Datasheet

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OUTPUT COUPLING CAPACITOR, C
C
f
c

1
2 R
L
C
C
(7)
1

C
B
 25 kΩ


1

C
I
R
I


1
R
L
C
C
(8)
SHUTDOWN MODE
USING LOW-ESR CAPACITORS
TPA302
SLOS174C – JANUARY 1997 – REVISED JUNE 2004
APPLICATION INFORMATION (continued)
In the typical single-supply, single-ended (SE) configuration, an output coupling capacitor (C
C
) is required to
block the dc bias at the output of the amplifier thus preventing dc currents in the load. As with the input coupling
capacitor, the output coupling capacitor and impedance of the load form a high-pass filter governed by
Equation 7 .
The main disadvantage, from a performance standpoint, is that the load impedances are typically small, which
drives the low-frequency corner higher. Large values of C
C
are required to pass low frequencies into the load.
Consider the example where a C
C
of 68 µF is chosen and loads vary from 8 Ω, 32 Ω, and 47 kΩ. Table 1
summarizes the frequency response characteristics of each configuration.
Table 1. Common Load Impedances vs Low Frequency
Output Characteristics in SE Mode
R
L
C
C
LOWEST FREQUENCY
8 Ω 68 µF 293 Hz
32 Ω 68 µF 73 Hz
47,000 Ω 68 µF 0.05 Hz
As Table 1 indicates, most of the bass response is attenuated into 8-Ω loads while headphone response is
adequate and drive into line level inputs (a home stereo for example) is good.
The output coupling capacitor required in single-supply, SE mode also places additional constraints on the
selection of other components in the amplifier circuit. The rules described previously still hold with the addition of
the following relationship:
The TPA302 employs a shutdown mode of operation designed to reduce quiescent supply current, I
DD(q)
, to the
absolute minimum level during periods of nonuse for battery-power conservation. For example, during device
sleep modes or when other audio-drive currents are used (i.e., headphone mode), the speaker drive is not
required. The SHUTDOWN input terminal should be held low during normal operation when the amplifier is in
use. Pulling SHUTDOWN high causes the outputs to mute and the amplifier to enter a low-current state,
I
DD
~ 0.6 µA. SHUTDOWN should never be left unconnected because amplifier operation would be
unpredictable.
Low-ESR capacitors are recommended throughout this applications section. A real capacitor can be modeled
simply as a resistor in series with an ideal capacitor. The voltage drop across this resistor minimizes the
beneficial effects of the capacitor in the circuit. The lower the equivalent value of this resistance the more the real
capacitor behaves like an ideal capacitor.
18