TPA0172
2-W STEREO AUDIO POWER AMPLIFIER
WITH I
2
C BUS
SLOS327C – AUGUST 2000 – REVISED MAY 2001
15
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
input capacitor, C
I
(continued)
In this example, C
I
is 0.47 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 (C
I
) and the
feedback network 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. Note that it is important to confirm the capacitor polarity in the application.
power supply decoupling, C
S
The TPA0172 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
BYP
The midrail bypass capacitor (C
BYP
) is the most critical capacitor and serves several important functions. During
start-up or recovery from shutdown mode, C
BYP
determines the rate at which the amplifier starts up. The second
function is to reduce noise produced by the power supply which is 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.
Bypass capacitor (C
BYP
) values of 0.47-µF to 1-µF ceramic or tantalum low-ESR capacitors are recommended
for the best THD and noise performance.
output coupling capacitor, C
C
In the typical single-supply 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 4.
(4)
f
c(high)
1
2 R
L
C
C
–3 dB
f
c
The main disadvantage, from a performance standpoint, is the load impedances are typically small, which drives
the low-frequency corner higher degrading the bass response. Large values of C
C
are required to pass low
frequencies into the load. Consider the example where a C
C
of 330 µF is chosen and loads vary from 3 Ω,
4 Ω, 8 Ω, 32 Ω, 10 kΩ, and 47 kΩ. Table 1 summarizes the frequency response characteristics of each
configuration.
