Datasheet MAX9727 (Maxim) - 8

Quad Audio Line Driver with 3VRMS Output Detailed Description
VOUT The MAX9727 is a quad audio line driver with an output of 3VRMS from a single +5V supply and 2VRMS from a single +3.3V supply. The device employs Maxim’s VDD DirectDrive architecture that produces a ground-refer- enced output from a single supply, eliminating the need for large DC-blocking capacitors. An internal V V
MAX9727
charge pump creates an internal negative supply volt- DD/2 DD age. This allows the amplifier outputs of the MAX9727 to be biased at GND, almost doubling dynamic range while operating from a single supply. GND An active-low shutdown input disables the amplifiers and reduces quiescent current consumption to less than 100nA. CONVENTIONAL DRIVER BIASING SCHEME The MAX9727 also features click-and-pop suppression circuitry that reduces audible clicks and pops during VOUT startup and shutdown.
DirectDrive
VDD Maxim’s DirectDrive architecture uses a charge pump to create an internal negative supply voltage, allowing the MAX9727 outputs to be biased about ground. This allows for a symmetrical output biased around 0V. The MAX9727’s charge pump requires two small ceramic capacitors, conserving board space, reducing cost, 2V and improving the frequency response of the ampli- GND DD fiers. See the Charge-Pump Output Voltage vs. Output Current graphs 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 offsets of the MAX9727 are typical- ly 500µV, which, when combined with a 1kΩ load, VDD results in less than 500nA of DC current flow to the line- in device. DirectDrive BIASING SCHEME
Charge Pump
Figure 1. Conventional Driver Output Waveform vs. MAX9727 The MAX9727 features a low-noise charge pump. The Output Waveform 300kHz switching frequency is well beyond the audio range and does not interfere with audio signals. The switching speed of the charge pump. Although not typ- switch drivers feature a controlled switching speed that ically required, additional high-frequency noise attenu- minimizes noise generated by turn-on and turn-off tran- ation can be achieved by increasing the value of C2 sients. The di/dt noise caused by the parasitic bond (see the Functional Diagram/Typical Operating Circuit). wire and trace inductance is minimized by limiting the
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