Detailed Description

The MAX9721 stereo headphone driver features Maxim’s

patented DirectDrive architecture, eliminating the large

output-coupling capacitors required by conventional sin-

gle-supply headphone drivers. The MAX9721 consists of

two 20mW Class AB headphone drivers, shutdown con-

trol, inverting charge pump, internal gain-setting resistors,

and comprehensive click-and-pop suppression circuitry

(see the Functional Diagram). A negative power supply

dynamic range of the drivers to almost twice that of other

1V single-supply drivers. This increase in dynamic range

allows for higher output power.

The outputs of the MAX9721 are biased about GND

(Figure 1). The benefit of this GND bias is that the driver

outputs do not have a DC component, thus large DC-

blocking capacitors are unnecessary. Eliminating the

DC-blocking capacitors on the output saves board

space, system cost, and improves frequency response.

DirectDrive

Conventional single-supply headphone drivers have their

outputs biased about a nominal DC voltage (typically half

the supply) for maximum dynamic range. Large coupling

capacitors are needed to block the DC bias from the

headphones. Without these capacitors, a significant

amount of DC current flows to the headphone, resulting

in unnecessary power dissipation and possible damage

to both headphone and headphone driver.

Maxim’s DirectDrive architecture uses a charge pump

to create an internal negative supply voltage. This

allows the MAX9721 outputs to be biased about GND,

increasing the dynamic range while operating from a

single supply. A conventional amplifier powered from

1.5V ideally provides 18mW to a 16Ω load. The

MAX9721 provides 25mW to a 16Ω load. The

DirectDrive architecture eliminates the need for two

large (220µF, typ) DC-blocking capacitors on the out-

put. The MAX9721 charge pump requires two small

ceramic capacitors, conserving board space, reducing

cost, and improving the frequency response of the

headphone driver. See the Output Power vs. Charge-

1V, Fixed-Gain, DirectDrive, Stereo Headphone

Amplifier with Shutdown

6

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PIN

BUMP

THIN

QFN

UCSP

NAME

FUNCTION

1A1

C1N

Flying Capacitor Negative Terminal. Connect a 1µF capacitor from C1P to C1N.

2A2

3A3

INL

Left-Channel Audio Input

4A4

INR

Right-Channel Audio Input

5B4

6B3

SGND

Signal Ground. SGND must be connected to PGND. SGND is the ground reference for the input and

output signal.

7C4

OUTR

Right-Channel Output

8C3

OUTL

Left-Channel Output

9C2

Positive Power-Supply Input. Bypass with a 1µF capacitor to PGND.

10

C1

C1P

Flying Capacitor Positive Terminal. Connect a 1µF capacitor from C1P to C1N.

11

B1

PGND

Power Ground. Ground reference for the internal charge pump. PGND must be connected to SGND.

12

B2

SHDN

charge pump.

EP

—

EP

Pin Description