MAX9723 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX9723

Stereo DirectDrive Headphone Amplifier with BassMax, Volume Control, and I2C

Maxim Integrated 

MAX9723

Stereo DirectDrive Headphone Amplifier

with BassMax, Volume Control, and I2C

VDD

VDD/2

GND

CONVENTIONAL AMPLIFIER BIASING SCHEME

+VDD

SGND

-VDD

DirectDrive BIASING SCHEME

Figure 1. Traditional Amplifier Output vs. MAX9723 DirectDrive

Output

MAX9723A and MAX9723B have a maximum amplifier

gain of 0dB while the MAX9723C and MAX9723D have

a maximum gain of +6dB. Amplifier volume is digitally

programmable to any one of 32 levels.

DirectDrive

Traditional single-supply headphone amplifiers have their

outputs biased at a nominal DC voltage, typically half

the supply, for maximum dynamic range. Large cou-

pling capacitors are needed to block this DC bias from

the headphone. 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 amplifier.

Maxim’s DirectDrive architecture uses a charge pump to

create an internal negative supply voltage. This allows

the MAX9723 headphone amplifier outputs to be biased

at 0V, almost doubling the dynamic range while operat-

ing from a single supply. With no DC component, there

is no need for the large DC-blocking capacitors. Instead

of two large (typically 220μF) tantalum capacitors, the

MAX9723 charge pump requires only two small 1μF

ceramic capacitors, thereby conserving board space,

reducing cost, and improving the low-frequency response

of the headphone amplifier. See the Output Power vs.

Charge-Pump Capacitance and Load Resistance graph

in the Typical Operating Characteristics for details of the

possible capacitor sizes.

In addition to the cost and size disadvantages, the

DC-blocking capacitors required by conventional head-

phone amplifiers limit low-frequency response and can

distort the audio signal.

Previous attempts at eliminating the output-coupling

capacitors involved biasing the headphone return (sleeve)

to the DC bias voltage of the headphone amplifiers. This

method raises some issues:

1) The sleeve is typically grounded to the chassis. Using

the midrail biasing approach, the sleeve must be

isolated from system ground, complicating product

design. The DirectDrive output biasing scheme allows

the sleeve to be grounded.

2) During an ESD strike, the amplifier’s ESD structure is

the only path to system ground. The amplifier must be

able to withstand the full ESD strike. The MAX9723

headphone outputs can withstand an ±8kV ESD strike

(HBM).

3) When using the headphone jack as a line out to other

equipment, the bias voltage on the sleeve may con-

flict with the ground potential from other equipment,

resulting in possible damage to the amplifiers. The

DirectDrive outputs of the MAX9723 can be directly

coupled to other ground-biased equipment.

Charge Pump

The MAX9723 features a low-noise charge pump. The

600kHz switching frequency is well beyond the audio

range, and does not interfere with the audio signals.

This enables the MAX9723 to achieve a 99dB SNR.

The switch drivers feature a controlled switching speed

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

transients. Limiting the switching speed of the charge

pump minimizes di/dt noise caused by the parasitic

bond wire and trace inductance. Although not typically

required, additional high-frequency noise attenuation

can be achieved by increasing the size of C2 (see the

Functional Diagram/Typical Operating Circuit).

Shutdown

The MAX9723 features a 5μA, low-power shutdown mode

that reduces quiescent current consumption and extends

battery life. Shutdown is controlled by a hardware or

software interface. Driving SHDN low disables the drive

amplifiers, bias circuitry, charge pump, and sets the

headphone amplifier output impedance to 20kΩ. Similarly,

the MAX9723 enters shutdown when bit seven (B7) in

the control register is reset. SHDN and B7 must be high

to enable the MAX9723. The I2C interface is active and

the contents of the command register are not affected

when in shutdown. This allows the master to write to the

MAX9723 while in shutdown.

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