TPA301
350-mW MONO AUDIO POWER AMPLIFIER
SLOS208C – JANUARY1998 – REVISED MARCH 2000
APPLICATION INFORMATION
component selection (continued)
In this example, C is 0.40 µF so one would likely choose a value in the range of 0.47 µF to 1 µF. A further
I
consideration for this capacitor is the leakage path from the input source through the input network (R , C ) and
I
I
thefeedbackresistor(R )totheload. Thisleakagecurrentcreatesadcoffsetvoltageattheinputtotheamplifier
F
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 V /2, which is likely higher
DD
than the source dc level. It is important to confirm the capacitor polarity in the application.
power supply decoupling, C
S
The TPA301 is a high-performance CMOS audio amplifier that requires adequate power supply decoupling to
ensure 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 µF, placed as close as possible to the device V
lower-frequency noise signals, a larger aluminum electrolytic capacitor of 10µF orgreaterplacedneartheaudio
lead, works best. For filtering
DD
power amplifier is recommended.
midrail bypass capacitor, C
B
The midrail bypass capacitor, C , is the most critical capacitor and serves several important functions. During
B
start-up or recovery from shutdown mode, C determines the rate at which the amplifier starts up. The second
B
function is to reduce noise produced by the power supply caused by coupling into the output drive signal. This
noise is from the midrail generation circuit internal to the amplifier, which appears as degraded PSRR and THD
+ N. The capacitor is fed from a 250-kΩ source inside the amplifier. To keep the start-up pop as low as possible,
the relationship shown in equation 10 should be maintained, which insures the input capacitor is fully charged
before the bypass capacitor is fully charged and the amplifier starts up.
10
250 kΩ
1
(10)
C
R
R
C
B
F
I
I
As an example, consider a circuit where C is 2.2 µF, C is 0.47 µF, R is 50 kΩ and R is 10 kΩ. Inserting these
B
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F
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values into the equation 10 we get:
18.2 35.5
which satisfies the rule. Bypass capacitor, C , values of 2.2 µF to 1 µF ceramic or tantalum low-ESR capacitors
B
are recommended for the best THD and noise performance.
using low-ESR capacitors
Low-ESR capacitors are recommended throughout this application. A real (as opposed to ideal) 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.
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