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The design of portable audio devices such as PDAs, MP3 players, and cellular phones is often affected by many factors such as power supply noise, limited space, and power/grounding shared with digital circuits. End users are also required to increase the performance of the audio while also requiring extended battery operation. Lifetime, these requirements often conflict with each other. Extending the battery's working time requires shutting down those circuits that are not working at any time, which will produce some significant audio interference. Therefore, portable product audio circuits must be designed to meet the requirements of small size, high performance, low noise, and ability to work with ASICs, processors, and DC-DC converters. Switched audio amplifiers (Class D) have higher efficiency, but since the Class D amplifier actually turns the audio signal into a non-linear pulse, the resulting distortion is obvious and requires an output filter to recover the audio from the amplifier output. At the same time, the filter also increases the cost and increases the size of the amplifier. Maxim's new Class D audio power amplifiers (MAX9700/MAX9712) offer some significant improvements to the switch mode amplification technology, eliminating the need for an output filter with a unique modulation structure that provides a smaller, less expensive, portable product. More efficient solution.
Low EMI modulation structure
The MAX9700/MAX9712 utilize a unique modulation architecture to provide Class AB amplifier performance in Class D efficiency, taking up very little board space. The MAX9712 delivers 500mW for 8W loads and the MAX9700 can deliver up to 1.2W for 8W loads. There are two comparators inside the amplifier that monitor their inputs and compare the complementary input voltage to the sawtooth waveform. When the sawtooth input amplitude exceeds the corresponding comparator input voltage, the comparator output flips. As the input voltage increases or decreases, the duration of the first tripped comparator output pulse increases while the other comparator transitions the duration of the output pulse to tON. For a certain input signal level, the comparator output is a pulse-width modulated square wave signal, the period is determined by the frequency of the sawtooth oscillator, and the PWM signal controls the H-bridge driver to turn on or off a pair of MOSFETs in opposite states, making the speaker The net voltage at both ends (VOUT+ - VOUT-) changes with the input signal, effectively collecting the audio input. The dynamic range of the amplifier is determined by the noise amplitude and the amplitude of the sawtooth signal.
Operating mode
The MAX9700/MAX9712 have two modes of operation: fixed frequency modulation (FFM) mode and spread spectrum modulation (SSM) mode. In FFM mode, the frequency can be selected by the SYNC pin. When SYNC=GND, the switching frequency is 1.1MHz, and when SYNC = FLOAT, the switching frequency is 1.45MHz. In addition, applying an external TTL clock of 800kHz to 2MHz on the SYNC pin allows the amplifier to be synchronized to the system clock (providing a fully synchronized system) or to distribute the spectral components of the switching harmonics to insensitive frequency bands. When SYNC = VDD, the amplifier operates in SSM mode. In SSM mode, the switching frequency varies randomly around the center frequency (1.22MHz)? 20kHz. The modulation scheme is the same, but the frequency of the sawtooth varies with the period, at which point the energy is spread over the entire bandwidth as the frequency increases, rather than concentrating a large amount of spectral energy at the multiple of the switching frequency. In bands above a few MHz, EMI is equivalent to white noise in the wideband spectrum, and the spread spectrum mode improves the radiation index by 5 dB compared to conventional solutions.
Filterless modulation
The MAX9700/MAX9712 feature a unique modulation scheme that eliminates the need for LC filters for traditional Class D amplifiers, increasing efficiency, reducing component count, and saving board space and system cost. When there is no signal at the input of the MAX9700/MAX9712, the output switch waveform is shown in Figure 1. Since the amplifier drives the speaker differentially, the two outputs cancel each other out, so the net voltage in the idle mode at both ends of the speaker is zero, reducing power consumption. In addition, because the output frequency of the MAX9712 far exceeds the bandwidth of most speakers, the offset of the audio coil caused by the switching frequency is very small, and the square wave can be obtained from the inductance of the speaker coil itself and the natural filtering effect of the speaker and the human ear. The audio component is restored in the output. For best results, an inductor greater than 10mH can also be used in series with the speaker.
Efficiency problem
The efficiency of a Class D amplifier is determined by the operating time of the output stage transistors. In a Class D amplifier, the output transistor acts like a current-regulating switch, and the extra power consumed is negligible. Any power loss associated with the Class D output stage is dominated by the MOSFET's on-resistance and the quiescent current.
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