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MAX97000 Datasheet(PDF) 22 Page - Maxim Integrated Products |
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MAX97000 Datasheet(HTML) 22 Page - Maxim Integrated Products |
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22 / 33 page 
Audio Subsystem with Mono Class D Speaker and Class H Headphone Amplifier 22 conserving board space, reducing cost, and improv- ing the frequency response of the headphone amplifier. See the Output Power vs. Load Resistance graph in the Typical Operating Characteristics for details of the possible capacitor sizes. There is a low DC voltage on the amplifier outputs due to amplifier offset. However, the offset of the MAX97000 is typically Q0.15mV, which, when combined with a 32I load, results in less than 5FA of DC current flow to the headphones. In addition to the cost and size disadvantages of the DC-blocking capacitors required by conventional headphone amplifiers, these capacitors limit the ampli- fier’s low-frequency response and can distort the audio signal. Previous attempts at eliminating the output-cou- pling capacitors involved biasing the headphone return (sleeve) to the DC bias voltage of the headphone ampli- fiers. This method raises some issues: • The sleeve is typically grounded to the chassis. Using the midrail biasing approach, the sleeve must be isolated from system ground, complicating prod- uct design. • During an ESD strike, the amplifier’s ESD structures are the only path to system ground. Thus, the ampli- fier must be able to withstand the full energy from an ESD strike. • When using the headphone jack as a line out to other equipment, the bias voltage on the sleeve may conflict with the ground potential from other equip- ment, resulting in possible damage to the amplifiers. Charge Pump The MAX97000’s dual-mode charge pump generates both the positive and negative power supply for the headphone amplifier. To maximize efficiency, both the charge pump’s switching frequency and output voltage change based on signal level. When the input signal level is less than 10% of VDD, the switching frequency is reduced to a low rate. This minimizes switching losses in the charge pump. When the input signal exceeds 10% of VDD, the switching fre- quency increases to support the load current. For input signals below 25% of VDD, the charge pump generates Q(VDD/2) to minimize the voltage drop across the amplifier’s power stage and thus improve efficiency. Input signals that exceed 25% of VDD cause the charge pump to output QVDD. The higher output voltage allows for full output power from the headphone amplifier. To prevent audible gliches when transitioning from the Q (VDD/2) output mode to the QVDD output mode, the charge pump transitions very quickly. This quick change draws significant current from VDD for the duration of the transition. The bypass capacitor on VDD supplies the required current and prevents droop on VDD. The charge pump’s dynamic switching mode can be turned off through the I2C interface. The charge pump can then be forced to output either Q(VDD/2) or QVDD regardless of input signal level. Class H Operation A Class H amplifier uses a Class AB output stage with power supplies that are modulated by the output signal. In the case of the MAX97000, two nominal power-supply differentials of 1.8V (+0.9V to -0.9V) and 3.6V (+1.8V to -1.8V) are available from the charge pump. Figure 7 shows the operation of the output-voltage-dependent power supply. Low-Power Mode To minimize power consumption when using the head- phone amplifier, enable the low-power mode. In this mode, the headphone mixers and volume control are bypassed and shut down. I2C Slave Address The MAX97000 uses a slave address of 0x9A or 1001101RW. The address is defined as the 7 most significant bits (MSBs) followed by the read/write bit. Set the read/write bit to 1 to configure the MAX97000 to read mode. Set the read/write bit to 0 to configure the MAX97000 to write mode. The address is the first byte of information sent to the MAX97000 after the START (S) condition. Figure 7. Class H Operation 32ms 1.8V 0.9V VTH_H VTH_L -0.9V -1.8V HPVDD HPVSS OUTPUT VOLTAGE 32ms |
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