Rainbow Electronics MAX13331 - Manual (Page 9)

MAX13330/MAX13331
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:
where R L is the impedance of the headphone and
C OUT is the value of the DC-blocking capacitor. The
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-
ues of C
OUT reduce this effect but result in physically
larger, more expensive capacitors. Figure 2 shows the
relationship between the size of C
OUT 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 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
R C
Hz
dB
L OUT
−
=
Ч Ч
( )
3
1
2 π
0
-30
10 100 1k 10k 100k
LOW-FREQUENCY ROLLOFF
(R L = 16 Ω)
-24
-27
-12
-15
-18
-21
-6
-9
-3
FREQUENCY (Hz)
ATTENUATION (dB)
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)
THD+N (%)
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