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C
i
IN
Z
i
Z
f
I
n
p
u
t
S
i
g
n
a
l
f=
1
2 Z C
p
i
i
(1)
INPUT CAPACITOR, C
I
In the typical application, an input capacitor (C
I
) is required to allow the amplifier to bias the input signal to the
proper dc level for optimum operation. In this case, C
I
and the input impedance of the amplifier (Z
I
) form a
high-pass filter with the corner frequency determined in
Equation 2
.
f
=
c
1
2 Z C
p
i
i
-3dB
f
c
(2)
C =
i
1
2 Z f
p
i c
(3)
Power Supply Decoupling, C
S
The TPA3100D2 is a high-performance CMOS audio amplifier that requires adequate power supply decoupling
to ensure that the output total harmonic distortion (THD) is as low as possible. Power supply decoupling also
prevents oscillations for long lead lengths between the amplifier and the speaker. The optimum decoupling is
achieved by using two capacitors of different types that target different types of noise on the power supply leads.
For higher frequency transients, spikes, or digital hash on the line, a good low equivalent-series-resistance (ESR)
ceramic capacitor, typically 0.1
μ
F to 1 μF placed as close as possible to the device V
CC
lead works best. For
filtering lower frequency noise signals, a larger aluminum electrolytic capacitor of 220
μ
F or greater placed near
the audio power amplifier is recommended. The 220
μ
F capacitor also serves as local storage capacitor for
supplying current during large signal transients on the amplifier outputs. The PVCC terminals provide the power
to the output transistors, so a 220 μF or larger capacitor should be placed on each PVCC terminal. A 10 μF
capacitor on the AVCC terminal is adequate.
TPA3100D2
SLOS469D–OCTOBER 2005–REVISED FEBRUARY 2006
The -3-dB frequency can be calculated using
Equation 1
. Use the Z
I
values given in
Table 1
.
The value of C
is important, as it directly affects the bass (low-frequency) performance of the circuit. Consider
the example where Z
I
is 20 k
and the specification calls for a flat bass response down to 20 Hz.
Equation 2
is
reconfigured as
Equation 3
.
In this example, C
I
is 0.4 μF; so, one would likely choose a value of 0.47
μ
F as this value is commonly used. If
the gain is known and is constant, use Z
I
from
Table 1
to calculate C
I
. A further consideration for this capacitor is
the leakage path from the input source through the input network (C
) and the feedback network to the load. This
leakage current creates a dc offset voltage at the input to the amplifier that reduces useful headroom, especially
in high gain applications. For this reason, a low-leakage tantalum or ceramic capacitor is the best choice. When
polarized capacitors are used, the positive side of the capacitor should face the amplifier input in most
applications as the dc level there is held at 2 V, which is likely higher than the source dc level. Note that it is
important to confirm the capacitor polarity in the application. Additionally, lead-free solder can create dc offset
voltages and it is important to ensure that boards are cleaned properly.
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