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Detailed Description

The MAX9727 is a quad audio line driver with an output

single +3.3V supply. The device employs Maxim’s

DirectDrive architecture that produces a ground-refer-

enced output from a single supply, eliminating the

need for large DC-blocking capacitors. An internal

charge pump creates an internal negative supply volt-

age. This allows the amplifier outputs of the MAX9727

to be biased at GND, almost doubling dynamic range

while operating from a single supply.

An active-low shutdown input disables the amplifiers

and reduces quiescent current consumption to less

than 100nA.

The MAX9727 also features click-and-pop suppression

circuitry that reduces audible clicks and pops during

startup and shutdown.

DirectDrive

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,

and improving the frequency response of the ampli-

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,

results in less than 500nA of DC current flow to the line-

in device.

Charge Pump

The MAX9727 features a low-noise charge pump. The

300kHz switching frequency is well beyond the audio

range and does not interfere with audio signals. The

switch drivers feature a controlled switching speed that

minimizes noise generated by turn-on and turn-off tran-

sients. The di/dt noise caused by the parasitic bond

wire and trace inductance is minimized by limiting the

switching speed of the charge pump. Although not typ-

ically required, additional high-frequency noise attenu-

ation can be achieved by increasing the value of C2

(see the

Functional Diagram/Typical Operating Circuit).

CONVENTIONAL DRIVER BIASING SCHEME

DirectDrive BIASING SCHEME

GND

GND

Figure 1. Conventional Driver Output Waveform vs. MAX9727

Output Waveform