MAX9720 查看數據表（PDF） - Maxim Integrated

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MAX9720

50mW, DirectDrive, Stereo Headphone Amplifier with SmartSense and Shutdown

Maxim Integrated 

50mW, DirectDrive, Stereo Headphone

Amplifier with SmartSense and Shutdown

VOUT

VDD

VDD/2

GND

CONVENTIONAL DRIVER-BIASING SCHEME

+VDD

VOUT

GND

-VDD

DirectDrive BIASING SCHEME

Figure 1. Conventional Amplifier Output Waveform vs.

MAX9720 Output Waveform

DirectDrive

Conventional single-supply headphone amplifiers have

their outputs biased about a nominal DC voltage (typical-

ly half the supply) for maximum dynamic range. Large

coupling capacitors are needed to block this DC bias

from the headphone. Without these capacitors, a signifi-

cant amount of DC current flows to the headphone,

resulting in unnecessary power dissipation and possible

damage to both headphone and headphone amplifier.

Maxim’s patented DirectDrive architecture uses a

charge pump to create an internal negative supply volt-

age. This allows the MAX9720 output to be biased

about GND, almost doubling dynamic range while

operating from a single supply. With no DC component,

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

Instead of two large capacitors (220µF typ), the

MAX9720 charge pump requires only two, small ceram-

ic capacitors (1µF typ), conserving board space,

reducing cost, and improving the 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.

Previous attempts to eliminate the output-coupling

capacitors involved biasing the headphone return

(sleeve) to the DC bias voltage of the headphone

amplifiers. This method raised some issues:

• The sleeve is typically grounded to the chassis.

Using this biasing approach, the sleeve must be

isolated from system ground, complicating product

design.

• During an ESD strike, the amplifier’s ESD structures

are the only path to system ground. The amplifier

must be able to withstand the full ESD strike.

• When using the headphone jack as a line out to

other equipment, the bias voltage on the sleeve

may conflict with the ground potential from other

equipment, resulting in large ground-loop current

and possible damage to the amplifiers.

• When using a combination microphone and speak-

er headset (in a cell phone or PDA application), the

microphone typically requires a GND return. Any

DC bias on the sleeve conflicts with the microphone

requirements (Figure 2).

Low-Frequency Response

In addition to the cost and size disadvantages, the DC-

blocking capacitors limit the low-frequency response of

the amplifier and distort the audio signal:

• The impedance of the headphone load and the DC-

blocking capacitor form a highpass filter with the

-3dB point determined by:

f−3dB

=

1

2πRLCOUT

where RL is the impedance of the headphone and

COUT is the value of the DC-blocking capacitor.

The highpass filter is required by conventional single-

ended, single-supply headphone amplifiers to block

the midrail DC component of the audio signal from the

headphones. Depending on the -3dB point, the filter

can attenuate low-frequency signals within the audio

band. Larger values of COUT reduce the attenuation,

but are physically larger, more expensive capacitors.

Figure 3 shows the relationship between the size of

COUT and the resulting low-frequency attenuation. Note

that the -3dB point for a 16Ω headphone with a 100µF

blocking capacitor is 100Hz, well within the audio

band.

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