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MAX9722A Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX9722A Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 21 page 2) The voltage coefficient of the DC-blocking capaci- tor contributes distortion to the reproduced audio signal as the capacitance value varies and the function of the voltage across the capacitor changes. The reactance of the capacitor dominates at frequencies below the -3dB point and the voltage coefficient appears as frequency-dependent distor- tion. Figure 5 shows the THD+N introduced by two different capacitor dielectric types. Note that below 100Hz, THD+N increases rapidly. The combination of low-frequency attenuation and frequency-depen- dent distortion compromises audio reproduction in portable audio equipment that emphasizes low-fre- quency effects such as in multimedia laptops, MP3, CD, and DVD players. By eliminating the DC-block- ing capacitors through DirectDrive technology, these capacitor-related deficiencies are eliminated. Charge Pump The MAX9722A/MAX9722B feature a low-noise charge pump. The 600kHz switching frequency is well beyond the audio range and, thus, does not interfere with the audio signals. Also, the 600kHz switching frequency does not interfere with the 450kHz AM transceivers. The switch drivers feature a controlled switching speed that minimizes noise generated by turn-on and turn-off transients. By limiting the switching speed of the charge pump, the di/dt noise caused by the parasitic bond wire and trace inductance is minimized. Although not typically required, additional high-frequency noise attenuation can be achieved by increasing the value of C2 (see Typical Application Circuit). Click-and-Pop Suppression In conventional single-supply audio amplifiers, the out- put-coupling capacitor is a major contributor of audible clicks and pops. Upon startup, the amplifier charges the coupling capacitor to its bias voltage, typically half the supply. Likewise, on shutdown, the capacitor is dis- charged to GND. This results in a DC shift across the capacitor, which, in turn, appears as an audible tran- sient at the speaker. Since the MAX9722A/MAX9722B do not require output-coupling capacitors, this problem does not arise. Additionally, the MAX9722A/MAX9722B feature exten- sive click-and-pop suppression that eliminates any audi- ble transient sources internal to the device. The Power-Up/Down Transient graph in the Typical Operating Characteristics shows that there is minimal DC shift and no spurious transients at the output upon startup or shutdown. In most applications, the output of the preamplifier driving the MAX9722A/MAX9722B has a DC bias of typically half the supply. At startup, the input-coupling capacitor is charged to the preamplifier’s DC-bias voltage through the feedback resistor of the MAX9722A/MAX9722B, resulting in a DC shift across the capacitor and an audible click/pop. Delaying the rise of SHDN 4 to 5 time con- stants (80ms to 100ms) based on RIN and CIN, relative to the startup of the preamplifier, eliminates this click/pop caused by the input filter. 5V, Differential Input, DirectDrive, 130mW Stereo Headphone Amplifiers with Shutdown 10 ______________________________________________________________________________________ 0 -30 10 100 1k 10k 100k LOW-FREQUENCY ROLLOFF (RL = 16Ω) -24 -27 -12 -15 -18 -21 -6 -9 -3 FREQUENCY (Hz) DirectDrive 330µF 220µF 100µF 33µF Figure 4. Low-Frequency Attenuation for Common DC-Blocking Capacitor Values ADDITIONAL THD+N DUE TO DC-BLOCKING CAPACITORS FREQUENCY (Hz) 10k 1k 100 0.001 0.01 0.1 1 10 0.0001 10 100k TANTALUM ALUM/ELEC Figure 5. Distortion Contributed by DC-Blocking Capacitors |
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