MAX13331 datasheet(9/15 Pages) AD

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Low-Frequency Response

In addition to the cost and size disadvantages of the

DCblocking 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:

( )

3dB

OUT

−

π×

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 singleend-

ed, single power-supply headphone amplifiers to

block the midrail DC-bias component of the audio sig-

nal 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 physi-

cally larger, more expensive capacitors. Figure 2

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 block-

ing capacitor is 100Hz, well within the normal audio

band, resulting in low-frequency attenuation of the re-

produced signal.

2) The voltage coefficient of the DC-blocking capacitor

contributes distortion to the reproduced audio signal

as the capacitance value varies and the function of

the voltage across the capacitor changes. The reac-

tance of the capacitor dominates at frequencies below

the -3dB point and the voltage coefficient appears as

frequency-dependent distortion. Figure 3 shows the

THD+N introduced by two different capacitor dielec-

tric types. Note that below 100Hz, THD+N increases

rapidly. The combination of low-frequency attenuation

and frequency-dependent distortion compromises au-

dio reproduction in portable audio equipment that em-

phasizes low-frequency effects such as in multimedia

laptops, MP3, CD, and DVD players. By eliminating the

DC-blocking capacitors through DirectDrive technolo-

gy, these capacitor-related deficiencies are eliminated.

Charge Pump

The MAX13330/MAX13331 feature a low-noise charge

pump. The 2.2MHz (typ) switching frequency is well

beyond the audio range. It does not interfere with the

audio signals and avoids AM band interference. The

switch drivers feature a controlled switching speed that

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

sients. By limiting the switching speed of the charge

pump, the di/dt noise caused by the parasitic bond wire

and trace inductance is minimized. Although not typi-

cally required, additional high-frequency noise attenuation

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

Typical Application Circuits).

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

Capacitor Values

Figure 3. Distortion Contributed by DC-Blocking Capacitors

-30

100

10k

100k

LOW-FREQUENCY ROLLOFF

(RL = 16Ω)

-24

-27

-12

-15

-18

-21

-6

-9

-3

FREQUENCY (Hz)

DirectDrive

330µF

220µF

100µF

33µF

ADDITIONAL THD+N DUE

TO DC-BLOCKING CAPACITORS

FREQUENCY (Hz)

10k

100

0.001

0.01

0.1

0.0001

100k

TANTALUM

ALUM/ELEC

MAX13330/MAX13331

Automotive DirectDrive Headphone Amplifiers

with Output Protection and Diagnostics

www.maximintegrated.com

Maxim Integrated │ 9

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