Loop D audio power amplifier design

Class D audio power amplifier with high efficiency, low power consumption advantages of Class D audio power amplifier using the device can improve the battery life, it is particularly suitable for wireless and handheld communications devices, mainly used in PDA, mobile phones and similar design of handheld mobile communications tools and products. The power output of the audio device of great power, so power output of audio devices using low-power Class D audio power amplifier is also very necessary, especially in the integration of high-quality audio performance and enhanced mixing ability to achieve both low power consumption.

This article describes the loop D audio power amplifier design, that the D audio power amplifier with high efficiency, low power consumption and low harmonic distortion characteristics.

Shown in Figure 1, the audio power amplifier includes an audio channel, an oscillator, a reference voltage current source and a current protection circuit. The audio channel includes a control unit, control unit converts the input audio signal pulse-width modulation (PWM) signal, and then drive the PWM signal switching audio power amplifier power level, through the switching power stage output signal is fed back to the points again the input side, the feedback loop to improve the audio power amplifier power supply rejection ratio (PSRR) and total harmonic distortion (THD). The output using a second-order low-pass filter to demodulate the audio signal and suppress high-frequency energy in the audio power amplifier designed to optimize the system design eliminated external low-pass output filter to reduce the system application cost.

Loop D audio power amplifier design

Protection module includes over-current protection, so that the audio power amplifier and the load resistance in the misuse case destroyed to protect the audio amplifier is not burned.

In the oscillator design, the resistors and capacitors integrated into all internal audio power amplifier, application can use the least when the external device, the application of cost savings, but the oscillator with the oscillation frequency of the oscillator external resistor oscillation frequency of speaking, the process variation would be even more.

The PWM modulation audio power amplifier is based on bilateral natural sampling. PWM signal can directly compare the audio input signal (audio input) and triangular wave signals (triangulaI waveform) are shown in Figure 2, the triangle wave frequency is called the carrier frequency, the input signal amplitude and the magnitude of the ratio of carrier signal, said of the modulation depth (modulation depth), PWM spectrum is not directly modulated signal contains harmonics, ie from the viewpoint of harmonic distortion consideration, it is very good.

Loop D audio power amplifier design

Consider only the audio range (20Hz-20kHz) signal within, PWM modulation gain of the output PWM input signal amplitude and triangular wave amplitude ratio:

Loop D audio power amplifier design

On the type of PWM output signal Vp is the rate, VT is a triangular wave input signal amplitude.

An audio channel of the circuit shown in Figure 3, the audio power amplifier using a feedback loop to suppress supply voltage fluctuations, switching power stage output deviation, and harmonic distortion, the audio power amplifier closed-loop gain:

Loop D audio power amplifier design

Resistors R1, R2, R3, Rfb must have good linearity and matching, to obtain good closed loop performance.

Loop D audio power amplifier design

Open-loop system when the situation shown in Figure 4, the entire open-loop loop gain introduced by the following formula:

By (3), (4) can come to this loop unity gain frequency is:

Loop D audio power amplifier design

Loop D audio power amplifier design

Figure 5 shows the internal loop for the amplifier signal, VE for the integrator output waveform, VT for the oscillator's output waveform, the two triangular wave penetrating each other, the output change direction. To make this audio power amplifier to work properly, the oscillator triangle wave amplitude should be greater than integrator output triangular wave amplitude, is more accurate to say that the slope of the triangular wave oscillator error of the triangular wave should be higher than the slope of a large, otherwise there will be divergence of the phenomenon:

Loop D audio power amplifier design

Loop D audio power amplifier design

From Figure 4 indicated that the input integrator capacitor current feedback and the input current Iin current Ifb sum. Zero input signal conditions, shown in Figure 5 (a) shows, the feedback current into the integrator capacitor Ifb alternative, the integrator output triangular wave with a slope of:

Loop D audio power amplifier design

When there is a positive input signal current into this loop, the integrator output triangular wave on the falling edge of the slope becomes greater, rising edge slope become smaller, in Figure 5 (b) shows, the output signal duty cycle ratio began to change.

Current Iin when the input signal is equal to the current Ifb feedback signal, the audio power amplifier is 100% modulation depth, the error triangle wave slope of the falling edge of VE is no input signal about twice the slope of the entire loop convergence and stability The standards are:

Loop D audio power amplifier design

Loop D audio power amplifier design

In this loop system, there is only one pole.

System includes a pole loop, loop stability of the standard is 0dB gain, the circuit's phase margin greater than 60 °. Can also be in the loop system, the introduction of a LPH zero to create a second loop, and first-order loop systems, the second-order loop system in the audio bandwidth has a higher gain, it will be in the audio range has better performance in THD and PSRR performance.

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