Datasheet

www.ti.com
SELECTION OF COSC AND ROSC
f
OSC
= 6.6 / (R
OSC
x C
OSC
)
(5)
INPUT RESISTANCE
C
i
IN
R
i
R
f
Input
Signal
f
3 dB

1
2 C
i
R
i
(6)
INPUT CAPACITOR, C
i
−3 dB
f
c
f
c

1
2 R
i
C
i
(7)
C
i

1
2 R
i
f
c
(8)
TPA2008D2
SLOS413C – JULY 2003 – REVISED MAY 2004
APPLICATION INFORMATION (continued)
The switching frequency is determined using the values of the components connected to ROSC (pin 11) and
COSC (pin 10) and may be calculated with the following equation:
The frequency may be varied from 200 kHz to 300 kHz by adjusting the values chosen for R
OSC
and C
OSC
. The
recommended values are C
OSC
= 220 pF, R
OSC
= 120 kΩ for a switching frequency of 250 kHz.
Each gain setting is achieved by varying the input resistance of the amplifier, which can range from its smallest
value to over five times that value. As a result, if a single capacitor is used in the input high-pass filter, the -3 dB
or cutoff frequency also changes by over five times.
The -3-dB frequency can be calculated using equation Equation 6 . See Figure 12 . Note that due to process
variation, the input resistance, R
i
, can change by up to 20%.
In a 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 resistance of the amplifier (R
i
) form a
high-pass filter with the corner frequency determined in equation Equation 7 .
The value of C
i
is important, as it directly affects the bass (low frequency) performance of the circuit. Consider
the example where R
i
is 50 kΩ and the specification calls for a flat bass response down to 30 Hz. Equation
Equation 5 is reconfigured as equation Equation 8 .
In this example, C
i
is 0.1 µF, so one would likely choose a value in the range of 0.1 µF to 1 µF. Figure 12 can be
used to determine the input impedance for a given gain and can serve to aid in the calculation of C
i
.
17