Loop design of Class D audio power amplifier

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As shown in Figure 1, this audio power amplifier contains an audio channel, an oscillator, a reference current source, and an overcurrent protection circuit. The audio channel includes a control unit that converts the input audio signal into a pulse width modulation (PWM) signal, and then the PWM signal drives the switching power level of the audio power amplifier, and the signal output through the switching power stage is fed back to the integral. At the input of the device, a feedback loop is used to improve the power supply rejection ratio (PSRR) and total harmonic distortion (THD) of the audio power amplifier. A low-pass second-order filter is used at the output to demodulate the audio signal and suppress high-frequency energy. The optimized system design in this audio power amplifier design eliminates the external low-pass output filter to reduce system application. cost.

The protection module mainly includes overcurrent protection, which enables the audio power amplifier to protect the audio power amplifier from being burned out due to misoperation and load resistance being burned.

In the design of the oscillator, the resistor and capacitor are all integrated into the audio power amplifier, and the application can use a minimum of external devices, which saves the application cost, but the oscillation frequency of this oscillator is relative to the oscillator of the external resistor. In terms of oscillation frequency, the influence of process deviation will be greater.

The PWM modulation method of this audio power amplifier is based on bilateral natural sampling techniques. The PWM signal can be directly obtained by comparing the audio input signal and the triangular wave signal (triangulaI waveform). As shown in Fig. 2, the frequency of the triangular wave is called the carrier frequency, and the ratio of the amplitude of the input signal to the amplitude of the carrier signal is called For the modulation depth, the PWM spectrum does not directly contain the harmonics of the modulated signal, which is ideal from the point of view of harmonic distortion.

Considering only the signal in the audio range (20Hz-20kHz), the gain of the PWM modulation is the ratio of the amplitude of the output PWM signal to the amplitude of the input triangle wave:

Vp in the above equation is the amplitude of the PWM output signal, and VT is the amplitude of the input triangular wave signal.

The circuit diagram of an audio channel is shown in Figure 3. This audio power amplifier uses a feedback loop to suppress supply voltage fluctuations, switching power stage output deviation, and harmonic distortion. The closed-loop gain of this audio power amplifier is:

The resistors R1, R2, R3, Rfb must have good linearity and matching to achieve good closed loop performance.

The situation when the system is open loop is shown in Figure 4. The gain of the entire open loop loop can be derived from:

From equations (3) and (4), the unity gain frequency of this loop is:

Figure 5 shows the internal loop signal of the amplifier, VE is the output waveform of the integrator, and VT is the output waveform of the oscillator. The two triangular waves pass through each other and the output changes direction. In order for this audio power amplifier to work properly, the amplitude of the triangle wave of this oscillator should be larger than the amplitude of the triangular wave of the integrator output. More accurately, the slope of the oscillator triangle wave should be larger than the slope of the error triangle wave, otherwise it will appear. Divergence phenomenon:

As can be seen from FIG. 4, the current input to the integrator capacitor is the sum of the feedback current Ifb and the input current Iin. When the input signal is zero, as shown in Fig. 5(a), the feedback current Ifb is alternately injected into the integrator capacitor, and the slope of the integrator output triangle wave is:

When a positive input signal current is injected into this loop, the slope of the falling edge of the integrator output triangle wave becomes larger, and the slope of the rising edge becomes smaller, as shown in Fig. 5(b), the duty of the output signal Change than the beginning.

When the current Iin of the input signal is equal to the current Ifb of the feedback signal, the modulation depth of the audio power amplifier is 100%, and the slope of the falling edge of the error triangle wave VE is about twice the slope when there is no input signal, and the entire loop is converged and stable. The standard is:

In this loop system, there is only one pole.

The system loop contains a pole. The stability criterion of the loop is that the circuit has a phase margin greater than 60° at a gain of 0 dB. It is also possible to introduce a LPH zero point in this loop system to create a second order loop. Compared to the first order loop system, the second order loop system has a higher gain in the audio bandwidth and will therefore be in the audio range. Better THD performance and PSRR performance.