Design of Current Mode Based Oscillator for Class D Audio Power Amplifiers

In recent years, Class D audio power amplifiers have become the preferred solution for portable audio systems such as MP3 and mobile phones due to their high efficiency and low power consumption. The oscillator is an important part of the class D audio amplifier. The oscillator has an important influence on the amplifier's sound quality, chip efficiency, electromagnetic interference and other indicators. To this end, this paper designs a current-controlled oscillator circuit for class D power amplifiers. The module is based on the current mode and mainly implements two functions: one is to provide a triangular wave signal whose amplitude is proportional to the power supply voltage; the other is to provide a square wave signal whose frequency is almost independent of the power supply voltage, and the duty ratio of the square wave signal is 50%.

Step 1: Current Mode Oscillator Principle

The working principle of the oscillator is to control the charging and discharging of the capacitor by the current source through the MOS switch tube to generate a triangular wave signal. A block diagram of a conventional current mode based oscillator is shown in Figure 1.

Design of Current Mode Based Oscillator for Class D Audio Power Amplifiers

In FIG. 1, R1, R2, R3, and R4 generate threshold voltages VH, VL and a reference voltage Vref by dividing a voltage of a power supply voltage. The reference voltage is then passed through an LDO structure of amplifiers OPA and MN1 to generate a reference current Iref that is proportional to the supply voltage. So there are:

MP1, MP2, and MP3 in this system can form a mirror current source to generate charging current IB1. The mirror current source composed of MP1, MP2, MN2, and MN3 generates a discharge current IB2. It is assumed that MP1, MP2, and MP3 have equal width to length ratios, and MN2 and MN3 have equal width to length ratios. Then there are:

When the oscillator is working, during the charging phase t1, CLK=1, the MP3 tube charges the capacitor with a constant current IB1. After that, the voltage at point A rises linearly. When the voltage at point A is greater than VH, the voltage at the output of cmp1 is turned to zero. The logic control module is mainly composed of RS flip-flops. When the output of cmp1 is 0, the output terminal CLK is inverted to a low level, and CLK is a high level. The oscillator enters the discharge phase t2, at which point the capacitor C begins to discharge at a constant current IB2, causing the voltage at point A to drop. When the voltage drops below VL, the output voltage of cmp2 becomes zero. The RS flip-flop flips, CLK goes high, and CLK goes low, completing a period of charge and discharge. Since IB1 and IB2 are equal, the charging and discharging times of the capacitor are equal. The rising edge slope of the A-point triangular wave is equal to the absolute value of the falling edge slope. Therefore, the CLK signal is a square wave signal with a duty ratio of 50%.

The output frequency of this oscillator is independent of the supply voltage, and the amplitude of the triangular wave is proportional to the supply voltage.

Step 2: Oscillator Circuit Implementation

The oscillator circuit design designed in this paper is shown in Figure 2. The circuit is divided into three parts: a threshold voltage generating circuit, a charging and discharging current generating circuit and a logic control circuit.

Design of Current Mode Based Oscillator for Class D Audio Power Amplifiers Figure 2 oscillator implementation circuit

2.1 Threshold voltage generation unit

The threshold voltage generating portion may be constituted by MN1 and four voltage dividing resistors R1, R2, R3 and R4 having equal resistance values. The MOS transistor MN1 is here used as a switching transistor. When no audio signal is input, the chip sets the CTRL terminal low, VH and VL are both 0V, and the oscillator stops working to reduce the static power consumption of the chip. When there is a signal input, CTRL is low, VH=3Vdd/4, VL=Vdd/4. Due to the high frequency operation of the comparator, if point B and point C are directly connected to the comparator input, electromagnetic interference may be generated to the threshold voltage through the parasitic capacitance of the MOS transistor. Therefore, this circuit connects point B and point C to the buffer. Circuit simulations show that the use of buffers can effectively isolate electromagnetic interference and stabilize the threshold voltage.

2.2 Generation of charge and discharge current

Current proportional to the supply voltage can be generated by OPA, MN2, and R5. Since the gain of the OPA is high, the voltage difference between Vref and V5 is negligible. Due to the channel modulation effect, the currents of MP11 and MN10 are affected by the source-drain voltage. Therefore, the charge-discharge current of the capacitor is no longer linear with the supply voltage. In this design, the current mirror uses cascode structure to stabilize the source-drain voltage of MP11 and MN10, and reduce the sensitivity to the power supply voltage. From an AC perspective, the cascode structure increases the output resistance of the current source (layer) and reduces the error in the output current. MN3, MN4, and MP5 are used to provide a bias voltage for the MP12. MP8, MP10, MN6 can provide bias voltage for MN9.

