TPA2000D4
STEREO 2W CLASSD AUDIO POWER AMPLIFIER
WITH STEREO HP AMPLIFIER
SLOS337B DECEMBER 2000 REVISED NOVEMBER 2005
19
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APPLICATION INFORMATION
input capacitor, CI
In the typical application an input capacitor, CI, is required to allow the amplifier to bias the input signal to the
proper dc level for optimum operation. In this case, CI and the input impedance of the amplifier, ZI, form a
high-pass filter with the corner frequency determined in equation 5.
f
c(highpass) +
1
2
pZ
I
C
I
3 dB
fc
(5)
The value of CI is important as it directly affects the bass (low frequency) performance of the circuit. Consider
the example where ZI is 20 k and the specification calls for a flat bass response down to 80 Hz. Equation 5
is reconfigured as equation 6.
C
I +
1
2
pZ
I fc
(6)
In this example, CI is 0.1 F, so one would likely choose a value in the range of 0.1 F to 1 F. If the gain is known
and is constant, use ZI from Table 1 to calculate CI. A further consideration for this capacitor is the leakage path
from the input source through the input network (CI) 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 VDD/2, which is likely higher than the source dc level. Note that it is important to
confirm the capacitor polarity in the application.
CI must be 10 times smaller than the bypass capacitor to reduce clicking and popping noise from power on/off
and entering and leaving shutdown. After sizing CI for a given cutoff frequency, size the bypass capacitor to 10
times that of the input capacitor.
(7)
CI ≤ CBYP / 10
switching frequency
The switching frequency is determined using the values of the components connected to ROSC (pin 18) and
COSC (pin 19) and is calculated with the following equation:
fs +
6.6
R
OSC
C
OSC
(8)
The switching frequency was chosen to be centered on 250 kHz. This frequency represents the optimization
of audio fidelity due to oversampling and the maximization of efficiency by minimizing the switching losses of
the amplifier. The recommended values are a resistance of 120 k
and a capacitance of 220 pF. Using these
component values, the amplifier operates properly by using 5% tolerance resistors and 10% tolerance
capacitors. The tolerance of the components can be changed, as long as the switching frequency remains
between 200 kHz and 300 kHz. Within this range, the internal circuitry of the device provides stable operation.