal can then be compared to the original signal input to the amplifier for error-correcting purposes.

Abstract: A pulse-width modulation (PWM) circuit in a Class D audio amplifier includes output-limiting logic and an automatic gain control (AGC) circuit. When an out-of-range, or overmodulated, input signal is received by the PWM, mono-stable multivibrator circuits provide discharge pulses that ensure that the PWM output will not spend excessive time in a single state. By using discrete mono-stable multivibrators, uniform and repeatable pulses can be generated at precise intervals. In addition, when an out-of-range input signal is detected, the AGC circuit lowers the gain on the input signal until it falls within the acceptable range of the PWM, enabling more faithful reproduction of the original signal.