Datasheet

C =
i
1
2 Z fp
i c
f =
c
1
2 Z Cp
i i
-3dB
f
c
f=
1
2 Z Cp
i i
C
i
IN
Z
i
Z
f
Input
Signal
TPA3112D1
SLOS654C –SEPTEMBER 2009–REVISED JULY 2012 www.ti.com
INPUT RESISTANCE
Changing the gain setting can vary the input resistance of the amplifier from its smallest value, 9 kΩ ±20%, to the
largest value, 60 kΩ ±20%. As a result, if a single capacitor is used in the input high-pass filter, the -3 dB or
cutoff frequency may change when changing gain steps.
The -3-dB frequency can be calculated using Equation 2. Use the Z
I
values given in Table 1.
(2)
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 3.
(3)
The value of C
I
is important, as it directly affects the bass (low-frequency) performance of the circuit. Consider
the example where Z
I
is 60 kΩ and the specification calls for a flat bass response down to 20 Hz. Equation 3 is
reconfigured as Equation 4.
(4)
In this example, C
I
is 0.13 µF; so, one would likely choose a value of 0.15 μF 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
I
) 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. If a
ceramic capacitor is used, use a high quality capacitor with good temperature and voltage coefficient. An X7R
type works well and if possible use a higher voltage rating than required. This will give a better C vs voltage
characteristic. 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 3 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.
POWER SUPPLY DECOUPLING, C
S
The TPA3112D1 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.
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