This article refers to the address: http:// The MAX9700/MAX9712 are a new generation of Class D audio power amplifiers with fully differential architecture and full-bridge output that delivers the high performance typically found in Class AB amplifiers, as well as complete click and click suppression circuitry. The device features up to 72dB Power Supply rejection ratio (PSRR), 0.01% total harmonic distortion and noise (THD+N), and SNR above 90 dB. Its short-circuit and over-temperature protection features protect the device from fault conditions. damage. Its wide voltage range of 2.5 to 5.5 V makes it easy to connect to microprocessors of various voltage standards. The MAX9712 is capable of delivering 0.5 W of power to an 8 Ω load with greater than 85% efficiency. The MAX9700 has a conversion efficiency of over 90% and provides 1.2 W for an 8 Ω load. In addition, they have a quiescent current of only 4 mA and a low-power shutdown mode of only 0.1μA. MAXIM's proprietary low EMI modulation eliminates the need for traditional Class D output filters and eliminates bulky heat sinks, saving board space and extending battery life. 1 Principle and function of MAX9700/MAX9712 1.1 Working principle Figure 1 shows the pinout of the MAX9700/MAX9712. Figure 2 shows the functional structure of the MAX9700/MAX9712. In Figure 2, the differential audio signal is input by the comparator, modulated by the D-type and amplified by the H-bridge. It works by monitoring the input of the MAX9700/MAX9712 through a comparator and comparing the complementary input voltage to the sawtooth waveform. When the sawtooth input amplitude exceeds the corresponding comparator input voltage, the comparator output flips. The two comparators are reset after a fixed period of time after the rising edge of the second comparator output transition, producing a pulse with the smallest pulse width at the output of the second comparator. Thus, as the input voltage increases or decreases, if one output pulse duration increases while the other output pulse duration remains the same, then the net voltage across the speaker will change. 1.2 Modulation method selection The MAX9700/MAX9712 have two modes of operation: fixed frequency modulation (FFM) mode and spread spectrum modulation (SSM) mode. The MAX9700/MAX9712 come in two FFM modes. When SYNC = GND, the switching frequency is 1.1 MHz; when SYNC = FLOAT, the switching frequency is 1.45 MHz. In FFM mode, the Class D output spectrum consists of the switching frequency fundamental and its associated harmonics. The MAX9712 allows a +32% change in switching frequency. The spread spectrum mode of the MAX9700/MAX9712 broadens the spectral content and reduces EMI emissions through the speaker or cable by 3dB. When SYNC=GND, the device is set to SSM mode. In SSM mode, the switching frequency varies ±120 kHz randomly around the center frequency of 1.22 MHz, at which point the energy is spread over the entire bandwidth as the frequency increases. At this time, although the modulation scheme is unchanged, the frequency of the sawtooth wave changes with the period, and its energy will be dispersed into the entire frequency bandwidth as the frequency increases, instead of concentrating a large amount of spectral energy at the multiple of the switching frequency. In fact, in bands above a few MHz, EMI is equivalent to white noise in the wideband spectrum. This method uses a spread spectrum mode to improve the radiation index by 5 dB compared to conventional approaches. 1.3 Filterless modulation / common mode idle mode The MAX9700/MAX9712's unique modulation scheme saves efficiency and reduces cost by eliminating the LC filters required for traditional Class D amplifiers. Because the output duty cycle of the conventional class D amplifier is 50% square wave when there is no signal input, if there is no filter, a DC voltage is generated and a load current is formed to increase the power consumption. The MAX9700/MAX9712 use a differential drive to drive the speaker, and the two outputs cancel each other out. In idle mode, the net voltage across the speaker is zero, which reduces power consumption. 1.4 Shutdown When the SHDN pin is asserted low, the MAX9700/MAX9712 enter a low-power (0.1μA) shutdown mode that extends battery life. In standard mode, this pin should be tied to VDD. 1.5 hum and click suppression The MAX9700/MAX9712 feature complete click and click suppression to eliminate transient noise during startup and shutdown. When turned off, the H-bridge is in a high-impedance state; at startup or power-up, the input amplifier is muted, and then after 35 ms of startup, the soft-start circuit releases the mute state of the input amplifier. 1.6 Low EMI modulation structure The MAX9700/MAX9712 offer Class AB amplifier performance in Class D efficiency with a unique modulation structure and take up very little board space. The MAX9712 delivers 500mW for an 8W load and the MAX9700 can deliver up to 1.2W for an 8W load. Inside these two amplifiers, two comparators 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. Generally for a certain input signal level, the comparator output is a pulse-width modulated square wave signal whose period is determined by the frequency of the sawtooth oscillator, and the PWM signal is used to control the H-bridge driver to open or close the opposite state. For the MOSFET, the net voltage across the speaker (VOUT+-VOUT-) changes with the input signal to effectively capture the audio input. The dynamic range of the amplifier is determined by the noise amplitude and the amplitude of the sawtooth signal. 