TPA302
300-mW STEREO AUDIO POWER AMPLIFIER
SLOS174B – JANUARY 1997 – REVISE MARCH 2000
17
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
power supply decoupling, CS
The TPA302 is a high-performance CMOS audio amplifier that requires adequate power supply decoupling to
ensure that 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 VDD lead, works best. For
filtering lower-frequency noise signals, a larger aluminum electrolytic capacitor of 10 µF or greater placed near
the power amplifier is recommended.
midrail bypass capacitor, CB
The midrail bypass capacitor, CB, serves several important functions. During startup or recovery from shutdown
mode, CB determines the rate at which the amplifier starts up. This helps to push the start-up pop noise into
the subaudible range (so low it can not be heard). 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. The capacitor is fed from a 25-kΩ source inside the amplifier. To keep the start-up pop as low
as possible, the relationship shown in equation 6 should be maintained.
(6)
1
ǒ
CB
25 kΩ
Ǔv
1
ǒ
CIRI
Ǔ
As an example, consider a circuit where CB is 0.1 µF, CI is 0.22 µF and RI is 10 kΩ. Inserting these values into
the equation 9 results in: 400 ≤ 454 which satisfies the rule. Bypass capacitor, CB, values of 0.1 µF to 1 µF
ceramic or tantalum low-ESR capacitors are recommended for the best THD and noise performance.
output coupling capacitor, CC
In the typical single-supply single-ended (SE) configuration, an output coupling capacitor (CC) 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.
(7)
fc
+
1
2
p
RLCC
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 CC 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.