MAX9700/MAX9712 Class D audio power amplifier and its application

MAX9700/MAX9712 is a fully differential structure and a new generation of full-bridge output Class D audio power amplifier provides Class AB amplifier is normally only has the performance, but also has a complete clicks and crackle suppression circuitry. The device has up to 72DB of the power supply rejection ratio (PSRR), 0.01% total harmonic distortion and noise (THD + N) and higher than 90 dB of SNR, the short-circuit and overheat protection function allows the device from fault conditions damage. 2.5 ~ 5.5 V and its wide voltage range with a variety of voltage can easily connect a standard microprocessor. MAX9712 can be higher than 85% efficiency of 0.5 W 8 Ω load power, MAX9700 conversion efficiency can reach 90%, 8 Ω load for 1.2 W of power. In addition, their quiescent current is only 4 mA, low-power shutdown mode is only 0.1μA. MAXIM's proprietary low-EMI modulation scheme eliminates the traditional class D output filter, also abandoned the heavy radiator, effectively saving board space, extends battery life.

1 MAX9700/MAX9712 Principle and Features

1.1 Work

Figure 1 is the MAX9700/MAX9712 pinout diagram. Figure 2 for the MAX9700/MAX9712 functional structure diagram. Figure 2, the difference audio signal from the comparator input, modulated by the Class D amplification through the H bridge output. Its working principle is monitored by comparing the MAX9700/MAX9712 input devices, and complementary input voltage compared with the sawtooth. When the sawtooth input amplitude than the corresponding comparator input voltage, the comparator output flip. Two comparators comparator output in the second transition after the rising edge of reset after a fixed time, so in the second comparator output generates a minimum pulse width. Thus, when the input voltage increases or decreases, if an output pulse duration increased, while the other output pulse duration remains the same, then the net voltage across the speaker will change.

MAX9700/MAX9712 Class D audio power amplifier and its application

1.2 Modulation Select

MAX9700/MAX9712 have two modes: fixed frequency modulation (FFM) mode and spread spectrum modulation (SSM) mode. MAX9700/MAX9712 have two ways FFM. When SYNC = GND, the switching frequency of 1.1 MHz; and SYNC = FLOAT, the switching frequency of 1.45 MHz. In FFM mode, D, the output spectrum from the fundamental switching frequency and associated harmonic component. MAX9712 allow the switching frequency of +32% change. The MAX9700/MAX9712 spread-spectrum mode to show elements of broadband spectrum, and can be made through the speakers or cables EMI radiation reduced 3dB. SYNC = GND, the device is set to the SSM model. In SSM mode, the switching frequency center frequency of 1.22 MHz in the vicinity of random changes in ± 120 kHz, then the energy will be spread over the entire bandwidth of the frequency increase. Although the modulation scheme at this time remain unchanged, but the sawtooth frequency will change with the cycle, its energy will spread with the growth of the entire frequency bandwidth, rather than a large number of spectral energy concentrated in the switching frequency of the harmonic Department. In fact, more than a few MHz of the band, EMI equivalent to the broadband spectrum of white noise. This approach compared with traditional programs, using spread-spectrum mode can improve the radiation target 5 dB.

1.3 No filter modulation / common mode idle mode

As the MAX9700/MAX9712 unique modulation scheme can save the traditional Class D amplifiers required for LC filter, which can improve efficiency and reduce costs. Because no signal input, the traditional class D amplifier output 50% duty cycle square wave, so that if there is no filter, it will produce a DC voltage and power consumption increases the formation of load current Er Shi. The MAX9700/MAX9712 are differential mode to drive the speaker, the two outputs can be offset, so the idle mode, the speaker at both ends of the net voltage is 0, it can reduce power consumption.

1.4 Shutdown

The SHDN pin is set low, MAX9700/MAX9712 enter low-power (0.1μA) shutdown mode, which extend battery life. In standard mode, the pin should be connected to VDD.

1.5 a click and Pop Suppression

MAX9700/MAX9712 have a complete clicks and pops sound suppression, may start and shutdown transient noise elimination. Shutdown, H bridge for the high impedance state; in starting or power, the input amplifier is muted, and then start the 35 ms, the soft-start circuitry to lift the mute status of input amplifiers.

1.6 low-EMI modulation structure

MAX9700/MAX9712 can use a unique modulation structure to provide efficient Class AB Class D amplifier's performance, but also takes up very little board space. MAX9712 8 W load for 500mW of power, MAX9700 can load up to 8 W 1.2 W of power. Both amplifiers are two-way internal comparator to monitor its input and complementary input voltage compared with the sawtooth. When the sawtooth input amplitude than the corresponding comparator input voltage, the comparator output flip. When the input voltage increases or decreases, the first transition of the comparator output pulse duration increased, while the other comparator output pulse after the jump duration tON. General for a certain input signal level, the comparator output is a pulse width modulated square wave signal, the cycle from the decision sawtooth oscillator frequency, PWM signal used to control the H bridge driver to open or close state of the opposite one on the MOSFET, so that the net voltage across the speaker (VOUT +-VOUT-) with the input signal changes, in order to effectively capture the audio input. Amplifier's dynamic range by the noise amplitude and the sawtooth signal amplitude decision.

