Automotive DirectDrive Headphone Amplifiers

with Output Protection and Diagnostics

_______________________________________________________________________________________

9

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:

highpass filter is required by conventional single-

ended, 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 val-

larger, more expensive capacitors. Figure 2 shows the

low-frequency attenuation. Note that the -3dB point for

a 16

Ω 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 and the function of the

voltage across the capacitor changes. The reactance

of the capacitor dominates at frequencies below the

-3dB point and the voltage coefficient appears as fre-

quency-dependent distortion. Figure 3 shows the

THD+N introduced by two different capacitor dielectric

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

ly. The combination of low-frequency attenuation and

frequency-dependent distortion compromises audio

reproduction in portable audio equipment that empha-

sizes low-frequency effects such as in multimedia lap-

tops, 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

typically required, additional high-frequency noise

attenuation can be achieved by increasing the value of

C2 (see the

Typical Application Circuits).

f

RC

Hz

dB

LOUT

−

=

××

()

3

1

2

π

0

-30

10

100

1k

10k

100k

LOW-FREQUENCY ROLLOFF

Ω)

-24

-27

-12

-15

-18

-21

-6

-9

-3

FREQUENCY (Hz)

DirectDrive

330

μF

220

μF

100

μF

33

μF

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

Capacitor Values

ADDITIONAL THD+N DUE

TO DC-BLOCKING CAPACITORS

FREQUENCY (Hz)

10k

1k

100

0.001

0.01

0.1

1

10

0.0001

10

100k

TANTALUM

ALUM/ELEC

Figure 3. Distortion Contributed by DC-Blocking Capacitors