1.7 High efficiency issues 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 (the extra power consumed is negligible), and the power loss associated with the Class D output stage is primarily the IR loss due to the MOSFET's on-resistance and quiescent current consumption. After the load is applied to the Class D amplifier, its output offset voltage does not significantly increase the quiescent current, which is the result of the power conversion of the Class D amplifier. In Class AB devices, an 8 mV dc offset voltage consumes an additional 1 mA through an 8 W load. For Class D devices, an 8 mV dc offset voltage consumes an additional 8 mW of power through an 8 W load. The theoretically optimal efficiency of a linear amplifier is 78%, but this efficiency only occurs at the peak of the output power. Typically at standard operating levels (typical music reproduction levels), efficiency drops below 30%, while under the same conditions, the MAX9700 maintains more than 80% efficiency. To further save power, the MAX9700/MAX9712 also offer a shutdown mode with a current consumption of only 0.1mA in shutdown mode, which reduces power consumption and extends battery life. In addition, the amplifier provides a complete noise suppression function to eliminate transient noise during startup and shutdown. When turned off, the H-bridge is in a high-impedance state; when the power is turned on or on, the input amplifier is muted, and the internal loop sets the modulator bias voltage to the correct level to avoid humming when the H-bridge is subsequently activated. Humming. After 35 ms of startup, the soft-start circuit releases the mute state of the input amplifier. 2 MAX9700/MAX9712 Application Circuit Traditional Class D amplifiers require an output filter to recover the audio signal from the amplifier's output signal, while the MAX9700/MAX9712 do not require an output filter. The MAX9700/MAX9712 devices use the speaker coil's own inductance and the natural filtering of the speaker and the human ear to recover the audio component from the square wave output, eliminating the need for an output filter, providing a smaller, cheaper, and more efficient Program. However, since the output frequency of the MAX9700/MAX9712 far exceeds the bandwidth of most speakers, and the offset of the audio coil caused by the square wave frequency is very small, for best results, inductors and speakers larger than 10μH should be used. In series. The MAX9700/MAX9712 feature a differential input structure that is compatible with many codecs and provides greater noise rejection than single-ended input amplifiers. The differential input cancels out any common-mode signals that are applied to the input. In cellular telephone devices, this feature can be used to remove the effects of high frequency common mode signals in the RF transmitter. The MAX9700/MAX9712 can also have either input connected to GND and drive the other input to configure it as a single-ended input amplifier. The coupling method can use either capacitive coupling or direct DC coupling, but the DC coupling eliminates the coupling capacitor, but also loses the low-frequency rejection of the capacitor. In addition, the MAX9700/MAX9712 can also be cascaded to form a stereo amplifier. The circuit structure is shown in Figure 3. In Figure 3, U1 is the main amplifier, and its unfiltered output can be used to drive the SYNC input of slave U2, which synchronizes the switching frequency of the two devices. The two synchronized MAX9700/MAX9712 ensure that the beat frequency does not occur in the audio spectrum. In fact, this configuration works regardless of whether the master is operating in FFM or SSM mode, because this SYNC connection provides excellent THD+N performance and low crosstalk between devices. U2 only tracks the signal frequency of SYNC, not the pulse width, so the feedback loop inside U2 ensures that the audio component of U1 output is suppressed. The specific application circuit of the MAX9700/MAX9712 is shown in Figure 4. In the figure, the MAX9700 or /MAX9712 can amplify the voice output signal of the 16-bit Sunplus MCU SPCE061A. SPCE061A is a SCM chip with distinctive speech features, and its speech compression algorithm library can be easily called through the AP1 function. This design uses the MAX9700 to amplify the SPCE061A voice signal and can receive good results. The SPCE061A has two voices that can form stereo. Only one of them (DAC1) is shown in the figure. SYNC can be set to SSM mode through the IOB1 port of the microcontroller, the switch can be controlled by IOB2, and the volume can be adjusted by the potentiometer CW. In addition, the voice quality of the RC path can improve the voice quality at power-on or stop. Therefore, in this design, both VDD and H-bridge supplies are bypassed to GND and PGND with 0.1μF capacitors, and GND and PGND are connected to the system ground with star wiring to minimize ground-line common-impedance interference. 3 Conclusion The voice output signal amplifying circuit designed by MAX9700/MAX9712 and Sunplus 16-bit single-chip microcomputer SPCE061A can be widely used in PDA, MP3 player, cellular phone and various applications requiring audio control and amplification, and this scheme has low EMI. It is worthy of promotion because it has no advantages such as filtering and occupying a small area.
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February 03, 2019