TPA321_07 Datasheet, PDF (18/26 Pages) Texas Instruments – 350-mW MONO AUDIO POWER AMPLIFIER WITH DIFFERENTIAL INPUTS

TPA321
SLOS312C – JUNE 2000 – REVISED JUNE 2004
fc
+
1
2 p RI CI
−3 dB
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f_c
(8)
The value of C_I is important to consider as it directly affects the bass (low-frequency) performance of the circuit.
Consider the example where R_I is 10 kΩ and the specification calls for a flat bass response down to 40 Hz.
Equation 8 is reconfigured as Equation 9.
CI
+
1
2 p RI fc
(9)
In this example, C_I is 0.40 µF, so one would likely choose a value in the range of 0.47 µF to 1 µF. A further
consideration for this capacitor is the leakage path from the input source through the input network (R_I, C_I) and
the feedback resistor (R_F) 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 V_DD/2, which is likely higher
than the source dc level. It is important to confirm the capacitor polarity in the application.
Power Supply Decoupling, C_S
The TPA321 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_DD lead, works best. For filtering
lower-frequency noise signals, a larger aluminum electrolytic capacitor of 10 µF or greater placed near the audio
power amplifier is recommended.
Midrail Bypass Capacitor, C_B
The midrail bypass capacitor, C_B, is the most critical capacitor and serves several important functions. During
start-up or recovery from shutdown mode, C_B determines the rate at which the amplifier starts up. The second
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.
ǒ_CB
10
250
kΩ^Ǔ
v
ǒ_RF
1
) RI^Ǔ
CI
(10)
As an example, consider a circuit where C_B is 2.2 µF, C_I is 0.47 µF, R_F is 50 kΩ, and R_I is 10 kΩ. Inserting these
values into the Equation 10 we get:
18.2 ≤ 35.5
which satisfies the rule. Bypass capacitor, C_B, values of 2.2-µF to 1-µF ceramic or tantalum low-ESR capacitors
are recommended for the best THD and noise performance.
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