2.3 Logic Control Section

The output CLK and CLK of the flip-flop are square wave signals with opposite phases, which can be used to control the opening and closing of MP13, MN11 and MP14, MN12. MP14 and MN11 act as switching transistors, which function as SW1 and SW2 in Figure 1. MN12 and MP13 act as auxiliary tubes, whose main function is to reduce the burrs of the charge and discharge current and eliminate the sharp-shooting phenomenon of triangular waves. The sharp-shoot phenomenon is mainly caused by the channel charge injection effect when the MOS transistor is in the state transition.

Assuming that MN12 and MP13 are removed, when CLK transitions from 0 to 1, MP14 is turned on to the off state, and the current source composed of MP11 and MP12 is forced to enter the deep linear region from the saturation region instantaneously, and MP11, MP12, MP13 are The channel charge is drawn out in a very short time, which causes a large glitch current, causing a spike voltage at point A. At the same time, MN11 jumps from the off state to the on state, and the current layers composed of MN10 and MN9 go from the deep linear region to the saturation region. The channel capacitance of these three tubes is charged in a short time, which also causes a large Burr current and spike voltage. Similarly, if the auxiliary pipe MN12 is removed, the MN11, MN10, and MN9 also generate a large glitch current and a spike voltage when the CLK is hopped. Although MP13 and MP14 have the same width-to-length ratio, the gate level is opposite, so MP13 and MP14 are alternately turned on. MP13 plays two main roles in eliminating the spike voltage. First, ensure that MP11 and MP12 work in the saturation region during the whole cycle to ensure the continuity of the current and avoid the sharp-shooting voltage caused by the current mirror. Second, make MP13 and MP14 form a complementary tube. Thus, at the moment of the CLK voltage change, the channel capacitance of one tube is charged, and the channel capacitance of the other tube is discharged, and the positive and negative charges cancel each other, thereby greatly reducing the glitch current. Similarly, the introduction of MN12 will play the same role.

2.4 Application of repair technology

The parameters of different batches of MOS tubes will vary between wafers. Under different process angles, the thickness of the oxide layer of the MOS tube will also be different, and the corresponding Cox will also change accordingly, causing the charge and discharge current to shift, causing the output frequency of the oscillator to change. In integrated circuit design, the trimming technology is mainly used to modify the resistor and resistor network (or capacitor network). Different resistor networks can be used to increase or decrease the resistance (or capacitance) to design different resistor networks (or capacitor networks). The charge and discharge currents IB1 and IB2 are mainly determined by the current Iref. And Iref=Vdd/2R5. Therefore, this design chooses to trim the resistor R5. The trimming network is shown in Figure 3. In the figure, all resistors are equal. In this design, the resistance of resistor R5 is 45kΩ. R5 is connected in series by ten small resistors with a resistance of 4.5kΩ. Fusing the wire between the two points A and B can increase the resistance of R5 by 2.5%, and fusing the wire between B and C can increase the resistance by 1.25%, between A, B and B, C. The fuses are all blown, which increases the resistance by 3.75%. The disadvantage of this trimming technique is that it can only increase the resistance value, but not the small one.

Figure 3 resistance repair network structure

Step 3: Simulation Results Analysis

This design can be implemented on CSMC's 0.5μm CMOS process and can be simulated with the Spectre tool.

3.1 Improvement of triangular wave by complementary switching tube

Figure 4 is a schematic diagram showing the improvement of the triangular wave by the complementary switch tube. It can be seen from Fig. 4 that the waveforms of MP13 and MN12 in this design have no obvious peaks when the slope changes, and the waveform sharpening phenomenon disappears after the auxiliary tube is added.

Figure 4 Improved waveform of the complementary switching tube to the triangular wave

3.2 Influence of power supply voltage and temperature

It can be seen from Figure 5 that the frequency of the oscillator changes to 1.86% when the power supply voltage changes from 3V to 5V. When the temperature changes from -40°C to 120°C, the oscillator frequency changes by 1.93%. It can be seen that when the temperature and the power supply voltage vary widely, the output frequency of the oscillator can remain stable, so that the normal operation of the chip can be ensured.

Figure 5 Effect of voltage and temperature on frequency

Step 4: Conclusion

This paper designs a current controlled oscillator for Class D audio power amplifiers. Typically, this oscillator can output square and triangular wave signals with a frequency of 250 kHz. Moreover, the output frequency of the oscillator can remain stable when the temperature and supply voltage vary widely. In addition, the spike voltage can also be removed by adding complementary switching transistors. By introducing a resistor network trimming technique, an accurate output frequency can be obtained in the presence of process variations. Currently, this oscillator has been used in a Class D audio amplifier.

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