High-efficiency 1.7

Efficiency Class D amplifier output stage transistors work by the time the decision. In the D class amplifier, output transistor as a current adjustment switch (the additional power consumed is negligible), and D type output stage are mainly related to the power loss by the MOSFET on-resistance and quiescent current consumption losses generated by IR. Class D amplifier with the load, the output offset voltage will not significantly increase the quiescent current, which is Class D amplifier power conversion results. If the AB class devices, 8 mV dc offset voltage through the consumption of 8 W load an additional 1 mA of current. While the device is Class D, 8 mV of DC offset voltage by 8 W load, consumes only 8 mW of additional power. Theoretically, the best linear amplifier efficiency of 78%, but the efficiency is only in the peak of output power. Usually work in the standard level (typical music reproduction levels), the efficiency will be reduced to below 30%, while under the same conditions, MAX9700 can be maintained above 80% efficiency.

To further save power, MAX9700/MAX9712 also provide the off mode, shutdown mode current consumption is only 0.1mA, it can effectively reduce power consumption and extend battery life. In addition, the amplifier also provides a complete interior noise suppression, you can start and shutdown transient noise elimination. Shutdown, H bridge for the high impedance state; start or power, the input amplifier is muted, the internal circuit will set the modulator bias voltage to the correct level, then start the H bridge to avoid a click generated when and pops. Started 35 ms after the lifting of the soft start circuit input amplifier muted.

2 MAX9700/MAX9712 application circuit

Traditional Class D amplifiers use output filters required output signal from the amplifier to recover the audio signal, while the MAX9700/MAX9712 are without output filter. MAX9700/MAX9712 devices using self-inductance of the speaker and the speaker coil and the role of the human ear's natural filter to recover from a square wave output of the audio component, thus eliminating the output filter, it can provide a smaller, cheaper, more efficient program. However, the MAX9700/MAX9712 output frequency far beyond the bandwidth of most speakers, while the square wave frequency shift caused by the audio coils and very small, therefore, for the best results, should be used more than 10μH inductor and the speaker series.

MAX9700/MAX9712 differential input structure, compatible with many codecs, and provides more than the single-ended input amplifier noise suppression. Differential input can be offset by any of the input common-mode signal. In the cellular phone device, this feature can be used to remove high frequency radio transmitter common mode signal. MAX9700/MAX9712 can also be any input termination to GND, and drive the other input to be configured for single-ended input amplifier. Coupling can be used both capacitive coupling can also be used directly to DC coupling, only way of eliminating the DC coupling capacitor coupling, it also lost the inhibitory effect of low frequency capacitance. In addition, MAX9700/MAX9712 can also be cascaded to the stereo amplifier, the circuit shown in Figure 3.

MAX9700/MAX9712 Class D audio power amplifier and its application

Figure 3, U1 is the main amplifier output without filtering device U2 can be used to drive from the SYNC input, so two devices can synchronize the switching frequency. Synchronization of two MAX9700/MAX9712 can be sure that the audio frequency spectrum does not appear within the beat frequency. In fact, both the main components work in FFM or SSM mode, this configuration can work, because the SYNC connection can get excellent THD + N performance, but a very small cross talk between devices. U2 SYNC signal frequency tracking only, not the pulse width, therefore, U2 internal feedback loop ensures that the output of the audio component U1 inhibition.

MAX9700/MAX9712 specific application circuit shown in Figure 4, diagram, MAX9700, or / MAX9712 can Sunplus 16-bit MCU SPCE061A speech output signal amplification. SPCE061A is a significant voice characteristics, single chip, the voice compression algorithm library can easily by APl function call. This design uses a MAX9700 on SPCE061A voice signal amplification, can receive good results. SPCE061A two way voice can form stereo. Figure only shows which way (DAC1). SYNC by SCM IOBl port is set to SSM mode, the switch can be IOB2 control volume can be adjusted through the potentiometer CW. In addition, because the RC channel speech input can improve the power or stop when the voice quality. Therefore, the design, VDD and the H bridge power all through the 0.1μF bypass capacitor to GND and PGND, GND and PGND star wiring and systems are used to connect to the ground impedance of interference to a minimum.

3 Conclusion

Using MAX9700/MAX9712 16-bit microcontroller with Sunplus SPCE061A designed voice output signal amplification circuit can be widely used in PDA, MP3 players, cell phones and a variety of needs of the audio control and amplification applications, and use the program with low EMI, filter-free, occupy an area of the Line, etc, and worthy of promotion.