MAX4410 データシートの表示（PDF） - Maxim Integrated

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MAX4410

80mW, DirectDrive Stereo Headphone Driver with Shutdown

Maxim Integrated 

80mW, DirectDrive Stereo Headphone Driver

with Shutdown

4) 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 drivers.

Low-Frequency Response

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

blocking capacitors required by conventional head-

phone amplifiers, these capacitors limit the amplifier’s

low-frequency response and can distort the audio signal.

1) The impedance of the headphone load and the DC-

blocking capacitor form a highpass filter with the

-3dB point set by:

f

−3dB =

1

2πRLCOUT

where RL is the headphone impedance and COUT is

the DC-blocking capacitor value. The highpass filter

is required by conventional single-ended, single

power-supply headphone drivers to block the midrail

DC bias component of the audio signal from the

headphones. The drawback to the filter is that it can

attenuate low-frequency signals. Larger values of

COUT reduce this effect but result in physically larg-

er, more expensive capacitors. Figure 2 shows the

relationship between the size of COUT and the result-

ing low-frequency attenuation. Note that the -3dB

point for a 16Ω headphone with a 100µF blocking

capacitor is 100Hz, well within the normal audio

band, resulting in low-frequency attenuation of the

reproduced signal.

2) The voltage coefficient of the DC-blocking capacitor

contributes distortion to the reproduced audio signal

as the capacitance value varies as a function of the

voltage change across the capacitor. At low fre-

quencies, the reactance of the capacitor dominates

at frequencies below the -3dB point and the voltage

coefficient appears as frequency-dependent distor-

tion. Figure 3 shows the THD + N introduced by two

different capacitor dielectric types. Note that below

100Hz, THD + N increases rapidly.

The combination of low-frequency attenuation and fre-

quency-dependent distortion compromises audio

reproduction in portable audio equipment that empha-

sizes low-frequency effects such as multimedia lap-

tops, as well as MP3, CD, and DVD players. By

eliminating the DC-blocking capacitors through

DirectDrive technology, these capacitor-related defi-

ciencies are eliminated.

Charge Pump

The MAX4410 features a low-noise charge pump. The

320kHz switching frequency is well beyond the audio

range, and thus does not interfere with the audio sig-

nals. The switch drivers feature a controlled switching

speed that minimizes noise generated by turn-on and

turn-off transients. By limiting the switching speed of the

switches, the di/dt noise caused by the parasitic bond

wire and trace inductance is minimized. Although not

typically required, additional high-frequency noise atten-

uation can be achieved by increasing the size of C2

(see Typical Application Circuit).

LF ROLL OFF (16Ω LOAD)

0

-3 330µF

-5

220µF

-10

100µF

-15

-3dB CORNER FOR

100µF IS 100Hz

33µF

-20

-25

-30

-35

10

100

1k

FREQUENCY (Hz)

Figure 2. Low-Frequency Attenuation for Common DC-Blocking

Capacitor Values

ADDITIONAL THD + N DUE

TO DC-BLOCKING CAPACITORS

10

1

0.1

TANTALUM

0.01

0.001

ALUM/ELEC

0.0001

10

100

1k

10k

100k

FREQUENCY (Hz)

Figure 3. Distortion Contributed by DC-Blocking Capacitors

______________________________________________________________________________________ 13

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