1 Introduction A common problem in portable audio applications is the limited supply voltage of the speaker amplifier. These audio systems are typically powered by Lithium-Ion (Li +) batteries with an output rating of 3.7V. Although a 3.7 V supply is sufficient to keep most of the system's components up and running, loudspeaker amplifiers require higher supply voltages to provide satisfactory sound pressure. As a result, most speaker amplifiers have a bridge-load power configuration that produces twice the supply voltage on the speakers. In most cases the speaker supply voltage is sufficient to meet the requirements, but some applications require more output power. Examples include piezoelectric speakers (which require higher voltage drive) or systems that require higher sound pressure (such as GPS devices). The only solution to these audio application needs is to increase the supply voltage, often requiring a separate DC-DC boost converter, increasing system cost and complexity. The MAX9730 and MAX9788 Class G amplifiers use charge pumps to boost the supply voltage to address the supply voltage. The MAX9730 applies traditional moving coil speakers, while the MAX9788 is suitable for ceramic speakers. 2 Innovative Charge Pump The MAX9730 and MAX9788 use a charge pump to double the output level compared to a standard 5-V amplifier. The charge pump is fully integrated within the amplifier, requiring only two external capacitors, which can be housed in a 0603 micro-surface-mount capacitor. An integrated charge pump generates a negative voltage of equal magnitude to the supply voltage, doubling the supply voltage and eventually doubling the output swing. Unlike DC-DC boost converters, the charge pump has a larger output impedance and can cause the negative supply to drop with a load. The MAX9730 and MAX9788 ensure that the output impedance of the charge pump is low enough to provide sufficient output power. The traditional amplifier delivers 700 mW of drive power to an 8 Ω load at 3.7 V while the MAX9730 delivers 1.3 W of output drive power under the same conditions. 3 Unique Class G Technology Since the MAX9730 and MAX9788 replace the common 5 V amplifiers found in portable devices with 10 V amplifiers, maintaining high efficiency is the key to extending battery life. Although Class D technology is highly efficient and commonly used in portable devices, it does not meet the drive requirements for ceramic speakers. The design challenge is very clear - ceramic speakers require additional drive technology. Therefore, Class G amplifier, an unknown amplifier technology, due to its perfect adaptability is to market. Class G amplifiers work similarly to class AB amplifiers, except that class G amplifiers are multi-supply voltages, not a single fixed voltage. As the input signal amplitude changes, Class G architecture automatically selects the appropriate power supply, which minimizes the voltage drop across the output transistor, greatly increasing efficiency. Class G amplifiers are typically powered by two positive supplies, high supplies for higher outputs, and low supplies for lower outputs. The Class G amplifiers, the MAX9730 and MAX9788, utilize Class G technology in a unique way that generates negative voltage from the charge pump instead of high and low positive power. When the amplifier produces a smaller output signal, the amplifier is powered by the battery voltage and ground. In this mode, the device operates in a similar way to the common 5 V Class AB amplifier (Figure 1a). When the output signal exceeds the supply voltage, the amplifier selects the battery voltage and the negative charge pump output to supply power (Figure 1b). As a result, Class G amplifier output signal is much higher than the traditional amplifier signal. The MAX9730 and MAX9788 ensure that switching between two supplies does not produce audible noise. The negative supply is automatically connected to the output stage when the output signal reaches the limits allowed by the VCC and GND supplies. In this way, the output signal does not appear shaved on the minus swing side and is still clamped to its positive swing. To correct this, the amplifier adds an additional correction signal to the negative output, as shown in Figure 2. When observing the positive and negative outputs respectively, the positive half-cycle waveform is significantly shaved, while the negative half cycle has obvious distortion. Despite the serious distortions of these signals, they are tightly controlled and take full advantage of the architecture. The actual output signal applied to the load is not distorted. 4 Conclusion G Class technology and negative charge pump combination, MAX9730 and MAX9788 audio design for the existence of common problems provide an effective solution. Most internal boost amplifiers require large inductors, while the MAX9730 and MAX9788 require only two small capacitors to help save PCB space and cost. The MAX9730 and MAX9788 utilize an efficient Class G architecture to reduce current consumption and improve existing designs.
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